Fiber Optic Network Optical Wavelength Transmission Bands & FOA

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Fiber Optic Network Optical Wavelength Transmission Bands As fiber optic networks have developed for longer distances, higher speeds and wavelength-division multiplexing (WDM), fibers have been used in new wavelength ranges, now called "bands," where fiber and transmission equipment can operate more efficiently. Singlemode fiber transmission began in the "O-band" just above the cutoff wavelength of the SM fiber developed to take advantage of the lower loss of the glass fiber at longer wavelengths and availablility of 1310 nm diode lasers. (Originally SM fibers were developed for 850 nm lasers where the fiber core was about half what it is for today's conventional SM fiber (5 microns as opposed to 8-9 microns at 1310 nm.) To take advantage of the lower loss at 1550 nm, fiber was developed for the C- band. As links became longer and fiber amplifiersbegan being used instead of optical-to-electronic-to-optical repeaters, the C-band became more

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Fiber Optic Network

Transcript of Fiber Optic Network Optical Wavelength Transmission Bands & FOA

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Fiber Optic Network Optical Wavelength Transmission Bands

As fiber optic networks have developed for longer distances, higher speeds and wavelength-division multiplexing (WDM), fibers have been used in new wavelength ranges, now called "bands," where fiber and transmission equipment can operate more efficiently. Singlemode fiber transmission began in the "O-band" just above the cutoff wavelength of the SM fiber developed to take advantage of the lower loss of the glass fiber at longer wavelengths and availablility of 1310 nm diode lasers. (Originally SM fibers were developed for 850 nm lasers where the fiber core was about half what it is for today's conventional SM fiber (5 microns as opposed to 8-9 microns at 1310 nm.) To take advantage of the lower loss at 1550 nm, fiber was developed for the C-band. As links became longer and fiber amplifiersbegan being used instead of optical-to-electronic-to-optical repeaters, the C-band became more important. With the advent of DWDM(dense wavelength-division multiplexing) which allowed multiple signals to share a single fiber, use of this band was expanded. Development of new fiber amplifiers (Raman and thullium-doped) promise to expand DWDM upward to the L-band. Several low-cost versions of WDM are in use, generally referred to as Coarse WDM or CWDM. Most do not work over long distances so do not require amplification, broading the wavelength choice. The most popular is FTTH PON systems, sending signals downstream to users at 1490 nm and using low cost 1310 nm transmission

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upstream. Early PON systems also use 1550 downstream for TV, but that is being replaced by IPTV on the downstream digital signal at 1490 nm. Other systems use a combination of S, C and L bands to carry signals because of the lower attenuation of the fiber. Some systems even use lasers at 20 nm spacings over the complete range of 1260 to 1660 nm but only with low water peak fibers.Manufacturers have been able to make fiber with low-water peaks, opening up a new transmission band (E-band), but it has not yet proven useful except for CWDM. It is probably mostly useful as an extension of the O-band but few applications have been proposed and it is very energy-intensive for manufacture.

DWDM Band Wavelength Range

Band Name Wavelengths Description

O-band1260 – 1360 nm

Original band, PON upstream

E-band1360 – 1460 nm

Water peak band

S-band1460 – 1530 nm

PON downstream

C-band1530 – 1565 nm

Lowest attenuation, original DWDM band, compatible with fiber amplifiers, CATV

L-band1565 – 1625 nm

Low attenuation, expanded DWDM band

U-band1625 – 1675 nm

Networks

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What Is A Network?

A network is basically a group of interconnected devices that share bandwidth and communicate with each other and probably devices in other networks too. Connections allow sending messages, sharing files and working in unison. The devices need a way to connect to each other first, with what is called a "physical layer" consisting of cable - copper or fiber- or radio - wireless. The physical layer can have several different architectures or connection schemes, generally called a a bus, ring or star. Then there are rules on how the devices identify themselves and share the bandwidth, and that's the basis of the network protocol.

All the devices in a network share the connection. One factor that is invariable is the sharing means that the total available bandwidth of the network is shared by the connected devices and is not available simultaneously to all devices. The possible exception is multiplexing schemes like wavelength-division multiplexing in fiber optics but WDM is actually separate networks using the same fiber infrastructure at different wavelengths. 

So a network is connected devices sharing bandwidth. Since they are sharing the network, they need unique identification, a way for one device to connect with another, a unique address to send communication from and to. They need rules for gaining access to the network to send messages without interfering with other messages or having their message interfered with. They need rules on how the data is formatted for transmission and transmitted on the network medium. Those rules are called the network protocol.

Several types of protocols are used. In a plain old telephone system (POTS), one phone was physically connected to another- literally in the early days operators patching wires at a switchboard, then through mechanical switches and finally electronic switches. This is called circuit switching. Switching electronically was done according to preprogrammed routines to efficiently allocate bandwidth along any given route and have alternatives if a route was busy.

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With the advent of digital phone systems, speeds were high enough that on top of physical switching, one could share a connection among many users by allocating a time slot on the connection of a faster network. A digitized phone call required only 64 kilobits per second. An early T1 phone line at 1.544 Mb/s could multiplex 24 calls at once. I will leave it as an exercise for the reader to figure how many calls a 10 gigabit/second phone link could carry! This is called "time-division multiplexing" (TDM - to introduce one of many TLAs (three letter acronyms) that will be used here.)

In the drawing below, note that the red signal has time slot 1 and 2, green 2 and 5, so this example is how a single connection shares bandwidth with 3 signals. Many more timeslots are possible. Note also it's synchronized, a signal is allocated a repeating time slot to guarantee its connection as long as the two communicating devices are connected.

The combination of physical switching and TDM was called "time-space-

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time"switching - TST for short. But TST was still inefficient. If there was silence on a phone call, the system still transmitted bits, even though there was no useful content to transmit, tying up otherwise useful bandwidth.

Along with data representing the digitized voice signal, every system needed to transmit data on addressing plus bits needed to synchronize the network. This overhead could use lots of valuable bandwidth. ATM, one phone protocol, had ~30% overhead. Others require time to process and encapsulate packets for transmission through networks using incompatible protocols. At one point Google declared that they only transmitted IP-Internet Protocol, reducing the overhead and latency - plus the power consumed in processing packets - along their networks. 

CATV - which means "community antenna TV" not "cable TV" - used a different multiplexing scheme to put many channels on one cable. They assigned a frequency to each channel just like broadcasting over the air, creating frequency division multiplexing or FDM. Digital data can be transmitted the same way by converting signals to frequency modulation or FM. This method was originally called "broadband" which can be confusing considering the current meaning which refers to any high speed connection.

It was not long after computers became available that users wanted them to talk to multiple peripherals and other computers in the office, thus was developed the local area network or LAN. The first networks used simple switching protocols like the phone system. That worked fine with a few devices connected to the network, but once minicomputers became common and PCs began being connected to networks, more efficient and faster methods were necessary.

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In the mid-70s, Xerox Palo Alto Research Labs invented Ethernet with a new type of protocol. All connected devices shared a coax cable data bus - the "ether." 

Rather than switch onto the network, a connected device would simply listen for traffic. If it heard none, it would transmit its message which was called a data packet. If two devices did so simultaneously, causing a collision, they would stop, wait a random time and try again. Like everything techie, this has a catchy acronym, CSMA/CD for "carrier sense multiple access with collision detection."

Unlike telephone connections, Ethernet packets are asynchronous. They do not require synchronous transmission like a phone network does for quality voice transmission. A packet of data is sent and a confirmation of receipt is returned. If there is more data to be sent, it goes in another packet until the entire data set is transmitted.

The problem with the Ethernet protocol was traffic. More connected devices means more collisions which means wasted bandwidth. With enough devices, one could not guarantee a device would ever get to send its data, a big problem for systems where some devices need priority, for alarms, for example. The solution is for the network to work faster - bandwidth, as we

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shall see, solves most network problems.

By the mid-1980s, a new connection architecture was being used to implement the Ethernet bus structure, but it was done with a "star" architecture and cabling system. Basically a coax bus is where each connected device is connected to the same physical cable so each device can listen to all the traffic. The star architecture adopted to replace the coax bus was a switched star, where each  device was connected to a hub, which was simply a repeater. By repeating the incoming signal to all connected devices, it could use the same protocol as the coax bus where all devices listened to the network all the time. A star with a hub had the same traffic problems that a coax bus had, collisions between data packets. So somebody got the idea of changing to a switch, back to the phone idea, where the switch looked at the address the data packet was being sent to and only used the proper links, keeping others open for other traffic. That was much more efficient.

IBM tried a different method. It connected devices in a ring. Each device would be granted access to the network by receiving a "token"- a data packet that gave permission to transmit data. Once finished transmitting, the device would release the token to the next user. Thus access to the network was guaranteed, but the overhead was higher.  Logically enough,

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IBM's network was called IBM Token Ring or TR. The token guaranteed access, so token ring was called a deterministic network. 

The token ring idea was used for the first all-fiber network, FDDI (Fiber Distributed Data Interface). FDDI was a 100Mb/s network with a dual counter-rotating ring for redundancy and reliability. The ring architecture was versatile. You could have dual-attach stations (DAS) connected to both rings, single-attached stations (SAS) connected to one ring through a concentrator.

So now we have three examples of networks, TST, CD/CSMA and TR. All are designed to allow users to share the available bandwidth of the network. 

The Internet added another protocol, Internet Protocol or IP, another packet-switched network. IP mainly differed by how addressing and switching are handled. The Internet was designed as a "mesh" network, with each switching unit connected to many others. When a packet reaches

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a switch, the switch has no fixed way of transmitting that packet along any route. Instead, it checks with other switches, now called routers, to find where the destination is and how to reach it. Routing allows for a more flexible network and a survivable one, since if a router fails, any others can carry the same message to its destination. You also don't get busy signals.

Data is broken into packets. Packets are individual batches of digital data with source and destination information, the data and some bits that are used to check to see if the data has been transmitted error-free. The source breaks the message into packets and the receiving device uses the information in the packet to reassemble it. Thus data may be transmitted without regard to its meaning. Data, video or even voice may be broken into packets and transmitted using IP.

Voice over IP (called VoIP) was troublesome at first. For voice to be transmitted with good audio quality, it needs to only be digitized at 64kb/s, very low bandwidth, but it needs to be send synchronously. If the signals are delayed or lost, the voice becomes garbled. Many schemes were tried to fix this but bandwidth solved it best. The faster the network, the lower the latency or signal delay, so the voice signal was clearer.

An interesting hybrid network has evolved recently. When telcos began installing FTTH (fiber to the home), it used a passive optic network (PON) architecture. PONs use an optical splitter to broadcast a downstream signal to all users and a collision avoidance protocol upstream. This is designed to reduce the cost of components as the downstream components in a central office are more expensive than the upstream ones which must be located at each subscriber. Because the downstream signal is sent to all users, it is encoded for each user to prevent eavesdropping. This same protocol is also being used for LANs (OLANs or optical LANs) that are proving to be much cheaper than other types of LANs.

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Bandwidth

Let's talk bandwidth. If you have a PC connected to a network over Gigabit Ethernet, do you have 1 gigabit of bandwidth to use as you see fit? Does it allow you to download large files like video at an average rate of 1 Gb/s? Not at all. You may be transmitting or receiving data at a rare of 1 Gb/s but you are on a network sharing the available bandwidth with others. You might be sharing this gigabit network with 100 users. Does that mean that your bandwidth is 1 Gb/s divided by 100 or 10 Mb/s? Not really, because network traffic is asynchronous, with bandwidth allocated to users based on timing and demand. Big data users need lots of bandwidth and small users only need a little.

One of the biggest advantages of higher bandwidth is latency. The faster

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the connection, less time is needed to transmit data so users tie up the network for less time. If the network has less bandwidth, traffic backs up but eventually gets through, just like heavy vehicular traffic. Data, which is not inherently time-sensitive, is not corrupted by this back-up, but voice and video can be if the delay is significant.

Likewise, noisy links can cause errors. Data carries flags to find errors caused during transmission and corrupted data is retransmitted. This does not work with voice or video as they require continuous data streaming. Voice or video packets with errors must be dropped and data continues streaming.

Thus quality of transmission requires high bandwidth and low BER - bit error rate. 

Now we have another issue. Data within an enterprise network will have different rates. The "wired" devices may connect at 100Mb/s or 1000Mb/s (1Gb/s) while WiFi may be much less, depending on the implementation. But overall, data rates are likely to be higher than the connection to the outside world by the Internet. It is likely the connection to the outside world is ~10-100Mb/s, although many companies still have T1 connections at 1.544Mb/s!

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The bandwidth "funnel effect"

This is not a problem for most companies using internal data centers where most of the traffic is within the corporate LAN, but the increase in video downloads and the advent of "Cloud Computing" can change the traffic patterns. Instead of data mostly flowing within the network, it must reach the data center through the Internet. If the corporate LAN has only a slow connection to the Internet, those traffic backups described above will affect data latency and voice and video quality. Thus "cloud computing" not only requires a full-time Internet connection, it requires a fast one.

Likewise, the entire path data follows from point to point has high and low bandwidth sections. A metropolitan US link might be 10-100Gb/s while

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portions of the link in rural areas might be down to T1 rates. Routers along the line could direct the data packet one way which has higher speeds at one time then direct it through a slower link at another time. Generally speaking, high traffic routes will be built as higher bandwidth routes, but traffic patterns change, so carriers will usually be trying to catch up with network usage. That is the big problem with cellular wireless now, as smartphones have been growing and using bandwidth faster than the cellular operators can provide it by connecting towers to high speed fiber backbones.

There is another solution to Internet backbone traffic problems - build more local data centers and store duplicates of all the data there so traffic is confined to a local area. This is widely done by Google for example, which represents a significant amount of Internet traffic. Netflix is another service that mirrors data locally, especially important since they can be a majority of all Internet traffic in the US in the evening.

If you have ever done a speed test on your Internet connection, you have seen the speed and latency at that point in time, but have you noticed it changes? For example, we have tested both cable modem and satellite connections at our home over the course of time. In the weekday mornings, when people are away at work or school, our connection speeds are typically ~17-18 Mb/s for both services. After ~3PM, when people get home from school and work, the speed drops to ~2Mb/s, a 90% decrease! That is because of all the video entertainment downloads, where the majority of Internet traffic by consumers is video and the peak loading occurs when they are home. 

How does this affect users? When the traffic is high, services that require low latency like Skype or other voice over IP (VoIP) services provide poor quality connections. Video requires more local buffering at the user. How do you fix this? Prioritize services or provide more bandwidth. 

Another factor affects perceived network speeds, server response in data centers. Users think of network speeds in terms of how long does it take to download web pages. But web page designers are sometimes unconcerned with downloading times, loading pages with graphics and scripts that mean large numbers of files need to be downloaded - We have seen 300+ files in one page! When a server in a data center gets a request for this page, it

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has to hunt down  all those files and forward them as separate files. 

Even if the web page only uses a handful of files, the time the data center servers takes to find those files, download them from servers and send them along to the requester is often the biggest delay the user sees. Tests have shown that Internet connection speeds above 10Mb/s provide little benefit for web browsing. Higher speeds mostly benefit video and to a lesser extent, VoIP.

Is the Internet The Best Solution For EVERYTHING?

Hardly. Telephone service worked best on a TST switched POTs network. CATV provided video best on a broadband (meaning FM FDM in this case) network that broadcast all channels simultaneously. Data flows best on IP. But enough bandwidth makes it possible for all three to share the Internet.

As you can see, networks have evolved to accommodate new technology and new services, not necessarily in the most efficient way possible. Voice worked best on a switched network. Data works best in packets. Video worked best on on a broadcast model, carried over into the CATV format. Now we force everything into a packetized IP network and the only to get good quality of service is to provide lots of bandwidth.

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Fiber Optic Data Links

The purpose of this document is to define a “fiber optic datalink,” its purpose, design and performance. It is intended to provide guidance for the designer of datalinks or communications systems and the installer of fiber optic systems who must verify the performance of the datalink including the cable plant installed for its operation.

Datalinks

A fiber optic datalink is a communications subsystem that connects inputs and outputs (I/O) from electronic subsystems and transmits those signals over optical fiber. In this function, a fiber optic datalink operates as an alternative to copper cabling or a wireless subsystem. In typical applications, a fiber optic datalink acts as a communications medium attached to electronics on either end that provide the other services necessary for communications over the link. In the OSI (Open Systems Interconnection) Network Model, the datalink is basically the first layer, called the Physical Layer or PHY.

Signals and Protocols

Fiber optic datalinks may transmit signals that are either analog or digital and of many different, usually standardized, protocols, depending on the communications system(s) it supports. Datalinks may be protocol transparent but may also include data encoding to provide more robust data communications. Datalinks may be specified by the application or standardized network (e.g. Ethernet) they are intended to support or by the types and bandwidth of signals they are designed to transmit.

Figure 1. Components Of A Fiber Optic Datalink

A fiber optic datalink consists of fiber optic transceivers or individual

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transmitters and receivers at either end that transmit over optical fibers. The typical datalink transmits over two fibers for full duplex links, one fiber in each direction. The fibers may be of any type, multimode (graded index or step index) or singlemode. 

Some links may use couplers and wavelength-division multiplexing to transmit bi-directionally over a single fiber as in FTTH PONs passive optical networks or OLANs, optical LANs. Some links may allow transmission at several wavelengths of light simultaneously over a single fiber in each direction, called wavelength-division multiplexing.

Extremely long cable plant lengths may require regeneration using repeaters, essentially datalinks in series.  Optical fiber amplifiers may be used as repeaters in long singlemode systems. Singlemode systems using fiber amplifiers and wavelength-division multiplexing may require concern for nonlinear effects from high optical power involved.

Analog or Digital?

Analog signals are continuously variable signals where the information in the signal is contained in the amplitude of the signal over time. Digital signals are sampled at regular time intervals and the amplitude converted to digital bytes so the information is a digital number. Analog signals are the natural form of most data, but are subject to degradation by noise in the transmission system. As an analog signal is attenuated in a  cable, the signal to noise ratio becomes worse so the quality of the signal degrades. Digital signals can be transmitted long distances without degradation as the signal is less sensitive to noise.

Fiber optic datalinks can be either analog or digital in nature, although most are digital. Both have some common critical parameters and some major differences. For both, the optical loss margin or power budget is most important. This is determined by connecting the link up with an adjustable attenuator in the cable plant and varying the loss between transmitter and receiver until one can generate the curve shown above. Analog datalinks will be tested for signal to noise ratio to determine link margin, while digital links

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use bit error rate as a measure of performance. Both links require testing over the full bandwidth specified for operation, but most data links are now specified for a specific network application, like AM CATV or RGB color monitors for analog links and SONET, Ethernet or Fibre Channel for digital links.

Transceiver

A fiber optic transceiver used on each end of a link includes a transmitter and receiver that convert electrical signals to optical signals and vice versa for transmission over optical fiber. Appropriate interfaces are included in the datalink to mate with the electrical and optical signals it connects with. These are typically standardized electrical and fiber optic connectors.

 

Figure 2. Fiber optic transceiver block diagram

The transmitter consists of an electrical input and signal conditioning circuitry to drive an optical source, a light-emitting diode or laser that provides the electrical to optical conversion and alignment hardware for coupling of the optical signal into an optical fiber for transmission.

The receiver consists of a detector that connects to the optical fiber to accept an optical signal, convert the optical signal that has been transmitted through the optical fiber to an electrical signal and conditioning circuitry that creates an electrical output compatible with the communications system.

Transceivers are dedicated to one type of fiber determined by the distance and bandwidth of the communications being transmitted. Multimode fiber may

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be used for shorter and/or slower datalinks while singlemode fiber is used for longer links. The source in the transceiver will also depend on the application. 

LEDs are used for slower (<~100 Mb/s) multimode links and VCSELs are used for faster multimode links.  Some standard networks have options for using singlemode 1310nm lasers on multimode fiber. There are currently four types of multimode fiber used for datalinks, designated OM1-4. OM-1 is a fiber with larger core (62.5 microns) used primarily in legacy systems with LEDs at 850 or 1300 nm wavelengths. Faster multimode links use OM-2, OM-3 or OM-4 fiber with a 50 micron core, generally the faster fibers designated OM-3 or OM-4 which are optimized for 850 nm VCSELs.

Singlemode systems use lasers at 1310 or ~1550nm. 1310nm lasers are used for shorter links. The longer wavelengths around 1550 nm are used for long links and those using wavelength-division multiplexing. There are several specialized singlemode fibers which are optimized for special applications.

More on fiber optic transceivers and their components

Power Budget

All datalinks are limited by the power budget of the link. The power budget is the difference between the output power of the transmitter and the input power requirements of the receiver. The receiver has an operating range determined by the signal-to-noise ratio (S/N) in the receiver. The S/N ratio is generally quoted for analog links while the bit-error-rate (BER) is used for digital links. BER is practically an inverse function of S/N.

Figure 3. BER vs received optical power for a fiber optic transceiver

The operating range of a data link will look like figure 3. There must be a

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minimum power at the receiver to provide an acceptable S/N or BER. As the power increases, the BER or S/N improves until the signal becomes so high it overloads the receiver and receiver performance degrades rapidly.

The power of the receiver is determined by the output power of the transmitter diminished by the loss in the cable plant primarily but other factors may affect power budget performance. When the power budget is inadequate for the loss in the cable plant, higher power transmitters or more sensitive receivers or higher bandwidth cable plant are required.

Figure 4. Contributions to link power budget.

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 Figure 5. Link power budget

Transceivers may be components intended to be built into electronic subsystems to create communications equipment. Standalone transceivers may be sold as media converters to convert signals from electrical on copper cabling to fiber optics. Built-in transceivers are generally powered by the communications equipment in which they are installed. Media converters are powered by separate sources of electrical power.

Fiber Optic Cable Plant

A fiber optic datalink transmits signals as pulses or varying light over optical fibers that are included in a fiber optic cable plant. The permanently installed cable plant consists of an optical fiber cable designed to protect the fibers, which is installed, spliced and terminated with proper hardware to mate with the datalink transceivers. The cable plant must be selected and installed to withstand the environment in which it is installed. The cable plant is typically terminated at outlets or patch panels near the communications equipment. The installed cable plant is typically connected to the transceivers by short patchcords.

