CN -EP Unit 5 and 6

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connection is reserved only for the two communicating parties, that unused bandwidth cannot be

&borrowed& for any other transmission.

"he most common form of circuit switching happens in that most familiar of networks, the

telephone system, but circuit switching is also used in some networks. %urrently available IS+N

lines, also known as narrowband ISDN , and the form of " known as switched T1 are both

e'amples of circuit-switched communications technologies.

Message Switching

nlike circuit switching, message switching does not involve a direct physical connection

between sender and receiver. When a network relies on message switching, the sender can fire

off a transmission/after addressing it appropriately/whenever it wants. "hat message is then

routed through intermediate stations or, possibly, to a central network computer. Along the way,

each intermediary accepts the entire message, scrutini0es the address, and then forwards the

message to the ne't party, which can be another intermediary or the destination node.

What1s especially notable about message-switching networks, and indeed happens to be one of

their defining features, is that the intermediaries aren1t re2uired to forward messages

immediately. Instead, they can hold messages before sending them on to their ne't destination.

"his is one of the advantages of message switching. *ecause the intermediate stations can wait

for an opportunity to transmit, the network can avoid, or at least reduce, heavy traffic periods,

and it has some control over the efficient use of communication lines.

Packet Switching

3acket switching, although it is also involved in routing data within and between #ANs such as

4thernet and "oken 5ing, is also the backbone of WAN routing. It1s not the highway on which

the data packets travel, but it is the dispatching system and to some e'tent the cargo containers

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that carry the data from place to place. In a sense, packet switching is the !ederal 4'press or

nited 3arcel Service of a WAN.

In packet switching, all transmissions are broken into units called packets, each of which

contains addressing information that identifies both the source and destination nodes. "hese

packets are then routed through various intermediaries, known as Packet Switching Exchanges

( PSE s), until they reach their destination. At each stop along the way, the intermediary inspects

the packet1s destination address, consults a routing table, and forwards the packet at the highest

possible speed to the ne't link in the chain leading to the recipient.

As they travel from link to link, packets are often carried on what are known as virtual circuits /

temporary allocations of bandwidth over which the sending and receiving stations communicate

after agreeing on certain &ground rules,& including packet si0e, flow control, and error control.

"hus, unlike circuit switching, packet switching typically does not tie up a line indefinitely for

the benefit of sender and receiver. "ransmissions re2uire only the bandwidth needed for

forwarding any given packet, and because packet switching is also based on multiple'ing

messages, many transmissions can be interleaved on the same networking medium at the same

time.

Connectionless and Connection-Oriented Services

So packet-switched networks transfer data over variable routes in little bundles called packets.

*ut how do these networks actually make the connection between the sender and the recipient6

"he sender can1t 7ust assume that a transmitted packet will eventually find its way to the correct

destination. "here has to be some kind of connection/some kind of link between the sender and

the recipient. "hat link can be based on either connectionless or connection-oriented services,

depending on the type of packet-switching network involved.

• In a (so to speak) connectionless &connection,& an actual communications link isn1t

established between sender and recipient before packets can be transmitted. 4ach

transmitted packet is considered an independent unit, unrelated to any other. As a result,

the packets making up a complete message can be routed over different paths to reach

their destination.

In a connection-oriented service, the communications link is made before any packets are

transmitted. *ecause the link is established before transmission begins, the packets

comprising a message all follow the same route to their destination. In establishing the

link between sender and recipient, a connection-oriented service can make use of either

switched virtual circuits (SVC s) or er!anent virtual circuits ( PVC s)

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o sing a switched virtual circuit is comparable to calling someone on the

telephone. "he caller connects to the called computer, they e'change information,

and then they terminate the connection.

o sing a permanent virtual circuit, on the other hand, is more like relying on a

leased line. "he line remains available for use at all times, even when no

transmissions are passing through it.

Types of Packet-Switching Networks

As you1ve seen, packet-based data transfer is what defines a packet-switching network. *ut/toconfuse the issue a bit/referring to a packet-switching network is a little like referring to tail-

wagging canines as dogs. Sure, they1re dogs. *ut any given dog can also be a collie or a 8erman

shepherd or a poodle. Similarly, a packet-switching network might be, for e'ample, an 9.:;network, a frame relay network, an A"$ (Asynchronous "ransfer $ode) network, an S$+S

(Switched $ultimegabit +ata Service), and so on.

