Sam_CN_UNIT- Ib 1 1/14/2014 THE PHYSICAL LAYER Transmission (physical ) media THE PHYSICAL LAYER...

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THE PHYSICAL LAYERTHE PHYSICAL LAYER

Transmission (physical ) mediaTransmission (physical ) media For the transmission of bit stream from one

machine to another, various physical media can be used

They differ in terms of bandwidth, delay, cost, easy of installation and maintenance


Transmission (physical) mediaTransmission (physical) media

Transmission media can be divided into 2 types, Transmission media can be divided into 2 types, guided media and unguided media guided media and unguided media

Guided media – Magnetic media, Twisted Pair Guided media – Magnetic media, Twisted Pair (copper wire), Coaxial Cable and fiber optics (copper wire), Coaxial Cable and fiber optics

Unguided media - radio, Microwave, Infrared Unguided media - radio, Microwave, Infrared and light wave transmissionand light wave transmission


Guided transmission mediaGuided transmission mediaMagnetic mediaMagnetic media

One of the most common ways to transport data One of the most common ways to transport data from one computer to another is to write them from one computer to another is to write them onto magnetic tapes or floppy disks, physically onto magnetic tapes or floppy disks, physically transport the tapes or disks to the destination transport the tapes or disks to the destination machine and read them back in againmachine and read them back in again


Guided Media: Twisted PairGuided Media: Twisted Pair

Twisted pair is the oldest and still most common Twisted pair is the oldest and still most common transmission medium used in telephone system transmission medium used in telephone system

It consists of two insulated copper wires, typically It consists of two insulated copper wires, typically about 1 mm thick. The wires are twisted together to about 1 mm thick. The wires are twisted together to reduce electrical interference from similar pairs reduce electrical interference from similar pairs close by close by

Twisted pairs can run several km without Twisted pairs can run several km without amplification, but for longer distances repeaters are amplification, but for longer distances repeaters are needed needed

Twisted pairs can be used for either analog or Twisted pairs can be used for either analog or digital transmission. The bandwidth depends on the digital transmission. The bandwidth depends on the thickness of the wire and the distance traveled thickness of the wire and the distance traveled (several mbps for a few km can be achieved) (several mbps for a few km can be achieved)


Twisted pair cabling comes in several varieties, Twisted pair cabling comes in several varieties,

two of which are important for computer two of which are important for computer

networks: networks:

Category 3 twisted pairsCategory 3 twisted pairs - gently twisted, 4 pairs - gently twisted, 4 pairs

typically grouped together in a plastic sheath typically grouped together in a plastic sheath

Category 5 twisted pairsCategory 5 twisted pairs - introduced in 1988. - introduced in 1988.

More twists per cm than category 3 and teflon More twists per cm than category 3 and teflon

insulation, which results in less crosstalk and insulation, which results in less crosstalk and

better quality signal over longer distances better quality signal over longer distances

(a) Category 3 TP(b) Category 5 TP


Guided Media: Coaxial CableGuided Media: Coaxial Cable

Outer conductor shields the inner conductor from picking up stray signal from the air

High bandwidth (1 to 2 Gbps) but lossy channel

Repeater is used to regenerate the weakened signals

Two kinds of coaxial cables are widely used:

Baseband(50-ohm) - used for digital transmissions

Broadband(75-ohms) - used for analog transmissions

CategoryCategory ImpedanceImpedance UseUse

RG-59RG-59 75 75 Cable TVCable TV

RG-58RG-58 50 50 Thin Thin EthernetEthernet

RG-11RG-11 50 50 Thick Thick EthernetEthernet

Coaxial cable or coax is a copper-cored cable surrounded by a heavy shielding and is used to connect computers


Guided media: Fiber OpticsGuided media: Fiber Optics Optical fibers use light to send information

through the optical medium. It uses the principal of total internal reflection Modulated light transmissions are used to

transmit the signal


Fiber Optic NetworksFiber Optic Networks Fiber optics can be used for LANs as well as for Fiber optics can be used for LANs as well as for

long-haul transmission long-haul transmission

Tapping onto it is more complex than connecting Tapping onto it is more complex than connecting

to a copper wire to a copper wire

Figure: A fiber optic ring with active repeatersFigure: A fiber optic ring with active repeaters


Guided media: Fiber Optics (cont’d)Guided media: Fiber Optics (cont’d)

Light travels through the optical media by the Light travels through the optical media by the way of total internal reflection way of total internal reflection