The fiber optic cable plant must be compatible with the performance parameters of the transceivers for the link to operate properly. This includes types of fiber and connectors, optical loss and bandwidth of the cable plant. For the cable plant, a loss budget must be calculated to estimate its loss and a power budget to determine if the planned communications system will operate over the cable plant. Fiber polarity is important for design and documentation. Since the transmitter must be connected to the receiver for the link to operate, there must be a crossover in the fiber pair at some point in the cable plant. Generally, the

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permanent cable plant is installed straight through, with fiber 1 connected to fiber 1 on each end, and so forth. Since the link requires one crossover, that is accomplished by a crossover patch cord on one end of the link. The location of crossover patchcords should be limited to one area, e.g. the main equipment room, so all duplex patch cords at that location are crossover cables while straight through cables are used at all other locations.

Cable Plant Performance Parameters

The factors that determine the required performance parameters for a fiber optic datalink are those that define the communications signals to be carried on the link (the data bitrate (digital transmission) or bandwidth (analog transmission) at which the link operates), the length of the link and the specifications (bandwidth and optical loss) of the fiber optic cable plant. These factors determine the types of transceivers and cable plant components that must be chosen for a communications system.

The two major factors of concern in link design and testing after installation are the loss of the cable plant and the bandwidth.

Cable Plant Loss

The loss of the cable plant is determined by the summation of the loss in the cable plant due to fiber attenuation, splice loss and connector loss. In some cases the fiber attenuation may be increased by improper installation of the cable. As a signal travels down the fiber, the signal will be attenuated by the optical fiber and reduced by the loss in connectors and splices.

 

Figure 6. Loss of signal by attenuation in the cable plant

Loss Budgets

For each cable plant designed, one must calculate a loss budget. The loss budget estimates the loss of the fiber in the cable plant by multiplying the length (km) by the attenuation coefficient (dB/km), then adding the loss from

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connectors and/or splices determined by the number of connectors and/or splices times the estimated loss each to get the total estimated loss of the cable plant. The cable plant loss budget must be lower than the power budget of the link transceivers (see below) for the link to work properly.

Dispersion

Dispersion or pulse spreading limits the bandwidth of the link. Transceivers have some dispersion caused by the limitations of the electronics and electro-optical components but most of the dispersion comes from the limited bandwidth of the fiber in the cable plant.

 

Figure 7. Dispersion of signal in the cable plant

Dispersion in multimode optical fiber occurs by modal dispersion or chromatic dispersion. Modal dispersion is caused by the different velocities of the various modes being transmitted in the fiber. Chromatic dispersion is caused by the different velocities of light at different wavelengths. 

Singlemode fiber also causes dispersion, but generally only in very long links. Chromatic dispersion has the same cause as multimode fiber, the differences in the speed of light at different wavelengths. Singlemode fiber may also suffer from polarization-mode dispersion causes by the different speeds of polarized light in the fibers.

The transceiver must be chosen to provide proper performance for the communications system’s requirements for bandwidth or bitrate and to provide an optical transmitter output of sufficient power and receiver of adequate sensitivity to operate over the optical loss caused by the cable plant of the communications system. The difference in the transmitter output and receiver sensitivity defines the optical power budget of the link.

The cable plant components, optical fiber, splices and connectors, are chosen to allow sufficient distance and bandwidth performance with the transceivers to meet the communications system’s optical power budget requirements. The power budget of the link defines the maximum loss budget for the cable plant. The maximum link length will be determined by the power budget and loss budget for low bit rate links that will be derated for dispersion for higher

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bandwidth links.

Most standardized communications systems will specify the performance of the components including interfaces to the electronic I/O and types of fiber supported for various distances. Systems standards may also include specifications for fiber optic connector type, primarily at the transceiver. Most communications systems with short links have options for both multimode and singlemode fiber while longer links use only singlemode fiber. All networks may provide guidance as to the types or grades of fiber needed to support certain applications.

Every manufacturer of datalinks components and systems specifies their link for receiver sensitivity (perhaps a minimum power required) and minimum power coupled into the fiber from the source. Typical values for these parameters are shown in the table below. In order for a manufacturer or system designer to test them properly, it is necessary to know the test conditions. For data link components, that includes input data frequency or bitrate and duty cycle, power supply voltages and the type of fiber coupled to the source. For systems, it will be the diagnostic software needed by the system.

Typical Fiber optic link/system performance parameters

Link typeSource/Fiber Type

Wave-

length (nm)

Transmit Power (dBm)

Receiver Sen- sitivity (dBm)

Margin (dB)

Telecom laser/SM 1300/1550 +3 to -6 -30 to -45 30 to 40

DWDM 1550 +20 to 0 -30 to -45 40 to 50

Datacom LED/VCSEL 850 -3 to -15 -15 to -30  3 to 25

LED/laser 1300 -0 to -20 -15 to -30 10 to 25

CATV(AM) laser/SM 1300/1550 +10 to 0 0 to -10 10 to 20

Within the world of datacommunications links and networks, there are many vendor-specific fiber optic systems, but there are also a number of industry standard networks such as Ethernet which have fiber optic standards. These networks have agreed upon specifications common to all manufacturers' products to insure interoperability. This page in FOA Tech Topics shows a summary of specifications for many of these systems.

Testing Datalink Performance

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Design and Manufacturing

During the design and manufacturing process, datalinks are tested for proper transmission over specified distances for optical loss and bandwidth. Loss testing is done by transmitting through a variable optical attenuator over short lengths of fiber (to eliminate dispersion effects) to create a diagram of bit error rate (BER) vs received optical power (or signal-to-noise ratio, SNR, the inverse of BER) which shows the range of optical loss the transmitter/receiver pair can operate over.

Figure 8. BER vs received optical power

In the drawing above, the link has a lower BER as the optical power increases since the S/N ratio improves. This generally continues until the receiver power becomes too high and the receiver overloads. If the transmitter power is high and the cable plant loss low, it is possible in some systems to overload the receiver. In that case an optical attenuator should be used at the receiver to lower power to acceptable levels.

Bandwidth testing is generally done by bit error rate testing or examining “eye diagrams” of signals for dispersion. The most common way to overcome dispersion effects is to increase optical power at the receiver by reducing the length of the cable plant and/or lowering the loss budget of the cable plant.

 

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Figure 9. Bandwidth Eye diagram

This type of testing will provide specifications for the datalink as to the types of fiber required and the loss budget of the link. It is those specifications that will determine the use of the datalink in a communications system design and what performance parameters will need testing after installation.

Installed Datalink Testing

Testing installed datalinks includes testing the cable plant first then testing the operation of the datalink transceivers over the cable plant. The cable plant will be tested after installation to ensure the installation was done properly before the communications system is installed and tested.

Multimode cable plants are rarely tested for bandwidth but require testing for optical loss (called insertion loss.)

Short singlemode cable plants are also tested for loss only, but long distance high bitrate singlemode systems may also require testing the cable plant for bandwidth, e.g. chromatic dispersion and polarization mode dispersion. Short links may also have problems with reflectance from connectors, so special non-reflective APC style connectors are usually specified since reflectance testing can be difficult.

Testing the operation of the transceivers with the cable plant includes optical power testing of the output of the transmitter and the receiver input power compared to specifications for the link.

Figure 10. Power testing of datalink.

After the datalink or communications system is installed, testing the BER or SNR of the complete communications link may be done to confirm that the link is operating properly.

FOA Standards for testing cable plant loss and optical power can be used to properly specify test requirements.

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Test Your Comprehension

After you study this page and "More on fiber optic transceivers and their components" you should test your comprehension here.  

Fiber Optic Transmitters and Receivers (Transceivers)

 Fiber Optic Datalink

Fiber optic transmission systems (datalinks) all work similar to the diagram shown above. They consist of a transmitter on one end of a fiber and a receiver on the other end. Most systems operate by transmitting in one direction on one fiber and in the reverse direction on another fiber for full duplex operation.

Fiber Optic Transceiver

Most systems use a "transceiver" which includes both transmission and receiver in a single module. The transmitter takes an electrical input and converts it to an optical output from a laser diode or LED. The light from the transmitter is coupled into the fiber with a connector and is transmitted through the fiber optic cable plant. The light from the end of the fiber is coupled to a receiver where a detector converts the light into an electrical signal which is then

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conditioned properly for use by the receivingequipment.

Sources for Fiber Optic Transmitters

The sources used for fiber optic transmitters need to meet several criteria: it has to be at the correct wavelength, be able to be modulated fast enough to transmit data and be efficiently coupled into fiber.

Four types of sources are commonly used, LEDs, fabry-perot (FP) lasers, distributed feedback (DFB) lasers and vertical cavity surface-emitting lasers (VCSELs). All convert electrical signals into optical signals, but are otherwise quite different devices. All three are tiny semiconductor devices (chips). LEDs and VCSELs are fabricated on semiconductor wafers such that they emit light from the surface of the chip, while f-p lasers emit from the side of the chip from a laser cavity created in the middle of the chip.

LEDs have much lower power outputs than lasers and their larger, diverging light output pattern makes them harder to couple into fibers, limiting them to use with multimode fibers. Laser have smaller tighter light outputs and are easily coupled to singlemode fibers, making them ideal for long distance high speed links. LEDs have much less bandwidth than lasers and are limited to systems operating up to about 250 MHz or around 200 Mb/s. Lasers have very high bandwidth capability, most being useful to well over 10 GHz or 10 Gb/s. 

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Because of their fabrication methods, LEDs and VCSELs are cheap to make. Lasers are more expensive because creating the laser cavity inside the device is more difficult, the chip must be separated from the semiconductor wafer and each end coated before the laser can even be tested to see if its good.

Typical Fiber Optic Source Specifications

Device TypeWavelength (nm)

Power intoFiber (dBm)

Bandwidth Fiber Types

LED 850, 1300-30 to -10

<250 MHz MM

Fabry-Perot Laser

850, 1310 (1280-1330) 1550 (1480-1650)

0 to +10 >10 GHz MM, SM

DFB Laser 1550 (1480-1650)

0 to +25 >10 GHz SM

VCSEL 850 -10 to 0 >10 GHz MM

LEDs have a limited bandwidth while all types of lasers are very fast. Another big difference between LEDs and both types of lasers is the spectral output. LEDs have a very broad spectral output which causes them to suffer chromatic dispersion in fiber, while lasers have a narrow spectral output that suffers very little chromatic dispersion. DFB lasers, which are used in long distance and DWDM systems, have the narrowest spectral width which minimizes chromatic dispersion on the longest links. DFB lasers are also highly linear (that is the light output directly follows the electrical input) so they can be used as sources in AM CATV systems.

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The choice of these devices is determined mainly by speed and fiber compatibility issues.  As many premises systems using multimode fiber have exceeded bit rates of 1 Gb/s, lasers (mostly VCSELs) have replaced LEDs. The output of the LED is very broad but lasers are very focused, and the sources will have very different modal fill in the fibers. The restricted launch of the VCSEL (or any laser) makes the effective bandwidth of the fiber higher, but laser-optimized fiber, usually OM3, is the choice for lasers.

The electronics for a transmitter are simple. They convert an incoming pulse (voltage) into a precise current pulse to drive the source. Lasers generally are biased with a low DC current and

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modulated above that bias current to maximize speed.

Detectors for Fiber Optic Receivers

Receivers use semiconductor detectors (photodiodes or photodetectors) to convert optical signals to electrical signals. Silicon photodiodes are used for short wavelength links (650 for POF and 850 for glass MM fiber). Long wavelength systems usually use InGaAs (indium gallium arsenide) detectors as they have lower noise than germanium which allows for more sensitive receivers.

Very high speed systems sometimes use avalanche photodiodes (APDs) that are biased at high voltage to create gain in the photodiode. These devices are more expensive and more complicated to use but offer significant gains in performance.

Packaging

Transcivers are usually packaged in industry standard packages like these XFP modules for gigabit datalinks(L) and Xenpak (R). The XFP modules connect to a duplex LC connector on the optical end and a standard electrical interface on the other end. The Xenpak are for 10 gigabit networks but use SC duplex connection. Both are similar to media converters but are powered from the equipment they are built into.

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Performance

Just as with copper wire or radio transmission, the performance of the fiber optic data link can be determined by how well the reconverted electrical signal out of the receiver matches the input to the transmitter. The discussion of performance on datalinks applies directly to transceivers which supply the optical to electrical conversion.

Every manufacturer of transcivers specifies their product for receiver sensitivity (perhaps a minimum power required) and minimum power coupled into the fiber from the source. Those specifications will end up being the datalink specifications on the final product used in the field.

  

Test Your ComprehensionAfter you study this page and "More on fiber optic datalinks", you should test your comprehension here.  

Page 32: Fiber Optic Network Optical Wavelength Transmission Bands & FOA

Wavelength Division Multiplexing (WDM)

Why Is WDM Used?With the exponential growth in communications, caused mainly by the wide acceptance of the Internet, many carriers are finding that their estimates of fiber needs have been highly underestimated. Although most cables included many spare fibers when installed, this growth has used many of them and new capacity is needed. Three methods exist for expanding capacity: 1) installing more cables, 2) increasing system bitrate to multiplex more signals or 3) wavelength division multiplexing.Installing more cables will be the preferred method in many cases, especially in metropolitan areas, since fiber has become incredibly inexpensive and installation methods more efficient (like mass fusion splicing.) But if conduit space is not available or major construction is necessary, this may not be the most cost effective.Increasing system bitrate may not prove cost effective either. Many systems are already running at SONET OC-48 rates (2.5 GB/s) and upgrading to OC-192 (10 GB/s) is expensive, requires changing out all the electronics in a network, and adds 4 times the capacity, more than may be necessary.The third alternative, wavelength division

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multiplexing (WDM), has proven more cost effective in many instances. It allows using current electronics and current fibers, but simply shares fibers by transmitting different channels at different wavelengths (colors) of light. Systems that already use fiber optic amplifiers as repeaters also do not require upgrading for most WDM systems.

How Does WDM Work?It is easy to understand WDM. Consider the fact that you can see many different colors of light - reg, green, yellow, blue, etc. all at once. The colors are transmitted through the air together and may mix, but they can be easily separated using a simple device like a prism, just like we separate the "white" light from the sun into a spectrum of colors with the prism.

Figure 1. Separating a beam of light into its colors

This technique was first demonstrated with optical fiber in the early 80s when telco fiber optic links still used multimode fiber. Light at 850 nm and 1300 nm was injected into the fiber at one end using a simple fused coupler. At the far end of the fiber, another coupler split the light into two fibers, one sent to a

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silicon detector more sensitive to 850 nm and one to a germanium or InGaAs detector more sensitive to 1300 nm. Filters removed the unwanted wavelengths, so each detector then was able to receive only the signal intended for it.

Figure 2. WDM with couplers and filters

By the late 80s, all telecom links were singlemode fiber, and coupler manufactures learned how to make fused couplers that could separate 1300nm and 1550 nm signals adequately to allow WDM with simple, inexpensive components. However, these had limited usefulness, as fiber was designed differently for 1300nm and 1550 nm, due to the dispersion characteristics of glass. Fiber optimized at 1300 nm was used for local loop links, while long haul and submarine cables used dispersion-shifted fiber optimized for performance at 1550 nm. This simple version of WDM is widely used in fiber to the home (FTTH) applications. Signals are sent downstream to the subscriber at 1490 nm (and 1550 for analog CATV if used) and upstream at

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1310 n. Read more on FTTH. With the advent of fiber optic amplifiers for repeaters in the late 80s, emphasis shifted to the 1550 nm transmission band. WDM only made sense if the multiplexed wavelengths were in the region of the fiber amplifiers operating range of 1520 to 1560 nm. It was not long before WDM equipment was able to put 4 signals into this band, with wavelengths about 10 nm apart.The input end of a WDM system is really quite simple. It is a simple coupler that combines all the inputs into one output fiber. These have been available for many years, offering 2, 4, 8, 16, 32 or even 64 inputs. It is the demultiplexer that is the difficult component to make.

Figure 3. WDM demultiplexer

The demultiplexer takes the input fiber and collimates the light into a narrow, parallel beam of light. It shines on a grating (a mirror like device that works like a prism, similar to the data side of a CD) which separates the light into the different

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wavelengths by sending them off at different angles. Optics capture each wavelength and focuses it into a fiber, creating separate outputs for each separate wavelength of light.

WDM to DWDMCurrent systems offer from 4 to 32 channels of wavelengths. The higher numbers of wavelengths has lead to the name Dense Wavelength Division Multiplexing or DWDM. The technical requirement is only that the lasers be of very specific wavelengths and the wavelengths are very stable, and the DWDM demultiplexers capable of distinguishing each wavelength without crosstalk.

Advantages of WDMA WDM system has some features that make them very useable. Each wavelength can be from a normal link, for example a OC-48 link, so you do not obsolete most of your current equipment. You merely need laser transmitterss chosen for wavelengths that match the WDM demultiplexer to make sure each channel is properly decoded at the receiving end. If you use an OC-48 SONET input, you can have 4X2.5 GB/s = 10 GB/s up to 32 X 2.5 GB/s = 80 GB/s. While 32 channels are the maximum today, future enhancements are expected to offer 80-128 channels!And you are not limited to SONET, you can use Gigabit Ethernet for example, or you can mix and match SONET and Gigabit Ethernet or any other digital signals! You can even mix in analog

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channels like CATV, as is done with  BPON FTTH systems.

RepeatersAnother technology that facilitates DWDM is the development of fiber optic amplifiers for use as repeaters. They can amplify numerous wavelengths of light simultaneously, as long as all are in the wavelength range of the FO amplifier. They work best in the range of 1520-1560 nm, so most DWDM systems are designed for that range. Now that fiber has been made with less effect from the OH absorption bands at 1400 nm and 1600 nm, the possible range of DWDM has broadened considerably. Technology needs development for wider range fiber amplifiers to take advantage of the new fibers.

ApplicationsTwo obvious applications are already in use, submarine cables and extending the lifetime of cables where all fibers are being used. For submarine cables, DWDM enhances the capacity without adding fibers, which create larger cables and bulkier and more complicated repeaters. Adding service in areas where cables are now full is another good application.But this technology may also reduce the cost on all land-based long distance communications links and new technology may lead to totally new network architectures.

Further Enhancements

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Imagine an all-optical network that uses DWDM, switches signals in the optical domain without converting signals to electronics, and can add or drop signals by inserting or withdrawing wavelengths at will. All this is being researched right now, and given the speed with which optical technology advances, an all-optical network may not be far in the future!

CWDMCoarse wavelength-division multiplexing is another variant of WDM. Generally CWDM refers to using lasers spaced 20 nm apart over the full range of 1260 to 1670 nm. It only works on low water peak fibers, where the high water absorption bands have been eliminated in the manufacture of the fiber.

(C) 2003-13, The Fiber Optic Association, Inc.

Return To The FOA Home Page

Specifications For Fiber Optic Networks

Per current standards and specs, maximum supportable distances and attenuation for optical fiber applications by fiber type.

Multimode Fiber Network Specifications

Application

Parameter Multimode Fiber Type

62.5/125 μm 50/125 μm

850 nm laser-optimized50/125 μm

850 nm laser-optimized50/125 μm

TIA 492AAAA (OM1)

TIA 492AAAB (OM2)

TIA 492AAAC(OM3)

TIA 492AAAD(OM4)

Nominal wavelen

850

1300

850 1300

850 1300

850 1300

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gth (nm)

Ethernet10/100BASE-SX

Channel attenuation (dB) 4.0 - 4.0 - 4.0 - 4.0 -Supportable distance m (ft)

300(984) -

300(984) -

300(984) -

300(984) -

Ethernet100BASE-FX

Channel attenuation (dB) - 11.0 - 6.0 - 6.0 - 6.0Supportable distance m (ft) -

2000(6560) -

2000(6560) -

2000(6560) -

2000(6560)

Ethernet1000BASE-SX

Channel attenuation (dB) 2.6 - 3.6 - 4.5 - 4.8 -Supportable distance m (ft)

275(900) -

550(1804) -

800(2625) -

880(2887) -

Ethernet1000BASE-LX

Channel attenuation (dB) - 2.3 - 2.3 - 2.3 - 2.3Supportable distance m (ft) -

550(1804) -

550(1804) -

550(1804) -

550(1804)

Ethernet10GBASE-S

Channel attenuation (dB) 2.4 - 2.3 - 2.6 - 3.1 -Supportable distance m (ft)

33(108) -

82(269) -

300(984) -

450(1476) -

Ethernet10GBASE-LX4

Channel attenuation (dB) - 2.5 - 2.0 - 2.0 - 2.0Supportable distance m (ft) -

300(984) -

300(984) -

300(984) -

300(984)

Ethernet10GBASE-LRM

Channel attenuation (dB) - 1.9 - 1.9 - 1.9 - 1.9Supportable distance

- 220(720)

- 220(720)

- 220(720)

- 220(720)

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m (ft)

Ethernet40GBASE-SR4

Channel attenuation (dB)  - - - - 1.9 - 1.9 -Supportable distance m (ft) - - - -

100(328) -

125(410) -

Ethernet100GBASE-SR10

Channel attenuation (dB) - - - - 1.9 - 1.9 -Supportable distance m (ft) - - - -

100(328) -

125(410) -

1G Fibre Channel100-MX-SN-I (1062 Mbaud)

Channel attenuation (dB) 3.0 - 3.9 - 4.6 - 4.6 -Supportable distance m (ft)

300(984) -

500(1640) -

860(2822) -

860(2822) -

2G Fibre Channel200-MX-SN-I (2125 Mbaud)

Channel attenuation (dB) 2.1 - 2.6 - 3.3 - 3.3 -Supportable distance m (ft)

150(492) -

300(984) -

500(1640) -

500(1640) -

4G Fibre Channel400-MX-SN-I (4250 Mbaud)

Channel attenuation (dB) 1.8 - 2.1 - 2.9 - 3.0 -Supportable distance m (ft)

70(230) -

150(492) -

380(1247) -

400(1312) -

10G Fibre Channel1200-MX-SN-I (10512 Mbaud)

Channel attenuation (dB) 2.4 - 2.2 2.6 - 2.6 -Supportable distance m (ft)

33(108) -

82(269) -

300(984) -

300(984) -

16G Fibre Channel1600-MX-SN (10512

Channel attenuation (dB) - - 1.6 - 1.9 - 1..9 -Supportable distance

- - 35(115)

- 100(328)

- 125(410)

-

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Mbaud)m (ft)

FDDI PMDANSI X3.166

Channel attenuation (dB) - 11.0 - 6.0 - 6.0 - 6.0Supportable distance m (ft) -

2000(6560) -

2000(6560) -

2000(6560) -

2000(6560)