X!" pac#et$switching networ#s

Originating in the <=>s, 9.:; is a connection-oriented, packet-switching protocol, originally

based on the use of ordinary analog telephone lines, that has remained a standard in networking

for about twenty years. %omputers on an 9.:; network carry on full-duple' communication,

which begins when one computer contacts the other and the called computer responds byaccepting the call.

Although 9.:; is a packet-switching protocol, its concern is not with the way packets are routed

from switch to switch between networks, but with defining the means by which sending andreceiving computers (known as +"4s) interface with the communications devices (+%4s)

through which the transmissions actually flow. 9.:; has no control over the actual path taken by

the packets making up any particular transmission, and as a result the packets e'changed between 9.:; networks are often shown as entering a cloud at the beginning of the route and

e'iting the cloud at the end.

A recommendation of the I" (formerly the %%I""), 9.:; relates to the lowest three network

layers/physical, data link, and network/ in the ISO reference model

• At the lowest (physical) layer, 9.:; specifies the means/electrical, mechanical, and so

on/by which communication takes place over the physical media. At this level, 9.:;

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covers standards such as 5S-:?:, the I"1s @.: specification for international

connections, and the I"1s @.?; recommendation for high-speed modem signaling over

multiple telephone circuits.

• At the ne't (data link) level, 9.:; covers the link access protocol, known as #A3* (#ink

Access 3rotocol, *alanced), that defines how packets are framed. "he #A3* ensures that

two communicating devices can establish an error-free connection.

• At the highest level (in terms of 9.:;), the network layer, the 9.:; protocol covers packet

formats and the routing and multiple'ing of transmissions between the communicating

devices.

On an 9.:; network, transmissions are typically broken into :B-byte packets. "hey can,

however, be as small as C bytes or as large as ><C bytes.

%T&s and %'&s As already mentioned, the sending and receiving computers on an 9.:;

network are not known as computers, hosts, gateways, or nodes. "hey are +"4s " In 9.:;

parlance, +"4s are devices that pass packets to +%4s, for forwarding through the links that

make up a WAN. +"4s thus sit at the two ends of a network connectionD in contrast, +%4s sit at

the two ends of a communications circuit, as shown in the following illustration.

(A%s So far so good. *ut since packets are as important to a packet-switching network as atomsare to matter, what about the devices that create and reassemble the packets themselves6 In some

cases, such as an 9.:; gateway computer (the +"4) that sits between a #AN and the WAN, the

gateway takes care of packeti0ing. In other cases, as with an ordinary 3% (another type of +"4),the 7ob is handled by a device known as a acket asse!bler and disasse!bler , or P#D. In this

case, the 3A+ sits between the computer and the network, packeti0ing data before transmission

and, when all packets have been received, reconstituting the original message by putting the

packets back together in the correct order.Is this work difficult6 Well, to a human it might be, because packets are sent along the best

possible route available at the time they are forwarded. "hus, it1s 2uite possible for the packets

representing a single message to travel over different links and to arrive at their destination out of order. %onsidering the amount of traffic flowing over a WAN, and considering the possible

number of transmitting and receiving nodes, it would seem that the 7ob of reconstructing any

given message represents a Eerculean task. Well, to people, it probably does. "o a 3A+, it doesnot. 3utting Eumpty +umpty back together again is all in a day1s work for the 3A+. It does such

work over and over again.

Fra)e rela*

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!rame relay is a newer, faster, and less cumbersome form of packet switching than 9.:;. Often

referred to as a $ast acket switching technology, frame relay transfers variable-length packets up

to F* in si0e at ;C Fbps or " (.; or : $bps) speeds over permanent virtual circuits.Operating only at the data link layer, frame relay outpaces the 9.:; protocol by stripping away

much of the &accounting& overhead, such as error correction and network flow control, that is

needed in an 9.:; environment. Why is this6 *ecause frame relay, unlike 9.:; with its earlyreliance on often unreliable telephone connections, was designed to take advantage of newer

digital transmission capabilities, such as fiberoptic cable and IS+N. "hese offer reliability and

lowered error rates and thus make the types of checking and monitoring mechanisms in 9.:;unnecessary.