Modulation scheme used is intensity modulationModulation scheme used is intensity modulation Two types of Fiber media :Single and MultimodeTwo types of Fiber media :Single and Multimode Multimode Fiber can support less bandwidth Multimode Fiber can support less bandwidth

than Single mode Fiberthan Single mode Fiber Single mode Fiber has a very small core and Single mode Fiber has a very small core and

carry only one beam of light. It can support Gbps carry only one beam of light. It can support Gbps data rates over > 100 Km without using data rates over > 100 Km without using repeatersrepeaters


Single and Multimode FiberSingle and Multimode Fiber

Single-mode fiberSingle-mode fiberCarries light pulses Carries light pulses

along single pathalong single pathUses Laser Light Uses Laser Light

SourceSourceMultimode fiberMultimode fiber

Many pulses of Many pulses of light generated by light generated by LED travel at LED travel at different anglesdifferent angles


Comparison of Fiber Optics and Copper wireComparison of Fiber Optics and Copper wire

Advantages of fibers: Advantages of fibers: much higher bandwidth low attenuation (30 km distance of repeaters vs. 5 km

for copper) noise-resistance not affected by corrosive chemicals much lighter than copper - easier installation and

maintenance difficult to tap - higher security Disadvantages of fiber:Disadvantages of fiber: unfamiliar technology so far unidirectional communication more expensive interfaces than electrical ones


Unguided (Wireless) Transmission media

Provides mobility to communication nodes

Right of way and cable laying costs can be reduced

Susceptible to rain, atmospheric variations and objects in transmission path

Very useful in difficult terrain where cable laying is not possible


Unguided (Wireless) Transmission media

Indoor : 10 – 50m : BlueTooth, WLAN Short range Outdoor : 50 – 200m: WLAN Mid Range Outdoor : 200m – 5 Km : GSM,

CDMA, WLAN Point-to-Point, Wi-Max Long Range Outdoor : 5 Km – 100 Km :

Microwave Point-to-Point Long Distance Communication : Across

Continents : Satellite Communication


Unguided (Wireless) Transmission mediaThe Electromagnetic Spectrum: The amount of information that an electromagnetic wave can carry is related to its bandwidth. With current technology, it is possible to encode a few bits per hertz at low frequencies


Unguided media: The Electromagnetic Spectrum

BandBand RangeRangePropagatioPropagatio

nnApplicationApplication

VLFVLF 3–30 KHz3–30 KHz GroundGround Long-range radio navigationLong-range radio navigation

LFLF 30–300 KHz30–300 KHz GroundGroundRadio beacons andRadio beacons and

navigational locatorsnavigational locators

MFMF 300 KHz–3 MHz300 KHz–3 MHz SkySky AM radioAM radio

HF HF 3–30 MHz3–30 MHz SkySkyCitizens band (CB),Citizens band (CB),

ship/aircraft communicationship/aircraft communication

VHF VHF 30–300 MHz30–300 MHzSky andSky and

line-of-sightline-of-sightVHF TV, VHF TV, FM radioFM radio

UHF UHF 300 MHz–3 GHz300 MHz–3 GHzLine-of-Line-of-

sightsightUHF TV, cellular phones, UHF TV, cellular phones,

paging, satellitepaging, satellite

SHF SHF 3–30 GHz3–30 GHzLine-of-Line-of-

sightsightSatellite communicationSatellite communication

EHFEHF 30–300 GHz30–300 GHzLine-of-Line-of-

sightsightLong-range radio navigationLong-range radio navigation


Unguided media: Radio wave transmission

Radio waves are easy to generate, can travel long distance, and penetrate buildings easily, so they are widely used for communication, both indoors and outdoors.

Radio waves are also omnidirectional, meaning that they travel in all directions from the source, so that the transmitter and receiver do not have to be carefully aligned physically



In the VLF, LF, and MF bands, radio waves follow the curvature of the earth



In the HF they bounce off the ionosphere


Unguided media: Microwave transmission

Above 100 MHz, the waves travel in straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam using a parabolic antenna gives a much higher signal to noise ratio

Since the microwaves travel in a straight line, if the towers are too far apart, the earth will get in the way. Consequently, repeaters are needed periodically


Unguided media: Microwave transmission

Disadvantages: do not pass through buildings well multipath fading problem (the delayed waves

cancel the signal) absorption by rain above 8 GHz severe shortage of spectrumAdvantages: no right way is needed (compared to wired

media) relatively inexpensive simple to install


Unguided media: Infrared and Millimeter Waves

Unguided infrared and millimeter waves are widely used for short-range communication.