- means Not Applicable

Singlemode Fiber Network Specifications

Application Parameter Single-mode

TIA 492CAAA(OS1)  orTIA 492CAAB (OS2)

Nominal wavelength (nm) 1310 1550

Ethernet1000BASE-LX

Channel attenuation (dB) 4.5 -Supportable distance m (ft)

5000(16405) -

Ethernet10GBASE-LX4

Channel attenuation (dB) 6.3 -Supportable distance m (ft)

10000(32810) -

Ethernet10GBASE-L

Channel attenuation (dB) 6.2 -Supportable distance m (ft)

10000(32810) -

Ethernet10GBASE-E

Channel attenuation (dB) 11.0Supportable distance m (ft)

40000(131240)

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Ethernet40GBASE-LR4

Channel attenuation (dB) 6.7Supportable distance m (ft)

10000(32810)

Ethernet100GBASE-LR4

Channel attenuation (dB) 6.3Supportable distance m (ft)

10000(32810)

1G Fibre Channel100-SM-LC-L

Channel attenuation (dB) 7.8 -Supportable distance m (ft)

10000(32810) -

2G Fibre Channel200-SM-LC-L

Channel attenuation (dB) 7.8 -Supportable distance m (ft)

10000(32810) -

4G Fibre Channel400-SM-LC-M

Channel attenuation (dB) 4.8 -Supportable distance m (ft)

4000(13124) -

4G Fibre Channel400-SM-LC-L

Channel attenuation (dB) 7.8 -Supportable distance m (ft)

10000(32810) -

8G Fibre Channel800-SM-LC-I

Channel attenuation (dB) 2.6 -Supportable distance m (ft)

1400(4593) -

8G Fibre Channel800-SM-LC-L (4250 Mbaud)

Channel attenuation (dB) 6.4 -Supportable distance m (ft)

10000(32810) -

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10G Fibre Channel1200-SM-LL-L

Channel attenuation (dB) 6.0 -Supportable distance m (ft)

10000(32810) -

16G Fibre Channel1600-SM-LC-L

Channel attenuation (dB) 6.4 -Supportable distance m (ft)

10000(32810) -

FDDI SMF-PMDANSI X3.184

Channel attenuation (dB) 10.0 -Supportable distance m (ft)

10000(32810) -

- means Not Applicable

Link Specifications for FTTx

Single-mode

Parameter TIA 492CAAB (OS2)

Application

Nominal wavelength (nm)

1270/1310 1490 1550

EPON(IEEE 802.3AH)PX10 1,2

Channel attenuation (dB)

3.5 3 -

Supportable distance m (ft)

10000(32808)

EPON(IEEE 802.3AH)PX20 1,2

Channel attenuation (dB)

7 6 -

Supportable distance m (ft)

20000(65616)

10G EPON(IEEE 802.3AV)PR10 /PRX10 1,2

Channel attenuation (dB)

35. 3 2.5

Supportable distance m (ft)

10000(32810)

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10G EPON(IEEE 802.3AV)PR20 /PRX20 1,2

Channel attenuation (dB)

7 6 5

Supportable distance m (ft)

20000(65616)

10G EPON(IEEE 802.3AV)PR30 /PRX30 1,2

Channel attenuation (dB)

10.5 9

Supportable distance m (ft)

30000(98424)

GPON(ITU G.983)Class B+ 1,2

Channel attenuation (dB)

7 6

Supportable distance m (ft)

20000(65616)

GPON(ITU G.984)Class C+ 1,2

Channel attenuation (dB)

10.5 9

Supportable distance m (ft)

30000(98424)

10GPON(ITU G.987)Class N1 1,2

Channel attenuation (dB)

7 5

Supportable distance m (ft)

20000(65616)

10GPON(ITU G.987)Class N2 1,2

Channel attenuation (dB)

10.5 9

Supportable distance m (ft)

30000(98424)

RFOG(SCTE IPS SP910) 1,2

Channel attenuation (dB)

7 5

Supportable distance m (ft)

20000(65616)

- means Not Applicable

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Specifications For Legacy Fiber Optic Networks

A listing of many fiber optic LANs and links available in the last 30 years, with basic operational specs.

 ApplicationWavelength

 Max distance (m)for fiber type

Link Margin (dB)for fiber type

 62.5/125

50/125

SM 62.5

50 SM

 10Base-F  850  2000 2000 - 12.5 7.8 - FOIRL 850 2000 - - 8 - -T oken Ring 4/16 850 2000 2000 - 13 8.3 - Demand Priority(100VG-AnyLAN)

 850 500  500 - 7.5 2.8 -

 Demand Priority(100VG-AnyLAN)

 1300 2000 2000 - 7.0 2.3  -

 100Base-FX(Fast Ethernet)

 1300  2000 2000 - 11 6.3  -

 10/100Base-SX 850  300 300 - 4.0 4.0  -

 FDDI  1300  2000 200040,000

11.0 6.310-32

 FDDI (low cost)  1300  500 500 NA 7.0 2.3 -

 ATM 52  1300 3000 300015,000

 10 5.3 7-12

 ATM 155  1300 2000 200015,000

10 5.3 7-12

 ATM 155  850(laser) 1000 1000 - 7.2 7.2 -

 ATM 622  1300 500 50015,000

6.0 1.3 7-12

 ATM 622  850(laser)  300 300 - 4.0 4.0 - Fibre Channel 266

 1300  1500 150010,000

6.0 5.5 6-14

 Fibre Channel 266

 850(laser) 700 2000 - 12.0 12.0 -

 Fibre Channel 1062

 850(laser) 300 500 - 4.0 4.0 -

 Fibre Channel 1062

 1300 - -10,000

- - 6-14

 1000Base-SX  850(laser)  220 550 - 3.2 3.9 - 1000Base-LX  1300 550 550 5000 4.0 3.5 4.7

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 ESCON 1300 3000  -20,000

11  - 16

- means Not Applicable

NS = Not Specified. Most LANs and links not specified to run on SM fiber have media converters available to allow them to run on SM

fiber.

© 2004-2010 The Fiber Optic Association, Inc.

Telephone Networks

Telephone networks were the first major users of fiber optics. Fiber optic links were used to replace copper or digital radio links between telephone switches, beginning with long distance links, called long lines, where fiber's distance and bandwidth capabilities made fiber significantly more cost effective. Telcos use fiber to connect all their central offices and long distance switches because it has thousands of times the bandwidth of copper wire and can carry signals hundreds of times further before needing a repeater - making the cost of a phone connection over fiber only a few percent of the cost of the same connection on copper.

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After long distance links were converted to fiber, telcos began replacing shorter links between switches with fiber, for example between switches in the same metropolitan area. Today, with the exception of some rugged or remote locations, the entire telephone backbone is fiber optics. Cables on the land are run underground, direct buried or aerially, depending on the geography and local regulations. Connections around the world are run primarily on undersea cables which now link every continent and most island nations with the exception of Antarctica.

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After long distance links were converted to fiber, telcos began replacing shorter links between switches with fiber, for example between switches in the same metropolitan area. Today, practically all the telephone networks have been converted to fiber. Telcos and other groups are now running fiber right to the home, (FTTH) using low cost passive optical network (PON) systems that use splitters to share the cost of some fiber optic components among as many as 32 subscribers. More on FTTH, FTTH PON types and FTTH network architecture. 

Even cell phone networks have fiber backbones. It's more efficient and less expensive than using precious wireless bandwidth for backbone connections. Cell phone towers with many antennas will have large cable trays or pedestals where fiber cables connect to the antenna electronics. More on wireless.

The InternetThe Internet has always been based on a fiber optic backbone. It started as part of the telephone

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network when it was primarily voice and data traffic was mixed into the total traffic. But data has become the largest communications network as data traffic has outgrown voice traffic. The Internet now transmits user communications, e.g. requesting and downloading web pages or email, peer-to-peer transmissions, streaming video and massive data transfers between data centers. Large Internet providers are moving toward dedicated Internet networks that do not have the high overhead of telco networks which are burdened with transporting dozens of different types of communications services still being supported by the telco system providers. Now the telcos are moving their voice communications to Internet protocol (IP) for lower costs.

CATV

Most CATV systems are using fiber backbones too. CATV companies use fiber because it give them greater reliability and the opportunity to offer new services, like phone service and Internet connections.CATV used to have a terrible reputation for reliability, not really a problem with service but with network topology. CATV uses very high frequency analog signals, up to 1 GHz, which has high attenuation over coax cable. For a city-wide system, CATV needed many amplifiers (repeaters) to reach the users at the end of the system; 15 or more we

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common. Amplifiers failed often, meaning that subscriber downstream of the failed amp lost signal. Finding and fixing failed amps was difficult and time consuming, causing subscriber complaints. 

The development of highly linear distributed feedback (DFB) lasers allowed CATV systems to be converted to analog optical systems. CATV companies "overbuild" with fiber. They connect their headends with fiber and then take fiber into the neighborhood. They lash the fiber cable onto the aerial "hardline" coax used for the rest of the network or pull it in the same conduit underground. The fiber allows them to break their network into smaller service areas, typically fewer than 4 amplifiers deep, that prevent large numbers of customers from being affected in an outage, making their network more reliable and easier to troubleshoot, providing better service and customer relations. 

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The fiber also gives CATV operators a return path which they use for Internet and telephone connections, increasing their revenue potential. Most current CATV systems still use AM (analog) systems which simply convert the electrical TV signals into optical signals. Look for them to convert to more digital transmission in the future.

CATV is even developing its own version of fiber to the home called RF over glass (RFOG.) This uses a interface at the home that is like a cable modem but with an optical input that accepts the same analog radio frequency (RF) signals that are used throughout the HFC network.

Security Systems

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Security systems can have a wider reach and are more secure on fiber. Practically any system today has a fiber optic option. CCTV cameras used for surveillance often use fiber for it's distance capability and security, especially in large buildings like airports and around cities with metropolitan networks. Fiber also has much more bandwidth than coax so several cameras can be multiplexed onto one fiber. Bidirectional links allow controlling pan, zoom and tilt (PZT) cameras. Other security devices like intrusion alarms or perimeter alarms can utilize fiber and some even use fiber optic sensors.

Metropolitan Networks

Many cities have incorporated fiber optics into their

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communications networks. Metropolitan networks use fiber for many other applications besides CCTV surveillance cameras, including connecting public service agencies such as fire, police and other emergency services, hospitals, schools, as well as connecting municipal WiFi and traffic management systems as shown in the photo. Cities can install cables to strategic locations so various services can share the fibers in the cables, saving installation costs. Cities are also learning to bury extra conduits every time a roadway is dug up so when cables need installing, no further construction is needed.

Utility Networks

Utilities use fiber for communications, CCTV surveillance and network management. Electrical utilities have used fiber optics for decades for communications and managing their distribution systems. They realized quickly that fiber’s immunity to electromagnetic interference would allow them to operate communications and control networks in close proximity to electrical circuits without

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problems. Electrical utilities take full advantage of fiber's immunity to noise also, even running fiber inside high voltage power distribution cables. Some utilities install fibers inside their high voltage distribution networks using optical power ground wire (OPGW) and lease fibers to other telecommunications companies. Utilities use fiber in one non-communications application; fiber optic sensors allow monitoring high voltage and current in their distribution systems. The interest in "smart grid" management of power distribution to enhance efficiency is based on using fiber optics for network management. 

Premises Networks

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Premises networks, mostly computer LANs (local area networks) use fiber optics primarily in the backbone but increasingly to the desk and to connect wireless access points. The LAN backbone often needs longer distances than copper cable (Cat 5/5e/6/6A) can provide and of course, the fiber offers higher bandwidth for future expansion. Fiber's ability to handle network upgrades meant that one fiber type outlived nine generations of copper cables in LANs. A new fiber type (OM3) offers future potential for upgrades while copper continues to struggle with network speed increases.Until recently large corporate LANs use fiber backbones with copper wire to the desktop. LAN switches and hubs are usually available with fiber optic ports but PCs have interfaces to Ethernet on copper. Inexpensive media converters allow connecting PCs to fiber. Fiber to the desk can be cost effective if properly designed using centralized fiber architecture without local switching in the telecom closet, but many users no longer want to be "tethered" to a network cable. Desktop computer sales are declining and laptops are the PC of choice for most users, with wireless connections to the network. Generally only high data users like engineers and graphics designers use desktop workstations; everybody else gets a wireless-connected laptop.More on premises networks and fiber in premises networks.

Centralized Fiber LANsWhen most contractors and end users look at fiber

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optics versus Category-rated UTP cabling for a LAN, they compare the same old copper LAN with fiber directly replacing the copper links. The installed cost of a fiber optic cable plant comparable to the cost of Cat 5/6/6A, but fiber often requires medial conversion electronics which add cost to the link for fiber.However, the real difference comes if you use a centralized fiber optic network - shown on the right of the diagram above. Since fiber does not have the 90 meter distance limitation of UTP cable, you can place all electronics in one location in or near the computer room. The telecom room is only used for passive connection of backbone fiber optic cables, so no power, UPS, ground or air conditioning is needed. These auxiliary services, necessary with Cat 5 hubs, cost a tremendous amount of money in eachtelecom room. If designing a new building, you do not even need the cost of the telecom room itself.In addition, having all the fiber optic hubs in one location means better utilization of the hardware, with fewer unused ports. Since ports in modular hubs must be added in modules of 8 or 16, it's not uncommon with a hub in a telecom closet to have many of the ports in a module empty . With a centralized fiber system, you can add modules more efficiently as you are supporting many more desktop locations but need never have more than a one module with open ports.

More on  fiber in premises networks and fiber versus copper, generally and in LANs

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Industrial NetworksIndustrial plants use fiber for it's ruggedness, distance and noise immunity. In an industrial environment, electromagnetic interference (EMI) is often a big problem. Motors, relays, welders and other industrial equipment generate a tremendous amount of electrical noise that can cause major problems with copper cabling, especially unshielded cable like UTP. In order to run copper cable in an industrial environment, it is often necessary to pull it through conduit to provide adequate shielding. Fiber is also very flexible, so many industrial robots use fiber for controls, often plastic fiber.Fiber optics has complete immunity to EMI. You only need to choose a cable type that is rugged enough for the installation, with breakout cable being a good choice for it's heavy-duty construction. The fiber optic cable can be installed easily from point to point, passing right next to major sources of EMI with no effect. Conversion from copper networks is easy with media converters, gadgets that convert most types of systems to fiber optics. Even with the cost of the media converters, the fiber optic network will be less than copper run in conduit.

Military and PlatformsThe military uses fiber everywhere, on bases, platforms (ships and planes), and on the battlefield because it's hard to damage, tap or jam. Airplanes use fiber for its reliability and noise immunity, but also like the lighter weight of fiber. Even millions of

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cars have fiber networks connecting all the electronics because fiber is immune to noise and saves weight.

Fiber Optic Links

Fiber optic links work by sending optical signals over fiber. Fiber optic transmission systems all use data links that work similar to the diagram shown above. Each fiber link consists of a transmitter on one end of a fiber and a receiver on the other end. Most systems operate by transmitting in one direction on one fiber and in the reverse direction on another fiber for full duplex operation. Transmitters are semiconductor LEDs or lasers and receivers are semiconductor photodetectors. For more information on fiber optic links, see the next section on Transmission Systems.

Designing Fiber Optic Networks

This is a big topic so we have a complete section on the subject of Design. Fiber's extra bandwidth and distance capability makes it possible to do things not possible with copper wire or wireless. First and foremost, it's necessary to understand thoroughly what signals are to be transmitted over the fiber and the specifications of the transmission equipment. Then map and visit the work site to understand where the fiber optic cable

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plant needs to be installed. Know the standards but use common sense in designing the installation. Consider what are the possible problems and work around or prevent them. Don't cut corners which may affect performance or reliability. Document everything completely. Plan for future expansion and restoration in case of problems. There is no substitute for experience and common sense here!

Test Your Comprehension

References

The FOA Reference Guide to Outside Plant Fiber Optics   

The FOA Online Reference Guide to Fiber Optics

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You can buy the printed version of the  The FOA Reference Guide to Outside Plant Fiber Optics from the FOA eStore or Amazon.

Testing FTTH    New network architectures (PONs or passive optical networks) have been developed that allow sharing expensive components for FTTH. A passive splitter that takes one input and broadcasts it to as many as 32 users cuts the cost of the links substantially by sharing, for example, one expensive laser with up to 32 homes and only requiring an inexpensive laser at each home. However, this architecture changes the methodology of testing the complete installed cable plant and links for proper operation. Of course, individual links are tested as usual, it is the PON coupler that creates the difference.

    Each home needs to be connected to the local central office with a single singlemode fiber, through a local PON splitter (or maybe two if

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the PON splitters are cascaded.) Every home will have a singlemode fiber link pulled or strung aerially to the phone company cables running down the street and a network interface device containing fiber optic transmitters and receivers will be installed on the outside of the house. The incoming cable needs to be terminated at the house, tested, connected to the interface and the service tested. See FTTH Architectures  for more information on typical FTTH installations.

FTTx Testing Issues    Testing FTTH network is similar to other OSP testing but the splitter and WDM add complexity. FTTP PON networks can be more complicated than simple OSP links, with WDM couplers, PON splitters, etc. in a single link, so complete testing can include some components and installation issues not familiar to the usual OSP tech. PON couplers add high loss, WDM couplers have different performance at different wavelengths and connector reflectance, not a problem in most systems, can be a problem in short links typical in FTTx. Many FTTx systems use APC (angled PC) connectors to reduce reflectance so test cables for both OLTS and OTDR need to have matching connectors.

    However, once installed,  users on a live network means testing cannot disrupt service. Thus testing may be as simple as checking power at the ONT on the subscriber’s house with a calibrated fiber optic power meter or just seeing if the ONT has a “green” connection light! The ONT at the home usually has some intelligence that can be accessed from a remote location, allowing a service tech to initiate a loopback test to verify connections at any user. If only one user has a problem, a service tech is then sent there, while if all users are down, the tech is sent to the central office.

   As with most fiber optic links, troubleshooting requires knowing the architecture of the system, expected link losses and optical signal levels and typical problems that may be encountered. As always, we emphasize the importance of having documentation on the system before testing and troubleshooting.

Link Testing    A link is a single run of fiber, e.g.: from CO to FDH or from FDH to ONT. The fiber run may have connectors or not, depending on whether the links are spliced or use connectors for terminations. Quite a few now use preterminated cables to speed installation. The

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loss of the PON splitter must be included in the loss budget for the link. See FTTH Architectures  for more information on PON splitter losses.

   You must measure loss with OLTS at all wavelengths and bidirectionally to check all operational modes - similar to how the transmission equipment will use the fiber.

    The installer may need to characterize each fiber with an OTDR, verifying fiber attenuation, termination losses and reflectance and splice quality. The OTDR will also show any bending losses caused during installation. OTDR traces should be filed for future reference.

    Optionally, the installer may test splitters at the FDH or the WDMs at the CO. If these are pretested, as they should have been, this may not be necessary or advisable, especially since it is time-consuming and costly. WDMs also require specialized test equipment.

    After the link is installed, it needs testing from end to end. The end-to-end loss includes the connectors on each end, the loss of the fiber in each link, the connectors or splices on the splitter and the loss of the splitter itself. Since the fibers are being used bi-directionally and connector or splice loss may be different in each direction if the fiber core diameter (mode field diameter for SM fiber) is different, testing in both directions is important too. Special FTTx PON OLTS are available that test the proper wavelengths in each direction, simplifying testing logistics. 

    Since PON links are generally short (<20km) chromatic dispersion (CD) and polarization mode dispersion (PMD) are not concerns. CD

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and PMD are generally only issues on very long links. 

BPON    Let’s consider the most complex version of PON testing, BPON. It’s similar to OSP testing but splitter and WDM add complexity as well as more loss and there are three wavelengths in use. Tests include each coupler, each link and end-to-end loss. Loss and reflectance are especially important if systems are using an AM video transmission system at 1550 nm, as it has a maximum tolerable loss and reflectance before signals are noticeably affected. Tests need to be done at all three wavelengths of operation: 1310 nm for upstream digital data, 1490 for downstream digital data and 1550 nm for AM video downstream (BPON).

    Insertion loss of the cable plant including the loss of the coupler is tested using an optical loss test set (special test sets for FTTH PONs are available that cover all 3 wavelengths of interest.) OTDRs can be used if length is adequately long, to determine connection reflectance, fiber attenuation and troubleshoot problems. Many systems will take OTDR traces and store for troubleshooting. The splitters can confuse the OTDR so one generally reverses OTDR test, taking traces from the subscriber upstream. 

OTDR Testing PONs    Using an OTDR to test every fiber in  an OSP link is traditional, as the OTDR provides a snapshot of the losses in the fiber, locates loss events (connectors, splices and bending losses from improper installation), aids installation troubleshooting and provides a trace which can be stored for later troubleshooting and restoration. On

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FTTH PON networks, the PON splitter causes some unusual traces on OTDRs, with the traces looking totally different when tested from each direction. Here are two traces from an actual system taken in two directions.

This trace is taken downstream from the CO to the subscriber:

This trace is taken upstream from the subscriber toward the CO.

In both traces, you can see the large loss of the PON coupler, best seen in the upstream trace at the bottom, on the left side of the trace. On the downstream trace, it is the large loss preceding the multiple peaks of the subscriber fibers, marked with the dashed marker line. Below we will show a simpler coupler and explain what you are seeing here.

OTDR Testing From CO  PON systems create problems for OTDRs. Shooting from the input of a PON splitter at the CO, the OTDR sees and adds together the backscatter traces from all the fibers. As a result, it becomes impossible to see detail on individual fibers, and an event (connector, splice of bending loss) cannot be easily assigned to any individual fiber unless the cable plant is carefully documented at installation. 

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   Consider the “X” shown in the network diagram below. If it was a loss or reflective event, it would show on the OTDR trace, but the operator would not know if if were in fiber 1,2,3 or 4. The only unambiguous part of the OTDR trace shown is the end of fiber 4, the longest fiber, beyond the length of the next longest fiber, #3. 

   It should be noted that FTTH links, because of their short lengths and the use of some high power transmitters, usually have APC connectors or fibers prepared to have minimal reflectance. That can make analyzing downstream OTDR traces very difficult when no reflective end is available to mark the fiber end and there are 32 fibers in the system. 

Here is an illustration of how a real trace can become very complex to analyze. This is an enlargement of the coupler to subscriber section of the downstream trace above which is outlined in red on the trace.