!or e'ample, frame relay does include a means of detecting corrupted transmissions through a

cyclic redundancy check, or %5%, which can detect whether any bits in the transmission have

changed between the source and destination. *ut it does not include any facilities for errorcorrection. Similarly, because it can depend on other, higher-layer protocols to worry about

ensuring that the sender does not overwhelm the recipient with too much data too soon, frame

relay is content to simply include a means of responding to &too much traffic right now&

messages from the network.In addition, because frame relay operates over permanent virtual circuits (3@%s), transmissions

follow a known path and there is no need for the transmitting devices to figure out which route is best to use at a particular time. "hey don1t really have a choice, because the routes used in frame

relay are based on 3@%s known as Data %ink Connection Identi$iers, or D%CI s. Although a

frame relay network can include a number of +#%Is, each must be associated permanently witha particular route to a particular destination.

Also adding to the speed e2uation is the fact that the devices on a frame relay network do not

have to worry about the possibility of having to repackage andGor reassemble frames as they

travel. In essence, frame relay provides end-to-end service over a known/and fast/digitalcommunications route, and it relies heavily on the reliability afforded by the digital technologies

on which it depends. #ike 9.:;, however, frame relay is based on the transmission of variablelength packets, and it defines the interface between +"4s and +%4s. It is also based on

multiple'ing a number of (virtual) circuits on a single communications line.

So how, e'actly, does frame relay work6 #ike 9.:;, frame relay switches rely on addressinginformation in each frame header to determine where packets are to be sent. "he network

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transfers these packets at a predetermined rate that it assumes allows for free flow of information

during normal operations.

Although frame relay networks do not themselves take on the task of controlling the flow offrames through the network, they do rely on special bits in the frame headers that enable them to

address congestion. "he first response to congestion is to re2uest the sending application to &cool

it& a little and slow its transmission speedD the second involves discarding frames flagged aslower-priority deliveries, and thus essentially reducing congestion by throwing away some of the

cargo.

!rame relay networks connecting #ANs to a WAN rely, of course, on routers and switchinge2uipment capable of providing appropriate frame-relay interfaces.

ATM

Hou1re focused on networks when A"$ no longer translates as &Automated "eller $achine& but

instead makes you immediately think &Asynchronous "ransfer $ode.& All right. So what isAsynchronous "ransfer $ode, and what is it good for6

"o begin with, A"$ is a transport method capable of delivering not only data but also voice and

video simultaneously, and over the same communications lines. 8enerally considered the wave

of the immediate future in terms of increasing both #AN and WAN capabilities, A"$ is aconnection-oriented networking technology, closely tied to the I"1s recommendation on

broadband ISDN ( &ISDN ) released in <BB.What A"$ is good for is high-speed #AN and WAN networking over a range of media types

from the traditional coa'ial cable, twisted pair, and fiberoptic to communications services of the

future, including !iber %hannel, !++I, and SON4" (described in later sections of this chapter).Although A"$ sounds like a dream, it1s not. It1s here, at least in large part.

'ell rela* A"$, like 9.:; and frame relay, is based on packet switching. nlike both 9.:; and

frame relay, however, A"$ relies on cell relay, a high-speed transmission method based on

fi'ed-si0e units (tiny ones only ;? bytes long) that are known as cells and that are multiple'edonto the carrier.

*ecause uniformly si0ed cells travel faster and can be routed faster

than variable-length packets, they are one reason/though certainlynot the only one/that A"$ is so fast. "ransmission speeds are

commonly ;C Fbps to .; $bps, but the I" has also defined A"$

speeds as high as C:: $bps (over fiberoptic cable).

+ow it wor#s Imagine a &universal& machine/one that can take in any materials, whether they

are delivered sporadically or in a constant stream, and turn those materials into lookalike

packages. "hat1s basically how A"$ works at the intake end. It takes in streams of data, voice,

videowhateverand packages the contents in uniform ;?-byte cells. At the output end, A"$sends its cells out onto a WAN in a steady stream for delivery, as shown in !igure B-.