The remote controls used on televisions, VCRs, and stereos all use infrared communication

They are relatively directional, cheap, and easy to build, but have a major drawback: they do not pass through solid objects

This property is also a plus. It means that an infrared system in one room will not interfere with a similar system in adjacent room. It is more secure against eavesdropping


Unguided media: Light wave transmission

Affected by fog or rain

A modern application is to connect LANs in two buildings via lasers mounted on their rooftops It offers very high BW and very low cost


Switching: Circuit switching Two different switching techniques are used inside

the telephone system: circuit switching and packet switching

When a user place a telephone call, the switching equipment within the telephone system seeks out a physical "copper" (including fiber and radio) path from the caller telephone to the callee telephone. This technique is called circuit switching (Fig.(a)).

An important property of circuit switching is the need to set up an end-to-end path before any data can be sent.


Switching: Circuit switching (cont’d)

It takes some set-up time during which there is no data transmission in progress. Long set-up times are for many computer applications undesirable


Message switchingMessage switching

An alternative switching strategy is message switching (Fig (b)).It was first used for telegram

In this case, no physical copper path is established in advance. Instead, the store-and-forward technique for the entire messages is applied.

With message switching, there is no limit on block size, which means that routers must have disks to buffer long blocks. It also means that a single block may tie up a router-router line for minutes, rendering message switching useless for interactive traffic


Packet switching

To get around these problems of circuit switching, packet switching was invented

Packet-switching networks place a tight upper limit on block size


Packet switching (cont’d)

So no user can monopolize any transmission line very long and therefore these networks are well suited to handle interactive traffic.

A further advantage of packet switching over message switching is (Fig.(c)) that the first packet of a multipacket message can be forwarded before the second has fully arrived, reducing delay and improving throughput.

For these reasons, computer networks are usually packet switched, occasionally circuit switched, but never message switched


(a) Circuit switching (b) Message switching (c) Packet switching


A comparison of circuit switched and packet-switched networks


Encoding techniques: Line codingEncoding techniques: Line coding

It is the process of converting binary data to a digital signal

0 1 0 1 1 1 00 1 0 1 1 1 0

The number of values allowed in a particular signal as the number of signal values

The number of values used to represent data as the number of data levels

Linecoding


0 0001 1 1 1

0 0001 1 0 1

Two signal levels , two data levels

Three signal levels, Two data levels


DC ComponentDC Component

Some line coding schemes leave a residual direct-current (dc) component (zero- frequency)

This component is undesirable for 2 reasons:

1.If the signal is to pass through a system that does not allow the passage of a dc component, the signal is distorted and may create errors in the output.


2.This component is extra residing on the line and is useless

0 0001 1 1 1

0 0001 1 1 1

Signal with dc component

Signal without dc component


DC ComponentDC Component

The first signal has a dc component; the positive The first signal has a dc component; the positive voltages are not cancelled by the –ve voltagesvoltages are not cancelled by the –ve voltages

The second has no dc component; the +ve The second has no dc component; the +ve voltages are canceled by any –ve voltagesvoltages are canceled by any –ve voltages

The first does not pass through a transformer The first does not pass through a transformer properly, the secondproperly, the second does


Line coding schemes

Line coding

Unipolar Polar Bipolar

Line coding

NRZ RZ Manchester DifferentialManchester


Unipolar: uses only one voltage level

0 0001 1 1 1

Polar:Uses 2 voltage levels. one positive and one negative


POLAR: NRZ ( Non-Return-to-Zero) Codes 2 popular forms are used, NRZ-L and NRZ-I Uses two different voltage levels (one positive

and one negative) as the signal elements for the two binary digits.

1.NRZ-L(NRZ-Level) +ve voltage – 0 -ve voltage – 1

The voltage is constant during the bit interval NRZ-L is used for short distances between

terminal and modem or terminal and computer


POLAR :NRZ- I (Non-Return-to-Zero-Invert on Non-Return-to-Zero-Invert on ones)ones)

2.NRZ-I:It is the transition between +ve and –ve voltage.

Signal change each time when a 1 is encountered.

0 bit is represented by no change.