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    As a result of the complexity of downstream traces, OTDRs are generally used on PONs from the subscriber end toward the CO to characterize the fiber path. However, the OTDR may also be used from the CO end, because, as you can see from the diagram above, it allows the operator to quickly characterize the length of each fiber link, providing actual fiber length to add to network diagrams for future troubleshooting. 

    Special PON OTDRs will test at 1310, 1490 and 1550 nm. Some also test  “out of band”  at 1650 nm, which is more sensitive to bending losses and allows in-service testing with a filter to remove signal wavelengths. Since PONs are short, the OTDR needs very high resolution, usually obtained by having the shortest test pulse that will give adequate range. 

    Testing PONs in the downstream direction is helped with launch and receive cables. The launch cable allows testing the initial connector on the link as well as allowing the initial overload of the OTDR to settle down as with any OTDR test. But on the receive end, if a cable of known length is used, say 100m or 500m, one can look back exactly that distance from the reflective end to see the loss of the end connector.

OTDR Testing From Subscriber    Testing from the subscriber end is easier. The fiber path will show events on just one fiber, like the “X” shown on fiber 3, and a high loss

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for the coupler. Here a 1:4 coupler will have 6 dB of splitting loss plus perhaps  1dB excess loss for a total of 7 dB loss. Using launch and receive cables allow testing connectors on both ends and measuring end to end loss.

   Here is a detailed trace from the upstream example above, showing how much simpler the trace is when the other subscriber links are not shown.

Other FTTx Testing Issues    Network equipment will be tested as the system is turned on or for

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troubleshooting. Will the network equipment transmit and receive properly? If the cable plant is installed correctly and tests within specifications for loss and reflectance, it should. Most FTTx equipment has extensive self-testing capability and that may prove sufficient for most testing. PON couplers may have a second port on the upstream side just for testing or unused downstream connectors may be useful for testing, especially with OTDRs. 

    The network equipment should be tested for optical power. The transmitter output should be within specifications, as should the receiver input, when tested with acalibrated optical power meter set at the proper wavelength(s). If testing is done while all three systems are operating at their respective wavelengths, a power meter with wavelength selective input is required.  Power at the receiver is critical. Too low and the signal-to-noise ratio will be too low; too high and the receiver will saturate. Both conditions will cause transmission errors. High power is not uncommon, so attenuators may be used in these links to reduce power to acceptable levels.

    Data transfer testing with a protocol analyzer is the final test. It will be done using specific protocol testers for the data formats being transmitted. Personnel doing these tests are probably not the same that test the cable plant as each have specific training and test equipment needs.

    Remember that ONTs are generally capable of loopback testing under remote control. This may mean more sophisticated testing is unnecessary for troubleshooting.

FTTx Safety Issues    FTTx safety issues include all the usual fiber installation issues, for example working with bare fibers, solvents and adhesives. But FTTx networks have several other potential problems. 

    Links carrying AM CATV signals will have high power from EDFAs, especially before the splitters. And links may have multiple equipment transmitting simultaneously. Either case can cause high optical power that can be dangerous to worker’s eyes. Care should be taken to not expose eyes to light from the fibers and to always use microscopes with infrared filters, just in case. Since systems may have multiple systems transmitting on the same fiber, it is harder to ensure that all systems are turned off for inspection or testing, also.

    And, since up to 32 users may be sharing the CO based network

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equipment, turning off systems for troubleshooting is not desirable, so testing may have to be done with equipment in service. Exercise care. More on fiber optic safety.

Fiber To The Home Installation   There is probably no way to generalize on the installation process for FTTx since every system is unique and, in some cases, every subscriber is different. Rather than telling you how to install FTTx here, we will try to illustrate some of the ways that others have installed their systems and offer advice on how to design and install systems most efficiently.

JargonThis drawing shows the location of the hardware used in creating a typical PON network. This drawing also defines the network jargon for cables: a "feeder" cable extends from the OLT (optical line terminal) in the CO (central office) to a FDH (fiber distribution hub) where the PON (passive optical network) splitter is housed. It then connects to "distribution" cables that go out toward the subscriber location where "drop" cables will be used to connect the final link to the ONT (optical network terminal).

Background   Perhaps we should start with some history. When FTTH using PONs first began being installed, it was considered a extension of

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regular telecom systems, where subscribers were being connected to a metropolitan system. Cabinets or pedestals containing the PON couplers were placed near a group of subscribers. Cables were pulled between the cabinet and the central office containing the PON system electronics and spliced on each end by the usual OSP installation crews and were tested as normally done. 

    On the subscriber end, drop cables were placed to the home and connected either by splicing or installing connectors (APC to prevent reflectance problems). Drop cables could be installed aerially, underground or buried. Installing the cables through customer's yards created a problem as it is time consuming and disruptive to the customer. Simple trenching was sometimes dropped in favor of directional boring, an expensive process. Connectors were installed either by fusion splicing on pigtails or using prepolished splice connectors. 

    After the cable plant was installed, the optical network terminal (ONT) was installed at the home. Some systems installed ONTs on the outside of the house, some inside garages, some inside the home. Some homebuilders built new homes with provision for the ONT inside the home and installed cabling and power to the same area to create a home prepared for broadband. See the examples below.

    After the ONT was installed and tested, it was necessary to complete the installation by connecting the customers phones, TVs and computers. In all, three or four groups of installers were needed to install a FTTH customer. 

Systems Evolving With Experience    After some experience with the systems, methods were tried to simplify the process and cut costs. A big breakthrough came with the development of prefabricated cabling systems (sometimes call pre-terminated cabling) that eliminated the need for most of the splicing. Cables with weatherproof connectors were purchased made to the lengths needed and pedestals were factory made with connectors for the drop to the home and a cable ready to splice onto the cable

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installed from the central office. The prefab drop cables could be run aerially, even lashed to current telephone wires. They were also small enough they could be pulled through small PVC conduit often installed to home in new construction. Most of the systems use multi-connector cables near the homes being connected so homes can be connected during the first install or later when more customers decide to take the service.

Aerial installation in Santa Monica, CA, using prefab cabling system.

Closeup of the six-port drop.

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  If the cable is underground, it will usually be pulled through conduit from connection to the distribution cable or the splitter to the home. Here a preterminated systems has two home drops connected to the distribution cable.

Underground installation of prefab cable system in Long Beach, CA.

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    Other systems used microduct installation which requires little or no digging to install underground or under a road. Microducts can accommodate small regular cables or use air-blown fibers, another technique used in some systems.

Microduct installation using a saw on a paved street.

The splitter can be housed in a central office or a pedestal in the neighborhood near the homes served. Here is a typical pedestal that

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has connections to the CO, splitters and fibers out to each home in a sealed enclosure. The advantage of PONs is that this pedestal is passive - it does not require any power as would a switch or node for fiber to the curb.

   A network interface device containing fiber optic transmitters and receivers will be installed at the house. Some are installed on the outside of the house, others are indoors. Some houses are now being built with cabinets in the house for connecting to the FTTH fiber and then distributing phone, TV and Internet connections throughout the house over state of the art cabling. The incoming cable needs to be terminated at the house, tested, connected to the interface and the service tested.

New Cable Types And Hardware For Subscriber DropsSeveral new cable types were developed for use in FTTH. Until FTTH, most single fiber cables were complicated structures with tight buffered fibers and aramid fiber strength members inside plastic jackets, usually 3mm in diameter or sometimes smaller. While these cables were adequate for factory termination into prefabricated assemblies, they were not ideal for field termination or use inside buildings. With the advent of bend-insensitive fibers that required less protection, a new type of drop cable was developed that molded a bend-insensitive fiber inside a small plastic structure surrounded by metal or aramid fiber strength members. This design could also be made as a "figure 8" cable with a messenger for support in aerial

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installation. Here are some photos of this type of cable.

FTTH Drop Cable, 1 fiber, showing steel strength member

FTTH Drop Cable, 2 fiber

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FTTH Drop Cable, 1 fiber with messenger for aerial support

To work with these cables, special fiber closures were developed that are more convenient for field installation. 

This closure has entries for distribution cables, including one coming in and one continuing on to another closure for daisy-chained cables.  There are multiple outputs for drop cables which are terminated in connectors. Some closures like this one have provision for splicing on pigtails to terminate the distribution cables while others

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are designed for direct termination using prepolished-splice connectors. Patching with connectors in a re-enterable closure allows adding new drops when needed. 

ONT being installed on the outside of the house

Modern house with cabinet inside the home for distributing services from the FTTH connection

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Multi-Dwelling UnitsMulti-dwelling units (MDUs in some suppliers jargon) are sometimes handled like FTTC, where fiber is brought into the building and individual units are connected over copper cables, but they are ideal for FTTH since there are many users in a very small space so fiber lengths are short. Besides using less fiber, MDUs generally require less time per drop to install. A major problem in older buildings has been finding places to run cables, but new types of bend-insensitive fiber and the special small drop cables shown above make it easy to route fibers along walls or place in stick-on raceways on the walls. Here is an example from Corning on how bend-insensitive fiber can

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be treated without problems, but we offer a caution regarding stapling these fibers - it could be a long-term problem and should be avoided in our opinion. 

Bend-insensitive fiber installed in older building (Corning).

SummaryLike most fiber optic networks, every FTTx installation is unique. It must be designed for the location it is to serve and choices on components and installation methods should be optimized for the system. Installation methods may include every type of OSP installation. Suppliers familiar with FTTx can advise customers on what other systems have done to make installations simpler, easier and inexpensive. Most systems prefer to use as many factory-made components as possible as they are generally less expensive than doing the same work in the field. New installation methods should be considered as well to reduce costs. 

Technical Information on FTTX  From The FOA Online Reference Guide:

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FTTH

FTTH Architectures,  MDUs (Multiple Dwelling Units)

FTTH PON Protocols

Installing FTTX   Testing FTTH Networks   

FTTx Online Tutorial

Here's links for more information on FTTx

Training & Certification

FOA Certification OverviewFOA FTTx Certification Requirements

Fiber To The Home Architectures   New network architectures have been developed to reduce the cost of installing high bandwidth services to the home, often lumped into the acronym FTTx for "fiber to the x". These include FTTC for fiber to the curb, also called FTTN or fiber to the node, FTTH for fiber to the home and FTTP for fiber to the premises, using "premises" to include homes, apartments, condos, small businesses, etc. Recently, we've even added FTTW for fiber to wireless. 

Let's begin by describing these network architectures.

FTTC: Fiber To The Curb (or Node, FTTN)   Fiber to the curb brings fiber to the curb, or just down the street, close enough for the copper wiring already connecting the home to carry DSL (digital subscriber line, or fast digital signals on copper.)

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   FTTC bandwidth depends on DSL performance where the bandwidth declines over long lengths from the node to the home. There are many types of DSL (ADSL, HDSL, RADSL, VDSL, UDSL, etc.) that offer varying performance over length and some "bond" more pairs of wires to improve the bandwidth. 

   Newer homes that have good copper and are near where the DSL switch is located can expect good service. Homes with older copper or longer distances away will have less available bandwidth.    FTTC is less expensive than FTTH when first installed but since performance depends on the quality of the copper wiring currently installed to the home and the length to reach from the node to the home, the level of service may be obsoleted quickly by customer demands. In older areas where the copper wires are of poorer quality or have degraded over time, DSL is difficult or impossible to implement. The good news it that FTTC is ready to upgrade to FTTH. 

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FTTW: Fiber to Wireless   Of course today's mobile device users depend on wireless connections for their laptops, smartphones and tablets. Even many homes and businesses are now using wireless connectivity, especially those outside areas where FTTH or FTTC are not available or considered economical for future installations. Options for wireless include cellular systems which are the most widely available wireless solution around the world, WiFi which has become available inside many businesses and even outdoors in areas served by municipal networks and satellite wireless, used in many rural areas where distances are so large that cabling or WiFi is unfeasible. Future options include WiMAX and Super WiFi, land-based wireless with longer ranges and higher bandwidth capability than most cellular systems and smaller cellular antennas with more localized coverage like this LightCube Radio from Alcatel-Lucent that can be placed anywhere and connected with fiber and power. All these options are aimed at providing more bandwidth to users more efficiently.

    All these wireless systems depend on the same fiber optic communications backbones that everyone else does. As they grow, higher bandwidth demands means more traffic to local antennas which makes fiber more attractive. Most cellular users are converting older antenna towers connected by copper cables or line-of-sight wireless over to fiber. Fiber is even being used for connections up

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towers to wireless antennas as it is smaller and lighter than the coax cables previously used. Read more on how wireless depends on fiber here. 

FTTH Active Star Network   The simplest way to connect homes with fiber is to have a fiber link connecting every home to the phone company switches, either in the nearest central office (CO) or to a local active switch. 

The drawing above shows a home run connection from the home directly to the CO, while below, the home is connected to a local switch, like FTTC upgraded to fiber to the home.

   A home run active star network has one fiber dedicated to each home (or premises in the case of businesses, apartments or condos.) This architecture offers the maximum amount of bandwidth and flexibility, but at a higher cost, both in electronics on each end (compared to a PON architecture, described below) and the dedicated fiber(s) required for each home. 

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FTTH PON: Passive Optical Network  A PON system allows sharing expensive components for FTTH. A passive splitter that takes one input and splits it to broadcast to many users cuts the cost of the links susbstantially by sharing, for example, one expensive laser with up to 32 homes. PON splitters are bi-directional, that is signals can be sent downstream from the central office, broadcast to all users, and signals from the users can be sent upstream and combined into one fiber to communicate with the central office.

    Because of all the splitters and short links, plus since some systems are designed for AM video like CATV systems, non-reflective connectors (like the SC-APC angle-polished connector) are generally used.

The splitter can be one unit in a single location as shown above or several splitters cascaded as shown below. Cascaded splitters can be used to reduce the amount of fiber needed in a network by placing splitters nearer the user. The split ratio is the split of each coupler multiplied together, so a 4-way splitter folllowed by a 8-way splitter would be a 32-way split. Cascading is usually done when houses being served are clustered in smaller groups. Splitters are sometimes housed in the central office and individual fibers run from the office to each subscriber. This can enhance serviceability of the network since all the network hardware is in one location at only a small penalty in

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overall cost for either dense urban areas or long rural systems.

        Most PON splitters  are 1X32 or 2X32 or some smaller number of splits in a binary sequence (2, 4,8, 16, 32, etc.). Couplers are basically symmetrical, say 32X32, but PON architecture doesn't need but one fiber connection on the central office side, or maybe 2 so one is available for monitoring, testing and as a spare, so the other fibers are cut off. Couplers work by splitting the signal equally into all the fibers on the other side of the coupler, Splitters add considerable loss to a FTTH link, limiting the distance of a FTTH link compared to typical point-to-point telco link. When designing a fiber optic network, here are guidelines on loss in PON couplers.

Splitter Ratio 1:2 1:4 1:8 1:16 1:32

Ideal Loss / Port (dB)

3 6 9 12 15

Excess Loss (dB)

1 1 2 3 4

Typical Loss (dB)

4 7 11 15 19

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    Each home needs to be connected to the local central office with singlemode fiber through an optical splitter. Every home will have a singlemode fiber link pulled into underground conduit or strung aerially to the phone company cables running down the street. Verizon has pioneered installing prefabricated fiber links that require little field splicing. 

   Here is a fiber distribution system that has been spliced into cables connected to the local central office. The preterminated drop cable to the home merely connects to the closure on the pole in the red circle and is usually lashed to the aerial telephone wire already connected to the home.

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   If the cable is underground, it will usually be pulled through conduit from connection to the distribution cable or the splitter to the home. Here a preterminated systems has two home drops connected to the distribution cable.

The splitter can be housed in a central office or a pedestal in the neighborhood near the homes served. Here is a typical pedestal that has connections to the CO, splitters and fibers out to each home in a sealed enclosure. The advantage of PONs is that this pedestal is passive - it does not require any power as would a switch or node for fiber to the curb.

   A network interface device containing fiber optic transmitters and receivers will be installed on the outside of the house. The incoming cable needs to be terminated at the house, tested, connected to the interface and the service tested.

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Below is the layout of a typical PON network with the equipment required at the CO, fiber distribution hub and the home. This drawing shows the location of the hardware used in creating a complete PON network. This drawing also defines the network jargon for cables: a "feeder" cable extends from the OLT (optical line terminal) in the CO (central office) to a FDH (fiber distribution hub) where the PON (passive optical network) splitter is housed. It then connects to "distribution" cables that go out toward the subscriber location where "drop" cables will be used to connect the final link to the ONT (optical network terminal).

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Triple Play Systems    Most FTTH systems are "triple play" systems offering voice (telephone), video (TV) and data (Internet access.) To provide all three services over one fiber, signals are sent bidirectionally over a single fiber using two or three separate wavelengths of light. Three different protocols are in use today, BPON, shown below, uses a third wavelength for AM video, while EPON and GPON use digital IPTV transmission. Read more on PON protocols.    Downstream digital signals from the CO through the splitter to the home are sent at 1490 or 1550 nm. This signal carries both voice and data to the home. Video on BPON systems uses the same technology as CATV, an analog modulated signal, broadcast separately using a 1550 nm laser which may require a fiber amplifier to provide enough signal power to overcome the loss of the optical splitter. Upstream digital signals for voice and data are sent back to the CO from the home using an inexpensive 1310 nm laser. WDM couplers separate the signals at both the home and the CO.

  

Powering FTTH    Traditionally, telephone services, at least what are called "POTS" or plain old telephone service, have been self-powered from the central office. POTS phones were on a current loop powered from batteries or some other type of uninterruptible power in the CO. When a subscriber had an electrical power outage, they expected to be able to still use their phone, to call the electrical utility to report the outage, of course! Obviously, FTTH is not going to operate the same way. Fiber does not easily deliver electrical power, although systems have been developed to power sensors over light in the fiber, it is

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inefficient and expensive. Many FTTH systems provide a battery backup at the customer premises powered from the customer electrical system to keep the system operational during power outages. Some systems use the old copper wires replaced by the fiber to deliver power to keep the backup charged, so that the FTTH system provider pays for the power needed by the system. And some systems, recognizing that most people have a mobile phone, do not address the issue of backup power at all.

FTTH PON: Passive Optical Network  A PON system utilizes a passive splitter that takes one input and splits it to "broadcast" signals to many users. This reduces the cost of the system substantially by sharing one set of electronics and an expensive laser with up to 32 homes. An inexpensive laser is used for the home to send signals back to the FTTH system in the central office. 

Triple Play Systems    Most FTTH systems are so-called "triple play" systems offering voice (telephone), video (TV) and data (Internet access.) To provide all three services over one fiber, signals are sent bidirectionally over a single fiber using several wavelengths of light.    BPON, or broadband PON, was the most popular current PON application in the beginning. BPON uses ATM as the protocol. ATM is widely used for telephone networks and the methods of transporting all data types (voice, Internet, video, etc.) are well known. BPON digital signals operate at ATM rates of 155, 622 and 1244 Mb/s.    

   Downstream digital signals from the CO through the splitter to the

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home are sent at 1490 nm. This signal carries both voice and data to the home. Video on the first systems used the same technology as CATV, an analog modulated signal, broadcast separately using a 1550 nm laser which may require a fiber amplifier to provide enough signal strength to overcome the loss of the optical splitter. Video could be upgraded to digital using IPTV, negating the need for the separate wavelength for video. Upstream digital signals for voice and data are sent back to the CO from the home using an inexpensive 1310 nm laser. WDM couplers separate the signals at both the home and the CO.

BPON architecture with analog TV 

   GPON, or gigabit-capable PON, uses an IP-based protocol and either ATM or GEM (GPON encapsulation method) encoding. Data rates of up to 2.5 Gb/s are specified and it is very flexible in what types of traffic it carries. GPON enables “triple play” (voice-data-video) and is the basis of most planned FTTP applications in the near future. In the diagram above, one merely drops the AM Video at the CO and carries digital video over the downstream digital link.

   EPON or Ethernet PON is based on the IEEE standard for Ethernet in the First Mile. It uses packet-based transmission at 1 Gb/s with 10 Gb/s under discussion. EPON is widely deployed in Asia. The system architecture is the same as GPON but data protocols are differenet.

PON System Specification Summary

BPON GPON EPON

Standard ITU-T G.983 ITU-T G.984 IEEE 802.3ah (1 Gb/s)IEEE 802.3av (10Gb/s)

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Downstream Bitrate

155, 622 Mb/s, 1.2 Gb/s

155, 622 Mb/s, 1.2, 2.5 Gb/s

1.25 Gb/s, 10.3 Gb/s

Upstream Bitrate

155, 622 Mb/s 155, 622 Mb/s, 1.2, 2.5 Gb/s

1.25 Gb/s, 1.25 or 10.3 Gb/s

Downstream Wavelength

1490, 1550 1490 1490, 1550

Upstream Wavelength

1310 1310 1310

Protocol ATM Ethernet over ATM/IP or TDM

Ethernet

Video RF at 1550 or IP at 1490

RF at 1550 or IP at 1490

IP Video

Max PON Splits 32 64 16

Coverage <20 km <60 km <20 km

 RFOG: CATV's FTTHCATV operators were the first broadband providers using a HFC (hybrid fiber coax) system with cable modems using RF signals. Today, some CATV operators see a need for a system to provide fiber to the home, which has lead to the development of RFOG (RF over Glass.) CATV standards have looked at PON architectures and the SCTE has proposed a standard for deploying a broadcast architecture of analog signals similar to PONs called RFoG for RF (radio frequency - i.e. FM) over Glass. RFOG is basically nothing more than an all-fiber HFC/cable modem system built with less expensive components now available thanks to the volume pricing of components used in FTTH. It’s designed to operate over a standard telco PON (passive optical network) fiber architecture with short fiber lengths and including the losses of a FTTH PON splitter. 

There is one interesting aspect of this approach. Now telcos and CATV companies can deliver the same services over the same cable plant using totally different technologies. But that means that office or

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apartment building owners, developers or even whole towns that might be considering installing FTTH infrastructure themselves and leasing the fiber to a service provider can have a choice of service providers. One cable network can support either CATV or telco systems – or even someone else for that matter. That opens up a big market for private fiber optic systems.

WDM and PONObviously, PON networks use WDM (wavelength-division multiplexing) with different wavelengths upstream and downstream. But the PON architecture can easily support more wavelengths, allowing greater bandwidth to the user but allocating one wavelength to a user or a group of users or greater security by having each user have their own wavelength. WDM PON architectures are being developed by many companies but no standards exist for them yet.