"hat all seems simple enough, but now take a look at the &magic& of A"$ in a little more

technical detail."o begin with, remember that A"$ is designed to satisfy the need to deliver multimedia. Well,

multimedia covers a number of different types of information that have different characteristics

and are handled differently, both by the devices that work with them and by higher-levelnetworking protocols. Het, in order to make use of A"$, something must interface with the

different devices and must package their different types of data in

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Figure ,$-: ATM .rea#s data strea)s into fi/ed$si0e cells and delivers the) over a WAN

1The 2converter2 here is not a real ATM switch3it4s )eant to suggest a hopper or funnel

into which the various data strea)s flow56ust an atte)pt to lighten things up7 .ut the

concept is accurate8

A"$ cells for transport. "hat something is an A"$-capable node that handles the conversions

specified in the three-layer A"$ model shown in the following illustration

"hese are the layers and what they do

• "he topmost layer, the #T' #datation %a(er ( ##%), sits between what you might

consider &A"$ proper& and the higher-level network devices and protocols that send and

receive the different types of information over the A"$ network. AA#, as the adatation

in its name suggests, mediates between the A"$ layer and higher-level protocols,

remodeling the services of one so that they fit the services of the other. It1s a fascinating

&place,& in that AA# takes in all the different forms of data (audio, video, data frames)

and hands the data over to comparable AA# services (audio, video, data frames) that

repackage the information into B-byte payloads before passing them along to the A"$

layer for further grooming.

• "he A"$ layer attaches headers to the A"$ payloads. "hat might seem simple enough,

but the header does not simply say, &this is a cell.& 3art of the header includes information

that identifies the paths and circuits over which those cells will travel and so enables

A"$ switches and routers to deliver the cells accurately to their intended destinations.

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"he A"$ layer also multiple'es the cells for transmission before passing them to the

physical layer. "his layer, as you can see, has a big 7ob to do. It1s somewhat reminiscent

of a busy airport, railroad stationor maybe a large department store during the holiday

season.

• "he physical layer, the lowest layer, corresponds to the physical layer in the ISOGOSI

5eference $odel. As in the OSI model, it is concerned with moving information/in this

case, the ;?-byte A"$ cells/into the communications medium. As already mentioned,

this medium can be any of a number of different physical transports, including the

fiberoptics-based SON4" (Synchronous Optical N4"work), a " or 4 line, or even a

modem. "he medium and the message in this case are clearly separable because A"$ is a

transport !ethod and is independent of the transmissions medium over which the

messages travel.

So what happens after A"$ filters information down through the AA#, A"$, and physical

layers6 Once the physical layer sends the cells on their way, they travel to their destinations over

connections that might switch them from one circuit to another. Along the way, the switches and

routers work to maintain connections that provide the network with at least the minimum

bandwidth necessary to provide users with the )ualit( o$ service (*+S ) guaranteed them

When the cells arrive at their destinations, they go through the reverse of the sending process.

"he A"$ layer forwards the cells to the appropriate services (voice, data, video, and so on) in

the AA#, where the cell contents are converted back to their original form, everything is checked

to be sure it arrived correctly, and the &reconstituted& information is delivered to the receiving

device.

Availa.ilit* So A"$ is a wonderful means of transmitting all kinds of information at high speed.

It is reliable, fle'ible, scalable, and fast because it relies on higher-level protocols for error

checking and correction. It can interface with both narrowband and broadband networks, and it is

especially suitable for use in a network backbone.

Is there a downside6 Well, yes. "o begin with, A"$ networks must be made up of A"$-

compatible devices, and they are both e'pensive and not yet widely available. In addition, there

is a chicken-or-egg dilemma facing serious A"$ deployment businesses are not likely to incur

the e'pense of investing in A"$-capable e2uipment if A"$ services are not readily availablethrough communications carriers over a wide area, yet carriers are reluctant to invest in A"$

networking solutions if there is not enough demand for the service.

4ventually, no doubt, A"$ will win over both carriers and users, and the world will be treated to

e'tremely fast, reliable A"$ delivery. ntil then, A"$ continues to mature, especially with the

help of an organi0ation known as the A"$ !orum/a group of vendors and other interested

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parties working together to develop standards, provide information, and generally encourage the

development of A"$-related technology. As time passes, A"$ is e'pected to build up a

complete head of steam and begin to fulfill its promise. %ertainly, with increasing reliance on

networking and growing demand for faster and more sophisticated methods of delivering

multimedia, there1s a place for this technology.