Manchester encodingManchester encoding

Uses an inversion at the middle of each bit interval for both synchronization and bit representation

A negative-to-positive transition represents a binary 1

A positive-to-negative transition represents a binary 0

Used in 802.3 baseband coaxial cable and CSMA/CD twisted pair


Differential Manchester EncodingDifferential Manchester Encoding

The inversion at the middle of the bit interval is used for synchronization, but the presence or absence of an additional transition at the beginning of the interval is used to identify the bit

Transition means 0 No transition means binary 1 This coding is the opposite convention from

NRZ-I Used in 802.5 (token ring) with twisted pair


0 0001 1 1 1

0 0001 1 1 1

MANCHESTER

DIFFERENTIALMANCHESTER


Bipolar encodingBipolar encoding

Uses three levels: +ve, zero and –ve. Zero level – 0 1s are represented by alternating positive and

negative voltages even if the 1s are not consecutive

A common bipolar encoding scheme is called bipolar alternate mark inversion (AMI)

Mark – 1. neutral zero voltage – 0


1 0 1 0 1 1 0 01UnipolarNRZ

NRZ-Inverted(DifferentialEncoding)

BipolarEncoding

DifferentialManchesterEncoding

Polar NRZ

ManchesterEncoding


Integrated Services Digital Network Integrated Services Digital Network (ISDN)(ISDN)

ISDN stands for Integrated Services Digital Network

Its primary goal is to integrate the voice and non-voice services

The key ISDN service will continue to be voice but with many enhanced features


ISDN servicesISDN services

Telephones with multiple buttons for instant call setup to arbitrary telephones anywhere in the world

displaying the caller's telephone number, name and address while ringing

connecting the telephone to a computer enabling the caller's database record to be displayed on the screen as the call comes in

call forwarding conference calls worldwide on line medical, burglar, and smoke alarms

giving the address to speed up response


ISDN ArchitectureISDN Architecture


ISDN Architecture (cont’d)ISDN Architecture (cont’d) The key idea behind ISDN is that of the digital bit

pipe between the customer and the carrier through which bits flow in both directions.

Whether the bits originate from a digital telephone, a digital terminal, a digital facsimile machine, or some other device is irrelevant.

The digital bit pipe can support multiple independent channels by time division multiplexing of the bit stream. Two principal standards for the bit pipe have been developed:

a low bandwidth standard for home use, and a higher bandwidth standard for business use that

supports multiple channels identical to the home use channels


ISDN Architecture (cont’d)ISDN Architecture (cont’d)

Normal configuration for a home consists of a Normal configuration for a home consists of a network terminating device NT1 (Fig. (a)) placed network terminating device NT1 (Fig. (a)) placed on the customer's premises and connected to on the customer's premises and connected to the ISDN exchange in the carrier's office using the ISDN exchange in the carrier's office using the twisted pair previously used to connect the the twisted pair previously used to connect the telephone.telephone.

The NT1 box has a connector into which a bus The NT1 box has a connector into which a bus cable can be inserted. Up to 8 ISDN telephones, cable can be inserted. Up to 8 ISDN telephones, terminals, alarms, and other devices can be terminals, alarms, and other devices can be connected to the cable.connected to the cable.

From the customer's point of view, the network From the customer's point of view, the network boundary is the connector on NT1boundary is the connector on NT1


ISDN Architecture (cont’d)ISDN Architecture (cont’d) For large businesses, the model of Fig. (b) is used.

There is a device NT2 called PBX (Private Branch eXchange - conceptually the same as an ISDN switch) there connected to NT1 and providing the interface for ISDN devices.

CCITT defined four reference points (Fig.): U reference point = connection between the ISDN

exchange and NT1, T reference point = connector on NT1 to the

customer, S reference point = interface between the ISDN PBX

and the ISDN terminal, R reference point = the connection between the

terminal adapter and non-ISDN terminal.


The ISDN InterfaceThe ISDN Interface

The ISDN bit pipe supports multiple channels The ISDN bit pipe supports multiple channels interleaved by time division multiplexing. Several interleaved by time division multiplexing. Several channel types have been standardized: channel types have been standardized:

A - 4 kHz analog telephone channel A - 4 kHz analog telephone channel B - 64 kbps digital PCM channel for voice or data B - 64 kbps digital PCM channel for voice or data C - 8 or 16 kbps digital channel for out-of-C - 8 or 16 kbps digital channel for out-of- band signaling D - 16 kbps digitalband signaling D - 16 kbps digital channel for out-of-band signaling channel for out-of-band signaling E - 64 kbps digital channel for internal E - 64 kbps digital channel for internal ISDN signaling ISDN signaling H - 384, 1536, or 1920 kbps digital channel. H - 384, 1536, or 1920 kbps digital channel.