Other Uses For PONsPONs offer low cost connectivity for a large number of users with high security and relatively low management needs. Some PON suppliers have been promoting PONs as an alternative to LANs (Local Area Networks), which are especially attractive to organizations with large numbers of users. Passive Optical LANs are claimed to be less expensive than traditional copper cabling for LANs but offer virtually unlimited future expansion. See Premises/Networks for more information on POLs.

FTTH in MDUs (Multiple Dwelling Units)

When we normally talk about FTTH, we assume we are installing the fiber to a “home” where it terminates in a optical line terminal (OLT) and services (voice, data and video) are delivered inside the subscriber’s home. But since we may have detached single-family homes, row houses or units in a large building, the situations can be quite different, requiring different architectures and installation practices. We should add that office buildings are often similar to MDUs, with the exception that floor plans are generally more flexible and units are larger, but the concepts are similar.

Let’s assume we have fiber to the building, then what’s next? We must decide how to deliver broadband to each unit in the building and then inside the unit. What are the options for delivering services to each unit from the entry facility?

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Options for connecting each unit in the building include using:1. Currently installed phone lines using xDSL technology2. Currently installed CATV or satellite coax using cable modem or MOCA (Multimedia Over Coax Alliance) technology3. Installing wireless access points at appropriate points in the building connected by Ethernet as is usually done in hotels4. Installing new category-rated UTP cable to each unit if it is within the 100m distance limit and use Ethernet5. Installing new coax cable to each unit and use cable modem or MOCA6. Installing fiber to each unit and mounting the ONT (Optical Network Terminal) at or inside the unit

Options 1 and 2 eliminate the need to install new cabling but assume the current cables are in good enough condition to carry the signal bandwidth required. Options 3, 4 and 5 require installing new cables and furthermore, option 3 assumes adequate bandwidth over the wireless for typical users. It would probably be overwhelmed by video users.

And options 1 through 5 require considerable investment in electronics, space to locate them and quality uninterruptible power at the building entrance facility. The actual network architecture is influenced by the choice of electronics. ONTs are available for single users or multiple users, allowing one to distribute ONTs in a building,

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for example to serve all the units on one floor with copper cables. However, these multi-user ONTs are going to divide up available bandwidth among the number of units served, perhaps not a problem if the system if offering Gb/s services to the ONT, but potentially a large problem, even today but certainly in the future, if the bandwidth allocated to multiple users is much lower.

With option 6, we would generally assume a GPON or EPON system, although a point to point (P2P) system can be used. In the case of a P2P system, fiber to the unit would entail either a switch in the MDU building itself or a large fiber count cable back to the central office or nearest switch.

Assuming a GPON or EPON network, option 6, installing fiber to every unit, has several variations that can be used and all have one big advantage: no matter how big the building and how many units, the size of the entrance facility is minimized and no power will be required except at each individual ONT at the unit. The options startwith where to place the PON splitters to optimize the cabling and installation then what kinds of cabling and hardware are needed to simplify the installation.

Options for connecting units with fiber include these architectures:1. PON splitters can be outside the building in a service provider facility and large fiber count cables brought into the building, then broken out in premises drop cables to units. This architecture also supports a P2P (point to point, not PON) system.2. PON splitters can be located in the entrance facility of building, minimizing the fiber count into the building, then drop cables run from that point to each unit.3. PON splitters can be cascaded from an initial PON splitter in the entrance facility to individual splitters on each floor or area of the building, supporting units on that floor or area.4. Theoretically, one could have a OLT (optical line terminal) installed in the building connecting to splitters distributed throughout the building or buildings. Sonce these units support thousands of users, dedicating a unit to one building would probably not be done except in large complexes.

The actual architecture will be influenced by the design of the MDU building and where and how it is convenient to install components for the FTTH systems. Component cost may need to be compromised to facilitate installation and reduce cost there.

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MDUs come in many varieties, of course, including rows of attached units, low-rise MDUs with only a few levels of units and high-rise MDUs. The first two are more horizontally distributed while high-rise buildings can have both many vertical levels and small to large horizontal distribution depending on the height and size of the building and the size of the units.

While some older units will allow cables to be installed on the exterior of the building, that is probably not going to be allowed on more modern buildings nor on high-rise buildings. However, most buildings will have facilities for cabling even if they are so old that they only had electrical and phone services originally.

Like any FTTH system, a “greenfield” installation offers much more flexibility for designing a building that simplifies cable and hardware installation. Plans can be made to include cable conduit and/or cable trays and facilities for other network hardware. But most large MDUs have provision for cabling for services like phones and CATV, perhaps even Internet if built more recently, that offer good options for FTTH fiber installation.

The PON splitter can be located in the building entrance facility and drop cables run to each unit. This mimics most phone wiring and pathways may be available to run drop cables. It uses more fiber and/or cables but does not require mounting as much hardware around the building nor splicing and/or termination.

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One reasonable option is to use cascaded splitters. The first splitter can be located in the entrance facility with multiple fibers going out to the separate floors where a splitter is installed to serve the floor. Alternatively, the first splitter can be placed on one of the served floors. If one has 8 units per floor, a total of 4 floors can be handled on 32 split ratio system with a 4 way splitter feeding 8 way splitters on each floor. Likewise, one could use a first splitter of 8 ways to serve 4 floors. Or a 16 way first splitter would serve 4 floors. The best option probably depends on the building and how cabling would be installed.

Recent developments on distribution and drop components make MDU installations easier. Perhaps the biggest development was bend-insensitive fibers that allow the manufacture of drop cables in extremely small sizes that can be run along wall or ceiling junctions, around corners, placed inside baseboard or molding and even made with an adhesive surface that can be stuck directly on walls. Bend-insensitive fibers also allow the manufacture of small cables that allow opening at any location to break out one or more fibers for termination at that point and allow the whole cable to continue to another location.

Small boxes or closures are available that contain couplers and patch panels allowing drop cables to be terminated with prepolished/splice connectors, either fusion- or mechanical-spliced, to complete the connections.

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Like any fiber or cabling installation, the actual project will be unique but be able to incorporate ideas that worked well in prior projects. If the building project is in the design stage itself, knowledgeable fiber optic designers can provide feedback that will make the installation easier, neater and much less expensive. 

The most important part of the design of a project in an existing buildingis a “walk through” to familiarize yourself with the building. Inspect for entrance facilities, cabling pathways and locations for equipment on every floor. Look at several units to see where it is feasible to enter the unit and place equipment. Having a familiarity with the building itself will make choosing a design much easier.

Another issue, of course, is the take rate for FTTH connections., which can affect planning as well as the ultimate cost. On older buildings, units may already have CATV or satellite connections and not be interested in FTTH, so one cannot assume a 100% take rate. The building owner can survey those living in the units to determine the take rate for planning purposes, but one also has to assume some number of future additions in doing the design. New construction may be easier, as the developer/builder may decide to make FTTH a selling feature and provide it to all the units.

Fiber Optics For Wireless

    Today's users of mobile devices depend on wireless connections for their voice, data and even video communications. Even homes and businesses may depend on wireless, especially those who are not in urban or suburban areas served by FTTH (fiber to the home) or FTTC (fiber to the curb.) Some of us in the business now use the term FTTW for fiber to wireless, since wireless depends on fiber for the communications backbone and increasingly the connection to the wireless antennas, no matter what kinds of wireless we use.    Wireless is not entirely wireless. The easiest way to understand wireless is to think of it as a link that replaces the cable that connects your cellular or wireless phone to the phone system or the patchcord that connects your computer or other portable Internet device to the network. To understand wireless, it is necessary to look at several different and unique types of wireless systems, including cellular wireless phones, wireless in premises cabling, municipal or private wireless links and even some of the short distance links used for computer peripheral connections.

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Cellular Wireless    Cellular phone systems have grown to dominate the telecommunications marketplace. Countries that have had extensive landline phone systems for a century now already have more cell phones than land lines. Countries that had not developed landline-based phone networks skipped them entirely and went directly to cellular wireless where the adoption rates have been extremely high.     While cellular wireless started out as a voice network, text messaging became very popular, eclipsing voice for most users. Smart phones brought the Internet to the phone, and soon data became the largest traffic generator for cellular networks. In the first 3-1/2 years of the iPhone, AT&T claimed their data traffic grew 8000% - 80 times! Now video is coming to these same devices, creating an even faster growth rate for cellular network traffic.

 

   To accomodate this traffic level, wireless needs new systems with more radio frequency spectrum. Current systems (CDMA for some systems, in the US, GSM for the rest of the US and the world) are evolving into new generations of systems (4G, LTE) that have more data bandwidth. Almost from the beginning, cellular towers were connected to the telco networks over fiber optics, just like any other  connection. Wireless towers have small huts at the base that connect to fiber backbones that connect towers to the various phone companies. As traffic grows, towers need more antennas. Instead of 3-4 antennas on a tower, now one sees dozens, so towers and buildings now look like this:

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or on buildings

 

   All these antennas on a tower or the side of a building have created another problem. In the past, each antenna has been connected by a large (2", 50mm) coax cable that carries both signal and power to the antenna. But with all these antennas, the size, weight and even wind resistance of these cables has become a big problem. These towers which have been upgraded to add many antennas show the problem with these large coax cables.

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    This is another application where copper cable is being replaced by optical fiber. One small fiber cable can replace all those coax cables and a separate power cable is used for the drivers on the antennas. These applications use mostly prefab cable assemblies since making terminations on top of the tower is difficult to say the least. Some applications use prefab at the top of the tower and conventional termination at the base. Many of these systems use multimode fiber because the distances are so short and the transceivers are much less expensive for MM fiber.

    Below are photos from Corning showing a remote antenna head end and antenna and the fiber terminal serving the antennas. Note the use of a prefab cable system at the top of the tower, making installation much easier. Some installations use a composite cable that includes both fiber and power conductors so only one cable need be installed up the tower.

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Photos courtesy of Corning. 

    Many cell towers are independently owned and space for antennas is rented to the service providers. Installation of fiber to the towers and fiber up to the antennas is generally done by independent contractors who specialize in this kind of work.

 Wireless In Premises Networks   Wireless in the corporate premises network is WiFi (IEEE 802.11), the common network built into most laptop or netbook computers,

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tablets, VoIP phones, many cellular phones and other portable devices. The wireless “antenna” in the network, called an "access point," is a lot more than that. It’s a radio transceiver and network adapter that connects to your laptop to allow access to the network, with some logic that implements part of the network protocols allowing access to the network. The transceiver in the antenna has limited power as does the transceiver in the portable device, so the distance from the antenna to your laptop is limited. Connection between devices and antennas can be affected by metal in a building that reflects or attenuates signals. Signals can even be absorbed by people in the building. A typical office building may need 4-8 antennas per floor to get consistent connections throughout the area.  

 

    The antenna is connected to the network just like a PC, using UTP or fiber optic cable to a local switch which connects it into the network backbone. Not only does the wireless antenna require a network cable to connect to the network, but it needs power – uninterruptible power, just like any network hub or switch – to operate.     So replacing a wired network with a wireless one doesn’t mean you don’t need cabling; you may in fact need more when you consider the power needs of the antennas.  Any advantage of a wireless network is not in the installation, it’s in the flexibility of users roaming but maintaining connections.

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    Bluetooth  (IEEE 802.15) is a limited distance network mainly used for consumer devices. It has been used to connect a wireless printer or mouse to a PC, wireless headsets to cell phones and stereos, cell phones to cars for hands-free operation, digital cameras to printers, etc. In terms of installation, Bluetooth is built-in to many devices and adapters can be plugged into USB ports or added as cards to PCs, not installed as access points like WiFi, so it is not generally of interest for cabling installers.

More on wireless in premises systems including wireless network standards .  

Metropolitan Wireless     Metropolitan wireless systems have had a rocky road. Initially, they were proposed as an inexpensive way of offering broadband to everyone, but providing support and competitive issues with other broadband suppliers ended most of these early trials. Now cities often install WiFi for public service use and free Internet access in parks and plazas, like these examples. The distances these access points are from the network connections require fiber, usually SM fiber available in the metro fiber network used for other communications.

 

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Downtown Santa Monica California, where you can see two separate systems, one public, one private for city services, on one pole. 

 

Istanbul, Turkey installed metro WiFi for visitors as the European City of the Year. 

WiMAX      WiMAX is a further development of wireless network technology that expands the data capacity of wireless to ~ 100 Mb/s and it’s distance capability to several miles. Unlike WiFi which was primarily a short distance network aimed at private networks, WiMAX appears aimed at communications carriers who could use it in place of landline networks, substituting WiMAX, for example, for Fiber To The Home, in areas needing upgrades of their networks or using it to allow notebook PC or PDA roaming in a metropolitan area. 

"Super WiFi" On Fiber      Bringing broadband to the rural areas of the US or any large country with sparse population can be very expensive. The US has a plan, however, that may make it more affordable. "Super WiFi" is not your usual WiFi. It is using WiFi protocols but broadcasting on frequencies of unused TV channels, called "white space." The FCC is ready to open up new frequencies to broadband to allow delivering broadband Internet and phone to rural areas where cabling is too expensive. 

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  An example of a super Wi-Fi antenna: Altai

    Super WiFi works at a lower frequency than either regular WiFi or cellular systems so it has more reach into areas that are too rugged for most wireless systems. Usage in more urban areas may be a problem however, if there are too many broadcast TV stations which can interfere with Super WiFi signals (and vice versa.) These antennas will also require fiber to connect into the network.More in MIT Technology Review  And the US Government Announcement.

Fiber Optics and Premises Cabling

Fiber Optic Architecture For Local Area Networks (LANs)It’s fairly obvious that fiber optics is not copper wiring. The advantages of fiber include the capability of going longer distances at higher speeds, plus immunity to electromagnetic radiation. These advantages overcame fiber’s disadvantage in cost to make it the cabling of choice for telcom and CATV.  While fiber is still primarily limited to LAN backbones in the premises cabling market, new methods and components are increasing its acceptance.

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From the beginning, the EIA/TIA committees wrote the “568” standards around copper wiring. There was no logical reason to limit fiber to the architecture of UTP copper. While copper was technically limited to 100 meter links, fiber could go 2,000 meters or more at 100 Mb/s. But the architecture of 568 was written around copper, with a backbone cable to a telecom closet connecting the desktop over a horizontal cable of no more than 100 meters. As LAN speeds continued rising to gigabits/second, fiber became the media of choice for backbones, allowing easy upgrades and more flexibility in placing telecom rooms, as they became known, instead of "closets."

Several years ago, fiber’s capabilities were recognized in the TIA-568 standard with the addition of a centralized fiber architecture standard. The standard covered a network architecture that would place fiber hubs in the computer room and run backbone fiber to the telecom room, then through passive interconnects to the desktop. By allowing a direct to the desktop connection, there was no need for electronics in the telecom room. This meant that a data ground, conditioned, uninterruptable power, and air conditioning were also not needed, greatly reducing the cost to the end user and making fiber cost effective compared to copper. 

The copper people learned a lesson from fiber and created zone cabling. Zone cabling adds an additional consolidation point near desktops. Rather than the usual run of up to 90 meters of

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permanently-installed horizontal cabling using a single cable for every desktop, zone cabling uses a backbone cable from the telecom closet to the consolidation point, then short individual cables to the desktop. 

Zone cabling creates a “mini telecom closet” near the user. It’s a terrific solution for modular furniture designs that include pathways for cables. When the offices move, the furniture can be unplugged and moved but the wiring from the telecom closet to the consolidation point remains for future use. The modular furniture can be moved to another point where it again connects to a local consolidation point.

Another useful gadget is the MUTOA or "multi-user telecom outlet assembly." It is a patch panel or box with up to 12 connections that can be installed near a number of users who make their connections to that instead of a wall outlet. With proper design, it can not only simplify installation, but it can make MACs (moves, adds and changes) much easier.

There can be a big advantage in installation, where only one cable is pulled to an area, then shorter cables are used to connect to the desktop. This works for fiber optics and multi-pair telephone cabling, but may not be possible for all copper data cabling. For example, one can buy 25 pair Cat 5E cable, but not Cat 6, so if the user decides to install Cat 6, the cable runs to the zone box must be individual Cat 6 cables.

Fiber optics and zone cabling work well together. Using multifiber cables, a single cable can connect multiple desktops to a backbone cable with minimal bulk and weight, often a big problem in offices with many desktop connections. Overhead cable trays can become filled with many Cat 5E or Cat 6 cables, but one small, lightweight fiber cable can connect dozens or even hundreds of desktops.

Zone cabling works well with prefabricated fiber optic cable systems also. Cables can be factory terminated and the connectors enclosed in a protective boot for pulling. After the cable is pulled and secured, the boot is removed and connected to the zone box or MUTOA, and the cable is ready for use. 

These prefabricated cable assemblies offer several advantages. They are faster to install and have no yield problems on the connectors since every one is factory made and tested. The total installed cost of the components is often less than field termination,

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but the price to the customer is the same, so they can be more profitable for the contractor. As a downside, they do require more care in installation to prevent damage to the connectors. 

End users choose zone cabling for fiber to the desk applications because it reduces cable clutter and saves them money. It is not hard to design such a network if you know the basic layout and choose cabling hardware early in the process. Cable installation is easy since fewer cables are required but may require additional terminations at the zone boxes or MUTOAs. Like all installations, careful planning will yield an easier, neater installation.

Is Wireless Replacing Cable To The Desktop?

For many LAN applications, it appears that the choice of copper vs. fiber to the desktop may be neither. While these two have been trading claims over cost, technology and power, WiFi wireless networks have been developed to provide high quality connections at speeds that are more than adequate for most users and security issues have been addressed. 

The move to wireless connectivity has happened because most corporate users now prefer tablets or laptops with built-in WiFi to desktop computers for mobility. Many people also own portable mobile devices like smart phones, iPhones and Blackberries with wireless access that are becoming almost as powerful as a laptop.

Within the enterprise LAN, the backbone is primarily fiber, with new installations mostly using OM3/OM4 laser-optimized 50/125 variety capable of being upgraded from today’s 1 to 10 gigabits per second to 40 or even 100 in the future. The remaining desktop computer users will still get Cat 5e or Cat 6 to the desktop since it’s cheaper. 

Two other options are FTTO, using singlemode fiber to connect to a small, usually 2 or 4 port Gigabit Ethernet switch in the office where devices (including wireless access points) may plug in with a simple Cat 5e patchcord and passive optical LANs (POLs) using fiber to the home (FTTH) passive optical network (PON) components to implement a secure LAN within the premises or campus. 

Here is a diagram for FTTO:

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This is a POL where the ONT is the desktop connection:

Both these solutions appear to be less expensive than traditional structured cabling. Here is more information. 

The new generation of higher bandwidth wireless requires more access points which have traditionally been connected over copper, and recently have been powered over the same copper cables also. But there seems to be some question about whether adequate power for these new wireless access points can be carried over the same copper cables that must provide Gigabit Ethernet connectivity. Fiber may prove to be a better choice here too.

Data Centers

Data centers are comprised of many computers acting as servers, pulling data from storage disks and sending it to routers for transmission over the Internet. Because of the massive amounts of data served, data center links use the highest speeds available,

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currently 1 gigabit or 10G Ethernet or Fibre Channel at 1, 2, 4, 8 or 10 Gb/s. 

At 1 Gb/s the choice for connections are fiber or UTP copper, with copper probably being more popular. As data centers migrate to links with speeds above 1 Gb/s, UTP cabling must be upgraded to Cat 6A (augmented Cat 6) which is expensive, much larger than Cat 5e or Cat 6, and requires transceivers that  consume 4-8 times as much power as fiber transceivers, a big problem in data centers where power consumption - and the heat generated - cause problems. Other choices are coax copper for short links or fiber for any length of link. Most data centers use those two choices, with the fiber being high bandwidth laser-optimized 50/125 OM3 fiber.

More on Data Centers.

Fiber Optic Network Design

Jump To:

The Communications System  Cabling Design  Choosing Transmission Equipment  Planning The Route  Choosing Components  Cable Plant Link Loss Budget Analysis  

Project Documentation  Planning for the Installation  Planning for Restoration  Managing A Fiber Optic Project

Fiber optic network design refers to the specialized processes leading to a successful installation and operation of a fiber optic network. It includes first

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determining the type of communication system(s) which will be carried over the network, the geographic layout (premises, campus, outside plant (OSP, etc.), the transmission equipment required and the fiber network over which it will operate. Designing a fiber optic network usually also requires interfacing to other networks which may be connected over copper cabling and wireless.

Next to consider are requirements for permits, easements, permissions and inspections. Once we get to that stage, we can consider actual component selection, placement, installation practices, testing, troubleshooting and network equipment installation and startup. Finally, we have to consider documentation, maintenance and planning for restoration in event of a future outage. 

The design of the network must precede not only the installation itself, but it must be completed to estimate the cost of the project and, for the contractor, bid on the job. Design not only affects the technical aspects of the installation, but the business aspects also.

Finally, a fiber optic network designer needs to understand the processes of installation. We recommend you review the FOA Guide sections on fiber optic installation covering basic fiber installation and OSP fiber installation.

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 Campus network design

Working With OthersDesigning a network requires working with other personnel involved in the project, even beyond the customer. These may include network engineers usually from IT (information technology) departments, architects and engineers overseeing a major project and contractors involved with building the projects. Other groups like engineers or designers involved in aspects of project design such as security, CATV or industrial system designers  or specialized designers for premises cabling may also be overseeing various parts of a project that involves the design and installation of fiber optic cable plants and systems. Even company non-technical management may become involved when parts of the system are desired to be on exhibit to visitors.

Qualifications For Fiber Optic Network

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DesignersIt’s the job of the designer to understand not only the technology of communications cabling, but also the technology of communications, and to keep abreast of the latest developments in not only the technology but the applications of both.

Designers should have an in-depth knowledge of fiber optic components and systems and installation processes as well as all applicable standards, codes and any other local regulations. They must also be familiar with most telecom technology (cabled or wireless), site surveys, local politics, codes and standards, and where to find experts in those fields when help is needed. Obviously, the fiber optic network designer must be familiar with electrical power systems, since the electronic hardware must be provided with high quality uninterruptible power at every location. And if they work for a contractor, estimating will be a very important issue, as that is where a profit or loss can be determined!