And that, in a nutshell, is A"$. Eowever, before leaving the sub7ect, it1s worth taking a 2uick

look at broadband IS+N, another immature but promising technology, and the one for which the

A"$ layers were defined.

9IS%N *IS+N is ne't-generation IS+N, a technology that can deliver all kinds of information

over the network. In *IS+N terms, this information is divided into two basic categories,

interactive services and distributed ,or distribution services.

• Interactive services include you-and -me types of transactions, such as videoconferencing,

messaging, and information retrieval.

• +istributed services include you-to-me types of information that are either delivered or

broadcast to the recipient. "hese services are further divided into those that the recipient

controls (for e'ample, e-mail, video telephony, and tele') and those that the recipient

cannot control other than by refusing to &tune in& (for e'ample, audio and television

broadcasts).

*ut, you might think, current narrowband IS+N is also capable of delivering data, voice, video,

and sound, so what1s the difference6 "he difference is in the method of delivery. Narrowband

IS+N transmissions are based on time division multiple'ing ("+$), which uses timing as the

key to interleaving multiple transmissions onto a single signal. In contrast, *IS+N uses A"$,

with its packet switching and its little ;?-byte cells, for delivery.

"hus, A"$ defines *IS+N, or at least the part of it concerned with delivering the goods. In a

sense, *IS+N is comparable to a catalog shopping service that delivers everything from food to

clothing, and A"$ is like the bo'es in which those products are packaged and delivered.

Developing Technologies

A"$ is but one e'ample of an advanced technology. A"$ is here, though not yet widely

available. So is it the only one to choose from6 No, there are others. One, !++I, is well known

and used in both #ANs and WANs. "wo others are SON4", another developing technology, and

S$+S, which is available through some carriers. All three/!++I, SON4", and S$+S/tie in

with A"$, at least in the sense of being high-speed networking technologies that are

recommended by the A"$ !orum as interfaces for A"$ networks.

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All three of the networks described in the following sections are, of course, designed for speed,

speed, and more speed. Along with reliability, of course.

FDDI

.DDI , variously pronounced either &fiddy& or &eff-dee-dee-eye,& is short for .iber Distributed Data Inter$ace. As you1ve no doubt guessed, it1s based on fiberoptic transmission. It1s also based

on a ring topology and token passing. It1s advanced technology, yes, in the form of token ring

over fiber.

!++I was developed for two primary reasons to support and help e'tend the capabilities of

older #ANs, such as 4thernet and "oken 5ing, and to provide a reliable infrastructure for

businesses moving even !ission-critical applications to networks. *ased on a standard produced

by an ANSI committee known as 9?"<.;, the !++I specification was released in <BC/a

relatively long time ago in networking terms.

Although !++I isn1t really a WAN technology (its rings are limited to a ma'imum length of >>

kilometers, or C: miles), the ground it can cover does make it suitable for use as a backbone

connecting a number of smaller #ANs, and it can provide the core of a network as large as a

$etropolitan Area Network ($AN). In that sense, !++I is more than #AN but less than WAN.

In addition, because !++I transfers information e'tremely 2uickly (>> $bps), it is often used

to connect high-end devices, such as mainframes, minicomputers, and peripherals, or to connect

high-performance devices within a #AN. 4ngineering or videoGgraphics workstations, for

instance, benefit from !++I because they need considerable bandwidth in order to transfer large

amounts of data at satisfactorily high speeds.

As its name indicates, !++I was developed around the idea of using optical cable. "his is, in

fact, the type of cable used, especially when high-speed transmission is needed over relatively

long distances (:>>> to >,>>> meters, or roughly to C miles). Eowever, over shorter distances

(about >> meters, or ??> feet), !++I can also be implemented on less e'pensive copper cable.

In all, !++I supports four different types of cable

• Multi)ode fi.eroptic ca.le "his type of cable can be used over a ma'imum of :>>>

meters and uses #4+s as a light source.

• Single )ode fi.eroptic ca.le "his can be used over a ma'imum of >,>>> meters and

uses lasers as a light source. Single mode cable is thinner at the core than multimode, but

it provides greater bandwidth because of the way the light impulse travels through the

cable.