The ISDN Interface (cont’d)The ISDN Interface (cont’d) Three combinations have been standardized so far: Basic rate: 2B + 1D. It should be viewed as a

replacement for POTS (Plain Old Telephone Service). Each of the 64 kbps B channels can handle a single PCM voice channel with 8 bits samples made 8000 times per second. D channel is for signaling (i.e., to inform the local ISDN exchange of the address of the destination). The separate channel for signaling results in a significantly faster setup time.

Primary rate: 23B + 1D (US and Japan) or 30B + 1D (Europe). It is intended for use at the T reference point for businesses with a PBX.

Hybrid: 1A + 1C


The ISDN Interface (cont’d)The ISDN Interface (cont’d)

Because ISDN is so focused on 64 kbps channels, it is referred to as N-ISDN (Narrowband ISDN), in contrast to broadband ISDN (ATM)

Fig. (a) Basic rate digital pipe. (b) Primary rate digital pipe.


Broadband ISDN and ATMBroadband ISDN and ATM

The N-ISDN was not going to solve the actual communication problems, it tried to think of a new service

The result was broadband ISDN (B-ISDN), basically a digital virtual circuit for moving fixed-sized packets (cells) at 155 Mbps

Broadband ISDN is based on ATM technology that is fundamentally a packet-switching technology


Virtual Circuits versus Circuit Switching

The basic broadband ISDN service is a compromise between pure circuit switching and pure packet switching

The actual service offered is connection oriented, but it is implemented internally with packet switching

Two kinds of connections are offered: Permanent virtual circuits - ordered by

customers at carriers, remain in place for long time

Switched virtual circuits - set up dynamically like telephone calls


Virtual Circuits versus Circuit Switching

The advantage of permanent over a switched virtual circuit is that there is no setup time, packets along permanent circuit can move instantly

In a virtual circuit network, like ATM, when a circuit is established, what really happens is that route is chosen from source to destination, and all the switches (i.e., routers) along the way make table entries so that they can route any packet on that virtual circuit (Fig.). When a packet comes along, the switch inspects the packet header to find out which virtual circuit it belongs to. Then it looks up that virtual circuit in its tables to determine which communication line to send on


Virtual circuitVirtual circuit

Fig. The dotted line shows a virtual circuit. It is simply defined by table entries inside the switches.


Transmission in ATM NetworksTransmission in ATM Networks

ATM stands for Asynchronous Transfer Mode. This mode ATM stands for Asynchronous Transfer Mode. This mode

can be contrasted with the synchronous T1 carrier (Fig.). can be contrasted with the synchronous T1 carrier (Fig.).



T1: frames are generated precisely every 125 sec. This rate is governed by a master clock. Slot k of each frame contains 1 byte of data from the same source.

ATM: has no requirements that cells rigidly alternate among the various sources. Cells arrive randomly from different sources. The stream of cells need not be continuous. Gaps between the data are filled by special idle cells.

ATM does not standardize the format for transmitting cells. Cells are allowed to be sent individually


In the original ATM standard, the primary rate was 155.52 Mbps, with an additional rate at four time that speed (622.08 Mbps).

These rates were chosen to be compatible with SONET. ATM over T3 (44.736 Mbps) and FDDI (100 Mbps) is also foreseen.

The transmission medium for ATM is normally fiber optics, but for runs under 100 m, coax or category 5 twisted pair are also acceptable.

Each link goes between a computer and an ATM switch, or between two ATM switches.

So, all ATM links are point-to-point. Each link is unidirectional. For full-duplex operation, two parallel links are needed.



B-ISDN ATM Reference ModelB-ISDN ATM Reference Model


B-ISDN ATM Reference ModelB-ISDN ATM Reference Model It consists of 3 layers: Physical, ATM and ATM

Adaptation Layer (AAL) plus what ever the user wants to put on top of that

Physical Layer: Deals with physical medium: voltages, bit timing, media.

It has 2 sub layers1.Transmission convergence sub layer: Frame

Generation.2:Physical Medium Dependent sub layer: Flow of

bits for cablesATM Layer: Deals with cells and cell transport. It

defines the layout of a cell and tells what the header field mean. It also deals with establishment and release of virtual circuits. Congestion control too


B-ISDN ATM Reference ModelB-ISDN ATM Reference ModelATM Adaptation Layer:ATM Adaptation Layer: As most of the applications doesn’t deal with cells

directly, a layer above the ATM layer has been defined to allow users to send packets larger than a cell.

The ATM interface segments these packets, transmits the cells individually and reassembles them at the other end

It has 2 sub layers CS: Convergence Sub layer: Application Support SAR: Segmentation and reassemble sub layer. It packs the information received from cs into cells for

transmission

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