Those involved in fiber optic project design should already have a background in fiber optics, such as having completed a FOA CFOT certification course, and may have other training in the specialties of cable plant design and/or electrical contracting. It’s also very important to know how to find in-depth information, mostly on the web, about products, standards, codes and, for the OSP networks, how to use online mapping services like Google Maps. Experience with CAD systems is a definite plus.

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The Communications SystemBefore one can begin to design a fiber optic cable plant, one needs to establish with the end user or network owner where the network will be built and what communications signals it will carry. The contractor should be familiar with premises networks, where computer networks (LANs or local area networks) and security systems use structured cabling systems built around well-defined industry standards. Once the cabling exits a building, even for short links for example in a campus or metropolitan network, requirements for fiber and cable types change. Long distance links for telecommunications, CATV or utility networks have other, more stringent requirements, necessary to support longer high speed links, that must be considered. 

But while the contractor generally considers the cabling requirements first, the real design starts with the communications system requirements established by the end user. One must first look at the types of equipment required for the communications systems, the  speed of the network and the distances to be covered before considering anything related to the cable plant. The communications equipment will determine if fiber is necessary or preferable and what type of fiber is required.

Premises Networks

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Premises cable systems are designed to carry computer networks (LANs, local area networks) based on Ethernet which currently may operate at speeds from 10 megabits per second to 10 gigabits per second. The typical LAN has copper and fiber sections and links to connect to wireless access points for universal WiFi connectivity. Data centers are unique applications that house multiple Internet servers and storage networks operating at very high speeds using combinations of short copper and fiber links. Other systems may carry security systems with digital or analog video, perimeter alarms or entry systems, which are usually low speeds, at least as far as fiber is concerned. Premises telephone systems can be carried on traditional twisted pair cables or, as is becoming more common, utilize LAN cabling with voice over IP (VoIP) technology. 

Premises networks are usually short, often less than the 100 meters (about 330 feet) used as the limit for standardized structured cabling systems that allow twisted pair copper or fiber optic cabling, with backbones on campus networks used in industrial complexes or institutions as long as 500 m or more, requiring optical fiber. 

Premises networks generally operate over multimode fiber. Multimode systems are less expensive than singlemode systems, not because the fiber is cheaper (it isn’t) nor because cable is cheaper (the same), but because the large core of multimode fiber allows the use of cheaper LED or

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VCSEL sources in transmitters, making the electronics much cheaper. Astute designers and end users often include both multimode and singlemode fibers in their backbone cables (called hybrid cables)  since singlemode fibers are very inexpensive and it provides a virtually unlimited ability to expand the systems. LANs and data centers operating at speeds over 10Gb/s are migrating to singlemode fiber so more premises cabling systems include singlemode.

Premises networks will include a entrance facility where outside plant and premises communications systems meet. This facility must include not only cabling connections but compatible communications equipment. Since it is indoors, it must consider issues for building and electrical codes, such as the common requirement that bare OSP cables can only come 50 feet (about 15 meters) before being terminated in fire-rated cables unless it is in conduit.

Outside Plant NetworksOutside plant networks refers to all systems that are outdoors, not inside buildings or campuses. They are typically longer networks uses for telecom, CATV, utilities, security, metropolitan networks, etc.

Telephone networks are mainly outside plant (OSP) systems, connecting buildings over distances as short as a few hundred meters to hundreds or thousands of kilometers. Data rates for telecom are typically 2.5 to 10 gigabits per second using very

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high power lasers that operate exclusively over singlemode fibers. The big push for telecom is now taking fiber directly to a commercial building or the home, since the signals are now too fast for traditional twisted copper pairs.

CATV also uses singlemode fibers with systems that are either hybrid fiber-coax (HFC) or digital where the backbone is fiber and the connection to the home is on coax. Coax still works for CATV since it has very high bandwidth itself. Some CATV providers have discussed or even tried some fiber to the home, but have not seen the economics become attractive yet.

Besides telecom and CATV, there are many other OSP applications of fiber. Intelligent highways are dotted with security cameras and signs and/or signals connected on fiber. Security monitoring systems in large buildings like airports, government and commercial buildings, casinos, etc. are generally connected on fiber due to the long distances involved. Like other networks, premises applications are usually multimode while OSP is singlemode to support longer links. 

Metropolitan networks owned and operated by cities can carry a variety of traffic, including surveillance cameras, emergency services, educational systems, telephone, LAN, security, traffic monitoring and control and sometimes even traffic for commercial interests using leased bandwidth on dark fibers or city-owned fibers.

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However, since most are designed to support longer links than premises or campus applications, singlemode is the fiber of choice.

For all except premises applications, fiber is the communications medium of choice, since its greater distance and bandwidth capabilities make it either the only choice or considerably less expensive than copper or wireless. Only inside buildings is there a choice to be made, and that choice is affected by economics, network architecture and the tradition of using copper inside buildings. Next, we’ll look at the fiber/copper/wireless choices in more detail.

Cabling Design

Copper, Fiber or Wireless?While discussions of which is better – copper, fiber or wireless – has enlivened cabling discussions for decades, it’s becoming moot. Communications technology and the end user market, it seems, have already made decisions that generally dictate the media and many networks combine all three. The designer of cabling networks, especially fiber optic networks, and their customers today generally have a pretty easy task deciding which media to use once the communications systems are chosen.

Long Distance and Outside Plant CablingOther than telco systems that still use copper for the final connection to the home, practically every

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cable in the telephone system is fiber optic. CATV companies use a high performance coax into the home, but it connects to a fiber optic backbone. The Internet backbone is all fiber. Most commercial buildings in populous areas have direct fiber connections from communications suppliers. Cities use SM fiber to connect municipal buildings, surveillance cameras, traffic signals and sometimes offer commercial and residential connections, all over singlemode fiber. Even cellular antenna towers along highways and on tall buildings usually have fiber connections. Remote areas such as central Africa depend on satellite communications since cables are too expensive to run long distances for the small amounts of traffic involved.

Designing long distance or outside plant applications generally means choosing cabling containing singlemode (SM) fiber over all other media. Most of these systems are designed to be used over distances and speeds that preclude anything but SM fiber. Occasionally other options may be more cost effective, for example if a company has two buildings on opposite sides of a highway, a line-of-sight or radio optical wireless network may be easier to use since they have lower cost of installation and are easier to obtain relevant permits.

The choice of the actual singlemode fiber, however, can depend on the application. Depending on the length of the link, the wavelength of the transmitters, data rate of the transmission and if

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CWDM or DWDM are planned, different types of fiber may be optimal. Refer to the section on on fiber for more details.

Premises CablingThe desire for mobility, along with the expansion of connected services, appears to lead to a new type of corporate network. Fiber optic backbone with copper to the desktop where people want direct connections and multiple wireless access points, more than is common in the past, for full coverage and maintaining a reasonable number of users per access point  is the new norm for corporate networks. Most building management systems use proprietary copper cabling, for example thermostat wiring and paging/audio speaker systems. Security monitoring and entry systems, certainly the lower cost ones, still depend on  coax copper cable, although high security  facilities like government and military installations often pay the additional cost for fiber’s more secure nature. 

Surveillance systems are becoming more prevalent in buildings, especially governmental, banking, or other buildings that are considered possible security risks. While coax connections are common in short links and structured cabling advocates say you can run cameras limited distances on Cat 5E or Cat 6 UPT like computer networks, fiber has become a much more common choice. Besides offering greater flexibility in camera placement because of its distance capability, fiber optic cabling is much

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smaller and lightweight, allowing easier installation, especially in older facilities like airports or large buildings that may have available spaces already filled with many generations of copper cabling.

When these premises communications systems connect to the outside world, it is generally to singlemode optical fiber. The entrance facility and equipment room must accommodate the equipment needed to make those connections.

Use of Cabling StandardsMany documents relating to cable plant design focus on industry standards for both communications systems and cable plants. US standards come from the TIA or Telcordia while worldwide standards may come from ISO/IEC or ITU. 

It is important to realize why and by whom these standards are written. These standards are written by manufacturers of products to ensure that various manufacturers’ products work together properly. Whenever users specify standards for any project, it is important that the contractor/installer understand what standards are being referenced and ensure that such standards are relevant to the job being done.

To better understand installation and testing requirements, FOA recommends designers and installers refer to the NECA/FOA-301 standard on installing fiber optic networks and the FOA

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standards since they are written by contractors, designers, installers and users for their needs.  

Choosing Transmission Equipment And Links

Choosing transmission equipment is the next step in designing a fiber optic network. This step will usually be a cooperative venture involving the customer, who knows what kinds of data they need to communicate, the designer and installer, and the manufacturers of transmission equipment. Transmission equipment and the cable plant are tightly interrelated. The distance and bandwidth will help determine the fiber type necessary and that

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will dictate the optical interfaces on the cable plant. The ease of choosing equipment may depend on the type of communications equipment needed.

Telecom has been standardized on fiber optics for 30 years now, so they have plenty of experience building and installing equipment. Since most telecom equipment uses industry conventions, you can usually find equipment for telecom transmission that will be available for short links (usually metropolitan networks, maybe up to 20-30 km), long distance and then really long distance like undersea runs. All run on singlemode fiber, but may specify different types of singlemode.

Shorter telecom links will use 1310 nm lasers on regular singlemode fiber, often referred to as G.652 fiber, it’s international standard. Longer links will use a dispersion-shifted fiber optimized for operation with 1550 nm lasers (G.653 or G.655 fiber). For most applications, one of these will be used. Most telco equipment companies offer both options.

Most CATV links are AM (analog) systems based on special highly linear lasers called distributed feedback (DFB) lasers using either 1310 nm or 1550 nm operating on regular singlemode fibers. As CATV moves to digital transmission, it will use technology more like telecom, which is already all digital.The choices become more complex when it comes to data and CCTV because the applications are so

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varied and standards may not exist. In addition, equipment may not be available with fiber optic transmission options, requiring conversion from copper ports to fiber using devices called media converters. 

In computer networks, the Ethernet standards, created by the IEEE 802.3 committee, are fully standardized. You can read the standards and see how far each equipment option can transmit over different types of fiber, choosing the one that meets your needs. Most network hardware like switches or routers are available with optional fiber optic interfaces, but PCs generally only come with UTP copper interfaces that require media converters. An Internet search for “fiber optic media converters” will provide you with dozens of sources of these inexpensive devices. Media converters will also allow the choice of media appropriate for the customer application, allowing use with multimode or singlemode fiber and may even offer transceiver options for the distance that must be covered by the link.

CCTV is a similar application. More cameras now come with fiber interfaces since so many CCTV systems are in locations like big buildings, airports, or areas where the distances exceed the capability of coax transmission. If not, video media converters, usually available from the same vendors as the Ethernet media converters, are readily available and also inexpensive. Again, choose converters that meet the link requirements

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set by the customer application, which in the case of video, not only includes distance but also functions, as some video links carry control signals to the camera for camera pan, zoom and tilt in addition to video back to a central location.

What about industrial data links? Many factories use fiber optics for its immunity to electromagnetic interference.  But industrial links may use proprietary means to send data converted from old copper standards like RS-232, the ancient serial interface once available on every PC, SCADA popular in the utility industry, or even simple relay closures. Many companies that build these control links offer fiber optic interfaces themselves in response to customer requests. Some of these links have been available for decades, as industrial applications were some of the first premises uses of fiber optics, dating back to before 1980. Most operate over regular graded-index multimode fiber although some have been designed around large core PCS (plastic-clad silica) fibers.

Whatever the application, it’s important for the end user and the cabling contractor to discuss the actual application with the manufacturer of the transmission hardware to ensure getting the proper equipment. While the telecom and CATV applications are cut and dried and the data (Ethernet) applications covered by standards, it is our experience that not all manufacturers specify their products in exactly the same way.

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One company in the industrial marketplace offered about fifteen different fiber optic products, mainly media converters for their control equipment. However, those fifteen products had been designed by at least a dozen different engineers, not all of whom were familiar with fiber optics and especially fiber jargon and specifications. As a result, one could not compare the products to make a choice or design them into a network based on specifications. Until their design, sales and applications engineers were trained in fiber optics and created guidelines for product applications, they suffered from continual problems in customer application. 

The only way to make sure you are choosing the proper transmission equipment is to make absolutely certain the customer and equipment vendor – and you – are communicating clearly what you are planning to do.

Planning The Route Having decided to use fiber optics and chosen equipment appropriate for the application, it’s time to determine exactly where the cable plant and hardware will be located. One thing to remember – every installation will be unique. The actual placement of the cable plant will be determined by the physical locations along the route, local building codes or laws and other individuals involved in the designs. As usual, premises and outside plant installations are different so we will consider them separately.

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Premises and campus installations can be simpler since the physical area involved is smaller and the options fewer. Start with a good set of architectural drawings and, if possible, contact the architect, contractor and/or building manager. Having access to them means you have someone to ask for information and advice. Hopefully the drawings are available as CAD files so you can have a copy to do the network cabling design in your computer, which makes tweaking and documenting the design so much easier.

If the building is still in the design stage, you may have the opportunity to provide inputs on the needs of the cable plant. Ideally, that means you can influence the location of equipment rooms, routing of cable trays and conduits, availability of adequate conditioned power and separate data grounds, sufficient air-conditioning and other needs of the network. For pre-existing  buildings, detailed architectural drawings will provide you with the ability to route cabling and network equipment around the obstacles invariably in your way.

Outside plant (OSP) cabling installations have enormous variety depending on the route the cable must take. The route may cross long lengths of open fields, run along paved rural or urban roads, cross roads, ravines, rivers or lakes, or, more likely, some combination of all of these.  It could require buried cables, aerial cables or underwater cables. Cable may be in conduit, innerduct or direct buried, aerial cables may be self-supporting or lashed to a

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messenger. Longer runs often include crossing water, so the cable may be underwater or be lashed across a bridge with other cables. 

Site VisitsAnd as soon as possible, you must visit the site or route where the network will be installed. Outside plant routes need to be driven or walked every foot of the way to determine the best options for cable placement, obstacles to be avoided or overcome,

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and to determine what local entities may have input into the routing. Often cities or other governments will know of available conduits or rules on using utility poles that can save design time and effort.

For installations inside current buildings, you should inspect every area to be absolutely certain you know what the building really looks like and then mark up drawings to reflect reality, especially all obstacles to running cabling and hardware and walls requiring firestopping that are not on the current drawings. Take pictures if you can. For buildings under construction, a site visit is still a good idea, just to get a feeling of what the final structure will be like and to get to know the construction managers you will be working with. They may be the best source of information on who the local authorities are who will be inspecting your work and what they expect.

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 OSP network route on satellite map

With all those options on OSP installations, where do you start? With a good map. Not just a road map or a topographical map, but satellite images overlaid on roads is much better, like “Google Maps” can provide. Creating a route map is the first step, noting other utilities along the route on that map, and checking with groups that document the current utilities to prevent contractors from damaging currently installed pipes and cables. 

Once you have marked up maps, the real “fun” begins: finding out whose permission you need to run your cabling. OSP installs are subject to approval by local, state and federal authorities who will influence heavily how your project is designed.

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Some cities, for example, ban aerial cables. Some have already buried conduit which you can use for specific routes. Since many municipalities have installed city-owned fiber networks, they may have fiber you can rent, rather than go through the hassle of installing your own. 

Unless you are doing work for a utility that has someone who already has the contacts and hopefully easements needed, you may get to know a whole new set of people who have control over your activities. And you have to plan for adequate time to get approval from everyone who is involved.

Call Before You Dig

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Digging safely is vitally important. The risk is not just interrupting communications, but the life-threatening risk of digging up high voltage or gas lines. Some obstacles may be found during site visits, where signs like these are visible. There are several services that maintain databases of the location of underground services that must be contacted before any digging occurs, but mapping these should be done during the design phase and double-checked before digging to ensure having the latest data.

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If all this sounds vague, it is. Every project is different and requires some careful analysis of the conditions before even beginning to choose fiber optic components and plan the actual installation. Experience is the best teacher.

Choosing Components

Choosing Components For Outside Plant InstallationsThe choice of outside plant fiber optic (OSP) components begins with developing the route the cable plant will follow. Once the route is set, one knows where cables will be run, where splices are located and where the cables will be terminated. All that determines what choices must be made on cable type, hardware and sometimes installation methodology. 

CablesWhen choosing components, most projects start with the choice of a cable. Cable designs are optimized for the application type. In OSP installations, cables may be underground, direct buried, aerial or submarine (or simply underwater.) More on OSP cable types. 

Underground cables are generally installed in conduit which is usually a 4 inch (10 cm) conduit with several innerducts for pulling cables. Here cables are designed for high pulling tension and lubricants are used to reduce friction on longer

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pulls. Automated pulling equipment that limits pulling tension protects the cables. Very long runs or those with more bends in the conduit may need intermediate pulls where cable is pulled, figure-8ed and then pulled to the next stage or intermediate pulling equipment is used. Splices on underground cables are generally stored above ground in a pedestal or in a vault underground. Sufficient excess cable is needed to allow splicing in a controlled environment, usually a splicing trailer, and the storage of excess cable must be considered in the planning stage.

Direct buried cable is placed underground without conduit. Here the cable must be designed to withstand the rigors of being buried in dirt, so it is generally a more rugged cable, armored to prevent harm from rodent chewing or the pressures of dirt and rocks in which it is buried. Direct burial is generally limited to areas where the ground is mostly soil with few rocks down to the depth required so trenching or plowing in cable is easily accomplished. Splices on direct buried cables can be stored above ground in a pedestal or buried underground. Sufficient excess cable is needed to allow splicing in a controlled environment, usually a splicing trailer, and the storage of excess cable must be considered.

Aerial installations go from pole to pole, but the method of securing cables can vary depending on the situation. Some cables are lashed to messengers or other cables, such as CATV where

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light fiber cables are often lashed to the heavy coax already in place. Cables are available in a “8” configuration with an attached steel messenger that provides the strength to withstand tension on the cable. Some cables are made to directly be supported without a messenger, called all-dielectric sefl-supporting cables that use special hardware on poles to hold the cables. 

Optical ground wire is used by utilities for high voltage distribution lines. This cable is an electrical cable with fibers in the middle in a hermetically-sealed metal tube. It is installed just like standard electrical conductors. Splices on aerial cables can be supported on the cables or placed on poles or towers, Most splices are done on the ground, although it is sometimes done in a bucket or even on a tent supported on the pole or tower. Hardware is available for coiling and storing excess cable.

Sometime OSP installations involve running cables across rivers or lakes where other routes are not possible. Special cables are available for this that are more rugged and sealed. Even underwater splice hardware is available. Landings on the shore need to be planned to prevent damage, generally by burying the cable close to shore and marking the landing. Transoceanic links are similar but much more complex, requiring special ships designed for cable laying.

Since OSP applications often use significant lengths of cables, the cables can be made to order,

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allowing optimization for that particular installation. This usually allows saving costs but requires more knowledge on the part of the user and more time to negotiate with several cable manufacturers. To begin specifying the cable, one must know how many fibers of what type will be included in each cable. It’s important to realize that fiber, especially singlemode fiber used in virtually all OSP installations, is cheap and installation is expensive. Installation of an OSP cable may cost a hundred times the cost of the cable itself. 

Choosing a singlemode fiber is easy, with basic 1300 nm singlemode (called G.652 fiber) adequate for all but the longest links or those using wavelength-division multiplexing. Those may need special fiber optimized at 1500-1600 nm (G.653 or G.654). For premises and campus cable plants, OM3 type laser-optimized 50/125 multimode fiber is probably the best choice for any multimode OSP runs, as its lower attenuation and higher bandwidth will make most networks work better.

Including more fibers in a cable will not increase the cable cost proportionally; the basic cost of making a cable is fixed but adding fibers will not increase the cost much at all. Choosing a standard design will help reduce costs too, as manufacturers may have the cable in stock or be able to make your cable at the same time as others of similar design. The only real cost for adding more fibers is additional splicing and termination costs, still small with respect to total installed cost. And remember that having additional

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fibers for future expansion, backup systems or in case of breaks involving individual fibers can save many future headaches.   

Common traits of all outside plant cables include strength and water or moisture protection. The necessary strength of the cable will depend on the installation method (see below.) All cables installed outdoors must be rated for moisture and water resistance. Until recently, most people chose a gel-filled cable, but now dry-water blocked cables are widely available and preferred by many users. These cables use water-absorbing tape and power that expands and seals the cable if any water enters the cable. Installers especially prefer the dry cables as it does not require the messy, tedious removal of the gel used in many cables, greatly reducing cable preparation for splicing or termination.

OSP cable construction types are specifically designed for strength depending on where they are to be direct buried, buried in conduit, placed underwater or run aerially on poles. The proper type must be chosen for the cable runs. Some applications may even use several types of cable. Having good construction plans will help in working with cable manufacturers to find the appropriate cable types and ordering sufficient quantities. One must always order more cable than route lengths, to allow for service loops, preparation for termination and excess to save for possible restoration needs in the future.

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Like cable types, cable plant hardware types are quite diverse and should be chosen to match the application type and cable types being used. With so many choices in hardware, working with cable manufacturers is the most expeditious way to chose hardware and ensure compatibility. Besides cable compatibility, the hardware must be appropriate for the location, which can be outdoors, hung on poles, buried, underwater, inside pedestals, vaults or buildings, etc. Sometimes the hardware will need to be compatible with local zoning, for example in subdivisions or business parks. The time consumed in choosing this hardware can be lengthy, but is very important for the long term reliability of the cable plant. 

Splicing And Termination HardwareSplicing and termination are the final category of components to be chosen. Most OSP singlemode fiber is fusion spliced for low loss, low reflectance and reliability. Multimode fiber, especially OM2, 3 and 4, is also easily fusion spliced, but if only a few splices are necessary, mechanical splicing may provide adequate performance and reliability. 

Finished splices are placed in a splice tray and placed in a splice closure outdoors or optionally in trays on patch panels indoors. They are sealed to prevent moisture reaching the splices and are designed to be re-entered for repair or re-routing fibers. Splice closures are available in hundreds of

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designs, depending on the placement of the closure, for example underground in a manhole or vault, above ground in a pedestal, buried underground or mounted on a pole. Closures must also be chosen by the number and types of cables being spliced and whether they enter at both ends or only one. The numbers of cables and splices that a closure can accommodate will determine the size of the closure, and those for high fiber count cables can get quite large. 

Splice trays generally hold twelve single fiber fusion splices but may hold fewer ribbon or mechanical splices. Each splice tray should securely hold the splice and have a cover to protect the fibers when stacked in the closure.

Singlemode fibers are best terminated by fusion spicing factory-made pigtails onto fibers in the cable and protecting the splices in a closure or patch panel tray. If termination is done directly on multimode OSP cables, breakout kits will be necessary to sleeve fibers for reliability when connectors are directly attached. This takes more installation time than splicing pre-terminated pigtails on the cables, as is common with singlemode fiber cables, and may not save any costs. Even complete preterminated outside cable plant systems are becoming available, reducing the time necessary for termination and splicing. Talk to the cable manufacturers to determine feasibility of this option. 

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Outdoor terminations are sometimes housed in pedestals or equipment housings such as those used for local phone switches or traffic control systems. Some of these closures may not be fully sealed from dust and moisture, in which case it is recommended that the fiber connections be inside a more protective housing to prevent future unreliability.

Choosing the proper components for OSP installations can take time, but is important for system operation. Once components are chosen, the materials lists are added to the documentation for purchase, installation and future reference. 

Choosing Components For Premises InstallationsPremises cabling and outside plant cabling will coexist in the entrance facility or the equipment room where the two are connected. The choice of premises fiber optic components are affected by several factors, including the choice of communications equipment, physical routing of the cable plant and building codes and regulations. If the design is a corporate network (LAN), the design will probably include a fiber optic backbone connecting computer rooms to wiring closets. The wiring closets house switches that convert the fiber backbone to UTP copper for cable connected desktops and either copper or fiber to wireless access points. Some desktops, especially in engineering or design departments, may require fiber to the desktop for it’s greater bandwidth. Extra

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cables or fibers may be needed for security systems (alarms, access systems or CCTV cameras) and building management systems also. 

Design of the fiber optic cable plant requires coordinating with everyone who is involved in the network in any way, including IT personnel, company management,  architects and engineers, etc. to ensure all cabling requirements are considered at one time, to allow sharing resources.

As in OSP design, consider the fiber choice first. Most premises networks use multimode fiber, but many users now install hybrid cables with singlemode fibers for future expansion. The 62.5/125 micron fiber (OM1 fiber) that has been used for almost two decades has mostly been superceded by the new 50/125 laser-optimized fiber (OM3 or OM4), as it offers substantial bandwidth/distance advantages. 

Virtually all equipment will operate over 50/125 OM3 or OM4 fiber just as well as it did on 62.5/125 OM1 fiber, but it’s always a good idea to check with the equipment manufacturers to be sure.  Remember in the design documentation to include directions to mark all cables and patchpanels with aqua-colored tags, indicative of OM3 or OM4 fiber.

Cable in premises applications is generally either distribution or breakout cable. Distribution cables have more fibers in a smaller diameter cable, but require termination inside patch panels or wall

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mounted boxes. Breakout cables are bulky, but they allow direct connection without hardware, making them convenient for industrial use. Fiber count can be an issue, as backbone cables now have many fibers for current use, future expansion and spares, making distribution cables the more popular choice. 

On all indoor cables, the cable must be rated as fire-retardant per the NEC, CEC or other building codes. In the NEC terminology, indoor cables are generally OFNR-rated (Riser) unless the cable in air-handling areas above ceilings, where OFNP (plenum) is needed. Cable jacket color for OM3 cables can be ordered in aqua for identification as both fiber optics and OM3 or OM4 50/125 fibers.

If the cable is going to be run between buildings, indoor/outdoor designs are now available that have dry water-blocking and a double jacket. The outer jacket is moisture-resistant for outdoor use but can be easily stripped, leaving the fire-rated inner jacket for indoor runs. 

Fiber optic connector choices are also changing. STs and even SCs are succumbing to the success of the smaller LC connector. Since most fast (gigabit and above) equipment uses LC connectors, using them in the cable plant means only one connector needs to be supported. The LC offers another big advantage for those users who are upgrading to  OM3 fiber. The LC connector is incompatible with SC and ST connectors, so using

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it on 50/125 fiber cable plants prevents mixing 50 and 62.5 fibers with high fiber mismatch losses.

Premises fiber optic cables need to be run separately from copper cables to prevent crushing. Sometimes they are hung carefully below copper cable trays or pulled in innerduct. Using innerduct can save installation time, since the duct (which can be purchased with pull tapes already inside) can be installed quickly without fear of damage and then the fiber optic cable pulled quickly and easily. Some applications may require installing fiber optic cables inside conduit, which requires care to minimize bends, provide intermediate pulls to limit pulling force or use fiber optic cable lubricants.

The hardware necessary for the installation will need to be chosen based on where the cables are terminated. Premises runs are generally point-to-point and are not spliced. Wherever possible, allow room for large radii in the patch panels or wall-mounted boxes to minimize stress on the fibers. Choose hardware that is easy to enter for moves, adds and changes but lockable to prevent intrusion.

In premises applications, it’s worth considering a preterminated system. These use backbone cables terminated in multifiber connectors and preterminated patch panel modules. If the facility layout is properly designed, the cable manufacturer can work with you to create a “plug and play” system that needs no on-site termination and the cost may be very competitive to a field-terminated

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system.

Creating A Materials ListFor every installation, a complete materials list must be created listing each component needed and quantities required. This list will be used by the installation crew, but first it will be used to estimate the cost of the project. 

It is very important to list every component. Some components can be estimated based on other quantities. Ducts for example will be ordered in lengths similar to the cable pulled into them. Each fiber needs termination on both ends of the cable plant. Splice trays and closures must be ordered according to the numbers of fibers in the cables.

You should include extra quantities for installation. Every splice point, for example, needs 10-20 meters extra cable for splicing in a splice trailer, stripping for the splice and service loops. Extra cable should also be ordered to be kept for possible future restoration. Extra connectors or pigtails are needed to replace those improperly installed during installation. Some contractors routinely order 2-5% more than they estimate is necessary for the job.

While ordering more components than necessary can be costly, it’s less expensive than being short during the actual installation, especially for special order items like cables or patchcords. Excess components should be packed and stored as part of a restoration kit.

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More on estimating fiber optic projects. 

Cable Plant Link Loss Budget Analysis

Loss budget analysis is the calculation and verification of a fiber optic system's operating characteristics. It is used to estimate the loss of a cable plant being installed, determine if the cable plant will work with any given transmission system and provide an estimate for comparison to actual test results. A link loss budget encompasses items such as the length of the link, fiber type, wavelengths, connectors and splices, and any other sources of loss in the link. Attenuation and bandwidth are the key parameters for budget loss analysis, but since we cannot test attenuation, we generally use limits for loss set by the standards for the systems or networks we are going to use on the link. (Here is a table of link losses from industry standards for many links.) The designer should analyze link loss early in the design stage prior to installing a fiber optic system to make certain the system will work over the proposed cable plant. 

From the system standpoint, we have a limit to the loss it can tolerate on the cable plant, called a power budget, determined from the output of the transmitter and the required input of the receiver. We define these errors for the system as "bit-error rate" and they may be caused by too little

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power or too much power at the receiver. It is important to note that most calculations focus on the cable plant loss being low enough for the system power budget. However, on some systems, especially laser-based singlemode systems, the receiver may not tolerate too low a loss which causes high power at the receiver and may overload it, causing transmission errors. Under such conditions, an attenuator is added at the receiver end of the link to lower the power to an acceptable level.

Both the passive and active components of the circuit can be included in the budget loss calculation. Passive loss is made up of fiber loss, connector loss, and splice loss. Don't forget any couplers or splitters in the link. If the specifications for a type of system or network are not known, industry generic or standard loss values for the fiber optic components can be used for calculating the loss budget for the cable plant. Prior to system turn up, test the insertion loss of the cable plant with a source and power meter to ensure that it is within the loss budget.

The idea of a loss budget is to ensure the network equipment will work over the installed fiber optic link. One issue is what values should one should use for component losses when making the calculations. One can use the values in some industry standards like TIA-568 which are considered very high, one can use typical values, one can use values from component manufacturers

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which may be nearer typical or the user may have values that they require, not unusual for sophisticated users like telcos. It is normal to be conservative over the specifications. Don't use the best possible specs for fiber attenuation or connector loss to allow some margin for installation and component degradation over time.

The best way to illustrate calculating a loss budget is to show how it's done for a typical cable plant, here a 2 km hybrid multimode/singlemode link with 5 connections (2 connectors at each end and 3 connections at patch panels in the link) and one splice in the middle. See the drawings below of the link layout and the instantaneous power in the link at any point along it's length, scaled exactly to the link drawing above it.

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Cable Plant Passive Component Loss

Step 1. Calculate fiber loss at the operating wavelengths (length times standard estimates of loss at each wavelength)

Cable Length (km)

2.0 2.0 2.0 2.0

Fiber Type

Multimode

Singlemode

Wavelen 850 130 1300 155

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gth (nm) 0 0

Fiber Atten. (dB/km)

3 [3.5]1 [1.5]

0.4 [1/0.5]

0.3 [1/0.5]

Total Fiber Loss (dB)

6.0 [7.0]

2.0 [3.0]

0.8 [2/10.6 [2/1]

(All specifications in brackets are maximum values per EIA/TIA 568 standard. For singlemode fiber, a higher loss is allowed for premises applications, 1 dB/km for premises, 0.5 dB/km for outside plant. )

Step 2. Connector LossMultimode connectors will have losses of 0.2-0.5 dB typically. Singlemode connectors, which are factory made and fusion spliced on will have losses of 0.1-0.2 dB. Field terminated singlemode connectors may have losses as high as 0.5-1.0 dB. Let's calculate it at both typical and worst case values.

Connector Loss

0.3 dB (typical adhesive/polish connector)

0.75 dB (prepolished/splice connector and TIA-568 max acceptable)

Total # of Connect

5 5

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ors

Total Connector Loss

1.5 dB 3.75 dB

(All connectors are allowed 0.75 max per EIA/TIA 568 standard)Many designers and technicians forget when doing a loss budget that the connectors on the end of the cable plant must be included in the loss budget. When the cable plant is tested, the reference cables will mate with those connectors and their loss will be included in the measurements.

Step 3. Splice LossMultimode splices are usually made with mechanical splices, although some fusion splicing is used. The larger core and multiple layers make fusion splicing abut the same loss as mechanical splicing, but fusion is more reliable in adverse environments. Figure 0.1-0.5 dB for multimode splices, 0.3 being a good average for an experienced installer. Fusion splicing of singlemode fiber will typically have less than 0.05 dB (that's right, less than a tenth of a dB!) 

Splice Loss 0.3 dB

Total # splices 1

Total Splice Loss 0.3 dB

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(For this loss budget calculation, all splices are allowed 0.3 max per EIA/TIA 568 standard)

Step 4. Total Cable Plant LossAdd together the fiber, connector and splice losses to get the total link loss of the cable plant.

Best Case [TIA 568 Max]

Best Case [TIA 568 Max]

Wavelength (nm)

8501300

1300 1550

Total Fiber Loss (dB)

6.0 [7.0]

2.0 [3.0]

0.8 [2/1] 0.6 [2/1]

Total Connector Loss (dB)

1.5 [3.75]

1.5 [3.75]

1.5 [3.75]

1.5 [3.75]

Total Splice Loss (dB)

0.3 [0.3]

0.3 [0.3]

0.3 [0.3] 0.3 [0.3]

Other (dB)

0 0 0 0

Total Link Loss (dB)

7.8 [11.05]

3.8 [7.05]

2.6 [6.05/5.05]

2.4 [6.05/5.05]

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These values of cable plant loss should be the criteria for testing. Allow +/- 0.2 -0.5 dB for measurement uncertainty and that becomes your pass/fail criterion.

 Equipment Link Loss Budget CalculationLink loss budget for network hardware depends on the dynamic range, the difference between the sensitivity of the receiver and the output of the source into the fiber. You need some margin for system degradation over time or environment, so subtract that margin (as much as 3dB) to get the loss budget for the link.

Step 5. Data From Manufacturer's Specification for Active Components (Typical 100 Mb/s multimode digital link using a 1300 nm LED source.)

Operating Wavelength (nm)

1300

Fiber Type MM

Receiver Sensitivity (dBm@ required BER)

-31

Average Transmitter Output (dBm)

-16

Dynamic Range (dB)

15

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Recommended Excess Margin (dB)

3

Step 6. Loss Margin Calculation

Dynamic Range (dB) (above)

15 15

Cable Plant Link Loss (dB @ 1300 nm)

3.8 (Typical)

7.05 (TIA)

Link Loss Margin (dB)

11.2 7.95

In the past, as a general rule, the Link Loss Margin was expected be greater than approximately 3 dB to allow for link degradation over time or splicing for restoration. LEDs or lasers in the transmitter may age and lose power, connectors or splices may degrade or connectors may get dirty if opened for rerouting or testing. If cables are accidentally cut, excess margin will be needed to accommodate splices for restoration. Today some systems, particularly high bit rate multimode LANs, have little margin due to the high bandwidth required. Some of these links require assuming fiber and connector loss to be extremely low to even accommodate the small power budget available. Under such conditions, one has to assume lower values, especially for connector loss, and, of course,

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require installers to be extremely careful in installation to meet these needs.

More on link loss budgets.  

FOA offers a free App for smartphones and tablets to calculate loss budgets. Check the App Store for "FOA LossCalc."

Project Documentation

Documentation of the cable plant is a necessary part of the design and installation process for a fiber optic network that is often overlooked. Documenting the installation properly during the planning process will save time and material in the installation. It will speed the cable installation and testing since the routing and terminations are already known. After component installation, the documentation should be completed with loss test data for acceptance by the end user. During troubleshooting, documentation eases tracing links and finding faults. Proper documentation is usually required for customer acceptance of the installation. All this documentation gets included in the project paperwork that includes a Scope of Work (SOW), Request For Proposal or Quotation (RFP or RFQ) and the project Contract between the contractor and the customer. More on the project paperwork is here. 

The documentation process begins at the initiation

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of the project and continues through to the finish. It must begin with the actual cable plant path or location. OSP cables require documentation as to the overall route, but also details on exact locations, e.g. on which side of streets, which cable on poles, where and how deep buried cables and splice closures lay  and if markers or tracing tape is buried with the cable. Premises cables require similar details inside a building in order for the cable to be located anywhere in the path.

Most of this data can be kept in CAD drawings and a database or commercial software that stores component, connection and test data. Long outside plant links that include splices may also have OTDR traces which should be stored as printouts and possibly in computer files archived on disks for later viewing in case of problems. A computer with proper software for viewing traces must be available, so a copy of the viewing program should be on the disks with the files. If the OTDR data is stored digitally, a listing of data files should be kept with the documentation to allow finding specific OTDR traces more  easily. 

The Documentation ProcessDocumentation begins with a basic layout for the network. A sketch on building blueprints may work for a small building but a large campus, metropolitan or long distance network will probably need a complete CAD layout.  The best way to set up the data is to use a facility drawing and add the locations of all cables and connection points.

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Identify all the cables and racks or panels in closets and then you are ready to transfer this data to a database. 

Fiber optic cables, especially backbone cables, may contain many fibers that connect a number of different links which may not all be going to the same place. The fiber optic cable plant, therefore, must be documented for cable location, the path of each fiber, interconnections and test results.  You should record the specifications on every cable and fiber: the manufacturer, the type of  cable and fiber, how many fibers, cable construction type, estimated length, and installation technique (buried, aerial, plenum, riser, etc.)

It will help to know what types of panels and hardware are being used, and what end equipment is to be connected. If you are installing a big cable plant with many dark (unused) fibers, some will probably be left open or unterminated at the panels, and that must be documented also. Whenever designing a network, it's a very good idea to have spare fibers and interconnection points in panels for future expansion, rerouting for repair or moving network equipment.

Documentation is more than records. All components should be labeled with color-coded permanent labels in accessible locations. Once a scheme of labeling fibers has been determined, each cable, accessible fiber and termination point requires some labeling for identification. A simple

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scheme is preferred and if possible, explanations provided on patch panels or inside the cover of termination boxes.

Protecting  RecordsCable plant documentation records are very important documents. Keep several backup copies of each document, whether it is stored in a computer or on paper, in different locations for safekeeping. If a copy is presented to the customer, the installer should maintain their own records for future work on the project. One complete set on paper should be kept with a “restoration kit” of appropriate components, tools directions in case of outages or cable damage. Documentation should be kept up to date to be useful so that task should be assigned to one on-site person with instructions to inform all parties keeping copies of the records of updates needed. Access to modify records should be restricted to stop unauthorized changes to the documentation. 

Planning for the Installation

Once the design of a fiber optic project is complete and documented, one might think the bulk of the design work is done. But in fact, it’s just beginning. The next step is to plan for the actual installation. Planning for the installation is a critical phase of any project as it involves coordinating activities of many people and companies. The best way to keep everything straight is probably to develop a

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checklist based on the design during the early stages of the project.

The Project ManagerPerhaps the most important issue is to have a person who is the main point of contact for the project. The project manager needs to be involved from the beginning, understands the aims of the project, the technical aspects, the physical layout, and is familiar with all the personnel and companies who will be involved. Likewise all the parties need to know this person, how to contact them (even 24/7 during the actual install) and who is the backup if one is needed. 

The backup person should also be involved to such a degree that they can answer most questions, may even be more technically savvy on the project, but may not have full decision-making authority. The backup on big jobs may well be the person maintaining the documentation and schedules, keeping track of purchases and deliveries, permits, subcontractors, etc. while the  project manager is more of a hands-on manager.

Design ChecklistPlanning for a project is critical to the success of the project. The best way is to develop a checklist before beginning the design process. The checklist below is comprehensive but each project will have some of its own unique requirements that need to be added. Not all steps need be done serially, as some can be done in parallel to reduce time

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required for designing the project. The designer must interface with many other people and organizations in designing a project so contacts for outside sources should be maintained with the design documentation.

Design processLink communications requirements Link route chosen, inspected, special requirements noted including inspections and permitsSpecify communications equipment and component requirements Specify cable plant componentsDetermine coordination with facilities, electrical and other personnelDocumentation completed and ready for installationWrite test plan Write restoration plans 

Contractor package for the installDocumentation, drawings, bills of materials, instructions Permits available for inspectionGuidelines to inspect workmanship at every step, test planDaily review of progress, test dataSafety rules to be posted on the job site(s) and reviewed with all supervisors and installation personnel

Requirements for completion of cable plant installation Final inspection

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Review test data on cable plantInstructions to set up and test communications systemFinal update of documentationUpdate and complete restoration plan, store components and documentation

Developing A Project ChecklistThe final project checklist will have many items, all of great importance. Each item needs a full description, where and when it will be needed and who is responsible for it. See Chapter 10 for a recommended project installation checklist. Components like cables and cable plant hardware should indicate vendors, delivery times  and where, when and sometimes how it needs to be delivered. Special installation equipment needs to be scheduled also, with notes of what is needed to be purchased and what will be rented. If the jobsite is not secure and the install will take more than a day, security guards at the jobsite(s) may need to be arranged.

A work plan should be developed that indicates what specialties are going to be needed, where and when. Outside plant installations (OSP) often have one crew pulling cable, especially specialty installs like direct burial, aerial or underwater, another crew splicing and perhaps even another testing. OSP installers often do just part of the job since they need skills and training on specialized equipment like fusion splicers or OTDRs and installation practices like climbing poles or plowing-in cables.

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Inputs from the installation crews can help determine the approximate time needed for each stage of the installation and what might go wrong that can affect the schedule.

And things will go wrong. All personnel working on the project should be briefed on the safety rules and preferably be given a written copy. Supervisors and workers should have contact numbers for the project manager, backup and all other personnel they may need to contact. Since some projects require working outside normal work hours, for example airports or busy government buildings where cabling is often done overnight, having a project manager available – preferably onsite – while the work is being done is very important.

During the installation itself, a knowledgeable person should be onsite to monitor the progress of installation, inspect workmanship, review test data, create daily progress reports and immediately notify the proper management if something looks awry. If the project manager is not technically qualified, having someone available who is technical is important. That person should have the authority to stop work or require fixes if major problems are found.

Licenses, Certifications, Facilities and Power/Ground issuesThis chapter primarily focuses on the unique aspects of fiber optic cable plant design and installation, but this process cannot be done in a

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vacuum. Projects may require the sign-offs of architects or PEs (Professional Engineers) who are licensed in the area and involved in the project. Cable plants may require working with cities or counties for permits or easements, cooperation from other organizations to allow access through their property and construction disruptions. Any communications system requires not only the cable plant but facilities for termination at each end, placing communications equipment, providing power (usually uninterruptible data quality power) and a separate data ground which may require the services of an electrical contractor. Inside the facility, connections must be made to the end users of the link. The installation may require inspections by building and/or electrical inspectors and contractors may require electrical and/or low voltage or fiber optic licenses from local authorities. Customers often require certifications such as the FOA CFOT for contractors and installers to work on a project.

The large number of options involved in almost every project make it impossible to summarize the issues in a few sentences, so let’s just say you must consider the final, complete design to gain cooperation and coordinate the final installation. One of the most valuable assets you can have when designing and installing a fiber optic project is an experienced contractor. 

Developing A Test PlanEvery installation requires confirmation that

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components are installed properly. The installer or contractor wants to ensure the work is done properly so the customer is satisfied and callbacks for repair will not be necessary. Customers generally require test results as well as a final visual inspection as part of the documentation of a proper installation before approving payment. 

In our experience, however, there is often confusion about exactly what should be tested and how documentation of test results is to be done on fiber optic projects. These issues should be agreed upon during the design phase of the project. Project paperwork should include specifications for testing, references to industry standards and acceptable test results based on a loss budget analysis done during the design phase of the project. FOA standards are an easy way to have test methods agreed among all parties.

The process of testing any fiber optic cable plant may require testing three times, testing cable on the reel before installation, testing the insertion loss of each segment as it is installed with an optical loss test set (OLTS, another name for a light source and power meter), perhaps verifying every splice as it is made using an OTDR and finally testing complete end to end  loss of every fiber in the cable plant.  Practical testing usually means testing only a few fibers on each cable reel for continuity before installation to ensure there has been no damage to the cable during shipment. Then each segment is tested as it is spliced and/or terminated by the

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installers. Finally the entire cable run is plugged together and tested for end-to-end loss for final documentation.

One should require visual inspection of cable reels upon acceptance and, if visible damage is detected, testing the cable on the reel for continuity before installing it, to ensure no damage was done in shipment from the manufacturer to the job site. Since the cost of installation usually is high, often much higher than the cost of materials, it only makes sense to ensure that one does not install bad cable, which would then have to be removed and replaced. It is generally sufficient to just test continuity with a fiber tracer or visual fault locator. However, long spools of cable may be tested with an OTDR if damage is suspected and one wants to document the damage or determine if some of the cable needs to be cut off and discarded (or retained to get credit for the damaged materials.)

After cable installation, splicing and termination, each segment of the cable plant should be tested individually as it is installed, to ensure each splice, connector and cable is good. One should never complete splicing cables without verifying the splices are properly done with an OTDR before sealing the splice closure. Finally  each end to end run (from equipment to equipment connected on the cable plant) should be tested for loss as required by all standards. Remember that each fiber in each cable will need to be tested, so the total number of tests to be performed is calculated from the number

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of cable segments times the number of fibers in each cable. This can be a time-consuming process.

Before finishing, it is important to ensure the fibers are documented and arranged properly. When equipment is installed, another parameter becomes important, polarity. Most fiber links use two fibers transmitting in opposite directions, so it's important to check that the transmitters are connected to the receivers which often requires a crossover somewhere in the cable plant. Documentation should show how the fibers are to be connected to equipment. If the contractor installs the communications equipment, it may be necessary to test each data link also.

Required vs. Optional TestingTesting the complete cable plant requires insertion loss testing with a source and power meter or optical loss test set (OLTS) per  standard test procedure. The test plan should specify the “0 dB”  reference method option (one, two or three reference cables) as this will affect the value of the loss. Some standards call for a one cable reference, but this may not be possible with all combinations of test sets and cable plant connectors. The required test methods need to be agreed upon by the contractor and user beforehand. FOA standards provide a simple solution to this problem.

OTDR testing is generally done on outside plant cables, but OTDR testing alone is often not

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acceptable for cable plant certification. Long lengths of outside plant cabling which include splices should be tested with an OTDR to verify splice performance and look for problems caused by stress on the cable during installation. 

While there are advocates of using OTDRs to test any cable plant installation, including short premises cables, it is generally not required by industry standards nor is it appropriate for short links common to premises cabling. The shorter lengths of premises cabling runs and frequent connections with high reflectance often create confusing OTDR traces that cause problems for the OTDR autotest function and are sometimes difficult for even experienced OTDR users to interpret properly. 

Long OSP cables may require special testing for spectral attenuation (S), chromatic dispersion (CD) and polarization mode dispersion (PMD). These are specialized tests required to ensure DWDM and high bit rate systems operate properly.

Coordinating Testing and DocumentationThe Test Plan should be coordinated with the cable plant documentation. The documentation must show what links need testing and what test results are expected based on loss budget calculations. The Test Plan should also specify how the test data are incorporated into the documentation for acceptance of the installation and for reference in case of future cabling problems that require

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emergency restoration.

Planning for Restoration

About once a day in the USA, a fiber optic cable is broken by a contractor digging around the cable, as this photo shows. Premises cables are not as vulnerable, except for damage caused by clumsy personnel or during the removal of abandoned cables. Any network is susceptible to damage so every installation needs a restoration plan.

Efficient fiber optic restoration depends on rapidly finding the problem, knowing how to fix it, having the right parts and getting the job done quickly and efficiently. Like any type of emergency,  planning ahead will minimize the problems encountered.

Documentation for RestorationDocumentation is the most helpful thing you can have when trying to troubleshoot a fiber network, especially during restoration. Start with the manufacturer’s datasheets on every component you

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use: electronics, cables, connectors, hardware like patch panels, splice closures and even mounting hardware. Along with the data, one should have manufacturer’s “help line” contact information, which will be of immense value during restoration.

During installation, mark every fiber in every cable at every connection and keep records using cable plant documentation software or a simple spreadsheet of where every fiber goes. When tested, add loss data taken with an optical loss test set (OLTS) and optical time domain reflectometer (OTDR) data when available. Someone must be in charge of this data, including keeping it up to date if anything changes.

Equipment For Restoration

Testing and TroubleshootingYou must have available proper test equipment to troubleshoot and restore a cable plant. An OLTS should also have a power meter to test the power of the signals to determine if the problem is in the electronics or cable plant. Total failure of all fibers in the cable plant means a break or cut in the cable. For premises cables, finding the location is  often simple if you have a visual fault locator or VFL, which is a bright red laser coupled into the optical fiber that allows testing continuity, tracing fibers or finding bad connectors at patch panels.

For longer cables, an OTDR will be useful.  Outside plant networks should use the OTDR to document

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the cable plant during installation, so during restoration a simple comparison of installation data with current traces will usually find problems. OTDRs can also find non-catastrophic problems, for example when a cable is kinked or stressed, so it only has higher loss, which can also cause network problems.

Tools and Components Once you find the problem, you have to repair it. Repair requires having the right tools, supplies and trained personnel available. Besides the test equipment needed for troubleshooting, you need tools for splicing and termination, which may include a fusion splicer for outside plant cables. You also need components necessary for restoring the cable plant. 

For every installation, a reasonable amount of excess cable and installation hardware should be set aside in storage for restoration. Some users store the restoration supplies along with documentation in a sealed container ready for use. Remember that the fiber optic patchcords that connect the electronics to the cable plant can be damaged also, but are not considered repairable. Just keep replacements available.

One big problem in restoring damaged cables is pulling the two cable ends close enough to allow splicing them together. You need several meters of cable on each end to strip the cable, splice the fibers and place them in a splice closure. Designing the cable plant with local service loops is

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recommended. If the cable ends are too short , or the damaged cable is underground or buried, you will have to splice in a new section of cable. Since the restoration cable must match the damaged cable or at least have a greater nuber of fibers, the best source of cable for restoration is cable leftover from the original installation. Manufacturers also can supply cable restoration kits that include cable and splice closures.

What else besides cables and cable plant hardware should be in a restoration kit? You should have a termination or mechanical splice kit and proper supplies. For splices, you need splice closures with adequate space for a number of splices equal to the fiber count in the cable.  All these should be placed in a clearly marked box with a copy of the cable plant documentation and stored in a safe place where those who will eventually need it can find it fast. 

Preparing PersonnelPersonnel must be properly trained to use this equipment and do the troubleshooting and restoration. And, of course, they must be available on a moments notice. The biggest delay in restoring a fiber optic communications link is often the chaos that ensues while personnel figure out what to do. Having a plan that is known to the responsible personnel is the most important issue.

Major users of fiber optics have restoration plans in place, personnel trained and kits of supplies ready for use. It’s doubtful that most premises users are

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ready for such contingencies. Users may find that the cost of owning all this expensive equipment is not economic. It may be preferable to keep an inexpensive test set consisting of a VFL and OLTS at each end of the link and having an experienced contractor on call for restoration.

Managing A Fiber Optic Project

Managing a fiber optic project can be easiest part of the installation if the design and planning have been done thoroughly and completely, or, if not, the hardest. But even assuming everything has been done right, things will still probably go wrong, so planning for the unexpected is also very important. Here are some guidelines for managing the project that can minimize the problems and help in their speedy solution.

On Site Management and SupervisionFirst, someone has to be in charge, and everyone involved must know they are the boss, including them. During the project, they must be readily available for consultation and updates. While this may sound obvious, sometimes the network user’s representative has other responsibilities (like managing an IT department) and may not be able or willing to direct full attention to the project. Whoever is assigned the task of managing the project must be involved and available, preferably on the job site, full time. If necessary, delegate responsibility to the contracting construction supervisor with requirements for daily reports and

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personal updates.

Make certain that everyone responsible for parts of the project have appropriate documentation and have reviewed the installation plan. Everyone should have toured the relevant job sites and be familiar with locations. They must also know who to contact about questions on the sites, within the network user, the contractor and any outside organizations such as local governments or utilities. Everyone needs to have contact information for each other (cell phones usually, since email may be too slow and instant messaging will probably not be available to field workers.) The onsite supervisor should have a digital camera and take plenty of photos of the installation to be filed with the documentation for future reference and restoration.

Locations of components, tools and supplies should be known to all personnel. On larger jobs, managing equipment and materials may be a full time job. Special equipment, like splicing trailers or bucket trucks, should be scheduled as needed. Rental equipment should be double checked with the suppliers to ensure delivery to the job site on time. Contacts for vendor technical support should be noted on documentation for the inevitable questions arising during installation.

Contacts with Local AuthoritiesOutside plant installs may require local authorities to provide personnel for supervision or police for protection or traffic management on public job sites, so they must also become involved in the

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scheduling. If job inspections are required, arrangements should be made so that the job interruptions for inspections are minimized. Supervisory personnel must be responsible for job site safety and have appropriate contact information, including for public services like police, fire and ambulance.

If the project is large enough to last several days or more, daily meetings to review the day’s progress are advisable. At a minimum, it should involve the onsite construction supervisor and the network user’s person in charge of the project. As long as things are going well, such a meeting should be short. On longer projects, overnight security personnel at job sites should have contact information for the job manager who must be available 24/7 as well as public service contacts.

Continuous Inspection, Testing and CorrectionsInspection and testing of the installed cable plant should not be left until after the job is completed. Testing continually during installation can find and fix problems such as cable stresses or high termination losses before those problems become widespread.  Each installer doing testing should have documentation with loss budget calculations and acceptable losses to use for evaluating the test results. Installers should be double-checking each other’s work to ensure quality.

What do you do when (not if) things go wrong? Here judgment calls are important. When something happens, obviously it is the responsibility

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of the onsite supervisor to decide quickly if they can take care of it. If not, they must know who needs to be brought in and who needs to be notified. By reviewing progress regularly, disruptions can be minimized. Equipment failures, e.g. a fusion splicer, can slow progress, but other parts of the project like cable laying can continue, with splicing resumed as soon as replacement equipment is available. Problems with termination should be reviewed by an installer with lots of experience and the cure may require new supplies or turning termination over to more experienced personnel. Never hesitate to call vendor support when these kinds of questions or problems arise.

Following the completion of the install, all relevant personnel should meet, review the project results, update the documentation and decide if anything else needs to be done before closing the project.

References and TrainingReferences for the fiber optic designer’s bookshelf include the FOA texts,The FOA Online Reference Guide to Fiber Optics , printed textbooks or eBooks The FOA Reference Guide to Outside Plant Fiber Optics, The FOA Reference Guide to Fiber Optics, and The FOA Reference Guide to Premises Cabling. Reference standards include the FOA Standards and NECA/FOA-301 installation standard which are available free. When it comes to the NEC, Limited Energy Systems, a textbook published by the NFPA, is very useful. There are dozens of books on communications system

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design, but unfortunately, the fast pace of development in communications technologies means that many textbooks are hopelessly out of date unless it’s updated frequently. Better to rely on the web, especially the FOA Reference Guide site and websites of well-established manufacturers.

Getting trained specifically in fiber optic network design is available through some FOA-Approved schools. The material is covered in part in some advanced fiber optic courses offered by the FOA-approved schools and by large manufacturers who help you understand how to build networks using their products. The FOA has a fiber optic design certification (CFOS/D.) 

Additional reading on specific areas of cabling network design: Premises Cabling Design, Outside Plant (OSP) Fiber Optic Network Design

AAbsorption: That portion of fiber optic attenuation resulting of conversion of optical power to heat.Analog: Signals that are continually changing, as opposed to being digitally encoded.Attenuation Coefficient: Characteristic of the attenuation of an optical fiber per unit length, in dB/km.Attenuation: The reduction in optical power as it passes along a fiber, usually expressed in decibels (dB). See Loss, optical.Attenuator: A device that reduces signal power in a fiber optic link by inducing loss.Average power: The average over time of a modulated signal. B

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Back reflection, reflectance, optical return loss: Light reflected from the cleaved or polished end of a fiber caused by the difference of refractive indices of air and glass. Typically 4% of the incident light. Expressed in dB relative to incident power.Backscattering: The scattering of light in a fiber back toward the source, used to make OTDR measurements.Bandwidth: The range of signal frequencies or bit rate within which a fiber optic component, link or network will operate.Bending loss, microbending loss: Loss in fiber caused by stress on the fiber bent around a restrictive radius.Bit-error rate (BER): The fraction of data bits transmitted that are received in error.Bit: An electrical or optical pulse that carries information.Buffer: A protective coating applied directly on the fiber.CCable: One or more fibers enclosed in protective coverings and in some cable constructions, strength members, stiffeners, water blocking compounds or other components.Cable Plant, Fiber Optic: The combination of fiber optic cable sections, connectors and splices forming the optical path between two terminal devices.CATV: An abbreviation for Community Antenna Television or cable TV.Chromatic dispersion: The temporal spreading of a pulse in an optical waveguide caused by the wavelength dependence of the velocities of light.Cladding: The lower refractive index optical coating over the core of the fiber that "traps" light into the core. Connector: A device that provides for a demountable connection between two fibers or a fiber and an active device and provides protection for the fiber.

Connector: A device which terminates an optical fiber and allows temporary joining of fibers with like terminations.Core: The center of the optical fiber through which light is transmitted.Coupler: An optical device that splits or combines light from more than one fiber.Cutback method: A technique for measuring the loss of bare fiber by measuring the optical power transmitted through a long length then cutting back to the source and measuring the initial coupled power.Cutoff wavelength: The wavelength beyond which singlemode fiber only supports one mode of propagation.D

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dBm: Optical power referenced to 1 milliwatt.Decibel (dB): A unit of measurement of optical power which indicates relative power on a logarithmic scale, sometimes called dBr. dB=10 log ( power ratio)Detector: A photodiode that converts optical signals to electrical signals.Digital: Signals encoded into discrete bits.Dispersion: The temporal spreading of a pulse in an optical waveguide. May be caused by modal or chromatic effects.EEDFA: Erbium-doped fiber amplifier, an all optical amplifier for 1550 nm SM transmissionsystems.Edge-emitting diode (E-LED): A LED that emits from the edge of the semiconductor chip, producing higher power and narrower spectral width.End finish: The quality of the end surface of a fiber prepared for splicing or terminated in a connector.Equilibrium modal distribution (EMD): Steady state modal distribution in multimode fiber, achieved some distance from the source, where the relative power in the modes becomes stable with increasing distance.ESCON: IBM standard for connecting peripherals to a computer over fiber optics. Acronym for Enterprise System Connection.Excess loss: The amount of light lost in a coupler, beyond that inherent in the splitting to multiple output fibers.FFiber: see Optical Fiber.Fiber Amplifier: an all optical amplifier using erbium or other doped fibers and pump lasers to increase signal output power without electronic conversion.Fiber Distributed Data Interface, FDDI: 100 Mb/s ring architecture data network.Ferrule: A precision tube which holds a fiber for alignment for interconnection or termination. A ferrule may be part of a connector or mechanical splice.Fiber tracer: An instrument that couples visible light into the fiber to allow visual checking of continuity and tracing for correct connections.Fiber identifier: A device that clamps onto a fiber and couples light from the fiber by bending, to identify the fiber and detect high speed traffic of an operating link or a 2 kHz tone injected by a test source.Fiber optics: Light transmission through flexible transmissive fibers for communications or lighting.FO: Common abbreviation for "fiber optic."

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Fresnel reflection, reflection, back reflection, optical return loss: Light reflected from the cleaved or polished end of a fiber caused by the difference of refractive indices of air and glass. Typically 4% of the incident light.FTTH: fiber to the homeFusion splicer: An instrument that splices fibers by fusing or welding them, typically by electrical arc.GGraded index (GI): A type of multimode fiber which used a graded profile of refractive index in the core material to correct for dispersion.IIndex of refraction: A measure of the speed of light in a material.Index matching fluid: A liquid used of refractive index similar to glass used to match the materials at the ends of two fibers to reduce loss and back reflection.Index profile: The refractive index of a fiber as a function of cross section.Insertion loss: The loss caused by the insertion of a component such as a splice or connector in an optical fiber.JJacket: The protective outer coating of the cable.Jumper cable: A short single fiber cable with connectors on both ends used for interconnecting other cables or testing.LLaser diode, ILD: A semiconductor device that emits high powered, coherent light when stimulated by an electrical current. Used in transmitters for singlemode fiber links.Launch cable: A known good fiber optic jumper cable attached to a source and calibrated for output power used used as a reference cable for loss testing. This cable must be made of fiber and connectors of a matching type to the cables to be tested.Light-emitting diode, LED: A semiconductor device that emits light when stimulated by an electrical current. Used in transmitters for multimode fiber links.Link, fiber optic: A combination of transmitter, receiver and fiber optic cable connecting them capable of transmitting data. May be analog or digital.Long wavelength: A commonly used term for light in the 1300 and 1550 nm ranges.Loss,optical: The amount of optical power lost as light is transmitted through fiber, splices, couplers, etc.Loss budget: The amount of power lost in the link. Often used in terms of the maximum amount of loss that can be tolerated by a given link.

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MMargin: The additional amount of loss that can be tolerated in a link.Mechanical splice: A semi-permanent connection between two fibers made with an alignment device and index matching fluid or adhesive.Micron (*m): A unit of measure, 10-6 m, used to measure wavelength of light.Microscope, fiber optic inspection: A microscope used to inspect the end surface of a connector for flaws or contamination or a fiber for cleave quality.Modal dispersion: The temporal spreading of a pulse in an optical waveguide caused by modal effects.Mode field diameter: A measure of the core size in singlemode fiber.Mode filter: A device that removes optical power in higher order modes in fiber.Mode scrambler: A device that mixes optical power in fiber to achieve equal power distribution in all modes. Mode stripper: A device that removes light in the cladding of an optical fiber.Mode: A single electromagnetic field pattern that travels in fiber.Multimode fiber: A fiber with core diameter much larger than the wavelength of light transmitted that allows many modes of light to propagate. Commonly used with LED sources for lower speed, short distance links.NNanometer (nm): A unit of measure , 10-9 m, used to measure the wavelength of light.Network: A system of cables, hardware and equipment used for communications.Numerical aperture (NA): A measure of the light acceptance angle of the fiber.OOptical amplifier: A device that amplifies light without converting it to an electrical signal. Optical fiber: An optical waveguide, comprised of a light carrying core and cladding which traps light in the core.Optical loss test set (OLTS): An measurement instrument for optical loss that includes both a meter and source.Optical power: The amount of radiant energy per unit time, expressed in linear units of Watts or on a logarithmic scale, in dBm (where 0 dB = 1 mW) or dB* (where 0 dB*=1 microWatt).Optical return loss, back reflection: Light reflected from the cleaved or polished end of a fiber caused by the difference of refractive indices of air and glass. Typically 4% of the incident light. Expressed in dB relative to incident power.Optical switch: A device that routes an optical signal from one or

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more input ports to one or more output ports.Optical time domain reflectometer (OTDR): An instruments that used backscattered light to find faults in optical fiber and infer loss.Overfilled launch: A condition for launching light into the fiber where the incoming light has a spot size and NA larger than accepted by the fiber, filling all modes in the fiber.PPhotodiode: A semiconductor that converts light to an electrical signal, used in fiber optic receivers.Pigtail: A short length of fiber attached to a fiber optic component such as a laser or coupler.Plastic optical fiber (POF): An optical fiber made of plastic.Plastic-clad silica (PCS) fiber: A fiber made with a glass core and plastic cladding.POF: plastic optical fiber, optical fiber made from polymer materials.Power budget: The difference (in dB) between the transmitted optical power (in dBm) and the receiver sensitivity (in dBm).Power meter, fiber optic: An instrument that measures optical power emanating form the end of a fiber.Preform: The large diameter glass rod from which fiber is drawn.RReceive cable: A known good fiber optic jumper cable attached to a power meter used as a reference cable for loss testing. This cable must be made of fiber and connectors of a matching type to the cables to be tested.Receiver: A device containing a photodiode and signal conditioning circuitry that converts light to an electrical signal in fiber optic links.Reference cable: A known good fiber optic jumper cable attached to a light source or power meter used as a reference cable for loss testing.Reflectance: Light reflected from the cleaved or polished end of a fiber caused by the difference of refractive indices of air and glass.Refractive index: A property of optical materials that relates to the velocity of light in the material.Repeater, regenerator: A device that receives a fiber optic signal and regenerates it for retransmission, used in very long fiber optic links.SScattering: The change of direction of light after striking small particles that causes loss in optical fibers.Short wavelength: A commonly used term for light in the 665, 790, and 850 nm ranges.Singlemode fiber: A fiber with a small core, only a few times the wavelength of light transmitted, that only allows one mode of light to propagate. Commonly used with laser sources for high speed, long

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distance links.Source: A laser diode or LED used to inject an optical signal into fiber.Splice (fusion or mechanical): A device that provides for a connection between two fibers, typically intended to be permanent. Splitting ratio: The distribution of power among the output fibers of a coupler.Steady state modal distribution: Equilibrium modal distribution (EMD) in multimode fiber, achieved some distance from the source, where the relative power in the modes becomes stable with increasing distance.Step index fiber: A multimode fiber where the core is all the same index of refraction.Surface emitter LED: A LED that emits light perpendicular to the semiconductor chip. Most LEDs used in datacommunications are surface emitters.TTalkset, fiber optic: A communication device that allows conversation over unused fibers.Termination: Preparation of the end of a fiber to allow connection to another fiber or an active device, sometimes also called "connectorization".Test cable: A short single fiber jumper cable with connectors on both ends used for testing. This cable must be made of fiber and connectors of a matching type to the cables to be tested.Test kit: A kit of fiber optic instruments, typically including a power meter, source and test accessories used for measuring loss and power.Test source: A laser diode or LED used to inject an optical signal into fiber for testing loss of the fiber or other components.Total internal reflection: Confinement of light into the core of a fiber by the reflection off the core-cladding boundary.Transmitter: A device which includes a LED or laser source and signal conditioning electronics that is used to inject a signal into fiber.VVCSEL: vertical cavity surface emitting laser, a type of laser that emits light vertically out of the chip, not out the edge, widely used in fast multimode networks.Visual fault locator: A device that couples visible light into the fiber to allow visual tracing and testing of continuity. Some are bright enough to allow finding breaks in fiber through the cable jacket.WWatts: A linear measure of optical power, usually expressed in milliwatts (mW), microwatts (*W) or nanowatts (nW).

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Wavelength: A measure of the color of light, usually expressed in nanometers (nm) or microns (*m).Wavelength division multiplexing (WDM): A technique of sending signals of several different wavelengths of light into the fiber simultaneously.Working margin: The difference (in dB) between the power budget and the loss budget (i.e. the excess power margin).