• 'ategor* " nshielded Twisted (air copper wiring "his cable contains eight wires

and, like the ne't category, can be used over distances up to ?> meters.

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• I9M T*pe - Shielded Twisted (air copper wiring "his is a shielded cable that contains

two pairs of twisted wires, with each pair also shielded.

F%%I topolog* and fault tolerance

!++I topology and operation are similar to "oken 5ing, excet (there1s always an e'ception, is

there not6) that !++I is primarily based on optical transmission. In addition, !++I is

characteri0ed by two counter-rotating rings (known as a dual-ring toolog().

Why two rings6 "he second one is there mostly for insurance. Normally in a !++I network, one

ring (known as the primary ring) actually carries the tokens and data, and the secondary ring

remains idle and is used as a backup for fault tolerance/insurance. *ecause the secondary ring

is available if needed, whenever a nonfunctioning node causes a break in the primary ring, traffic

can &wrap& around the problem node and continue carrying data, only in the opposite direction

and on the secondary ring. "hat way, even if a node goes down, the network continues to

function.

Of course, it is also possible for two nodes to fail. When this happens, the wrap at both locations

effectively segments the one ring into two separate, noncommunicating rings. "o avoid this

potentially serious problem, !++I networks can rely on bypass devices known as concentrators.

"hese concentrators resemble hubs or $As in that multiple nodes plug into them. "hey are also

able to isolate any failed nodes, while keeping the network traffic flowing.

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Sometimes, however, both rings are used for data. In this case, the data travels in one direction

(clockwise) on one ring, and in the other direction (counterclockwise) on the other ring. sing

both rings to carry data means that twice as many frames can circulate at the same time and,

therefore, the speed of the network can double/from >> $bps to :>> $bps.

F%%I to#en passing

"oken passing on a !++I network works much the way it does on a "oken 5ing network. "hat

is, nodes pass a token around the ring, and only the node with the token is allowed to transmit a

frame. "here is a twist to this, however, that1s related to !++I1s fault tolerance. When a node on

the ring detects a problem, it doesn1t simply sit around and say, &gee, I can1t pass the token along,

I guess I1ll 7ust hang onto it.& Instead, it generates a frame known as a beacon and sends it on to

the network. As neighboring nodes detect the beacon, they too begin to transmit beacons, and so

it goes around the ring. When the node that started the process eventually receives its own

beacon back/usually after the network has switched to the secondary ring/it then assumes that

the problem has been isolated or resolved, generates a new token, and starts the ball rolling once

again.

Structure of a F%%I networ#

A !++I network, as already mentioned, cannot include rings longer than >> kilometers apiece.

Another restriction on a !++I network is that it cannot support more than ;>> nodes per ring.

Although the overall network topology must conform to a logical ring, the network doesn1t

actually have to look like a circle. It can include stars connected to hubs or concentrators, and it

can even include trees/collections of hubs connected in a hierarchy. As long as the stars andtrees connect in a logical ring, the !++I network is happy.

In terms of the nodes that connect to the network, they come in two varie-ties, depending on how

they are attached to the !++I ring. One variety, called a single attach!ent station, or S#S ,

connects to a concentrator and, through it, to the primary ring. *ecause an SAS connects to a

concentrator, the latter device can isolate the node from the rest of the ring if it happens to fail.

"he second type of node, called a dual attach!ent station, or D#S , has two connections to the

network. "hese can link it either to another node and a concentrator or/if their operation is

critical to the network/to two concentrators, one of which serves as a backup in case the otherfails. "his type of two-concentrator connection for a single resource, such as a mission-critical

server, is known as dual ho!ing and is used to provide the most fail-safe backup mechanism

possible.

In sum !++I is a high-speed, high-bandwidth network based on optical transmissions. It is

relatively e'pensive to implement, although the cost can be held down by the mi'ing of

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fiberoptic with copper cabling. *ecause it has been around for a few years, however, it has been

fine-tuned to a high level of stability. It is most often used as a network backbone, for connecting

high-end computers (mainframes, minicomputers, and peripherals), and for #ANs connecting

high-performance engineering, graphics, and other workstations that demand rapid transfer of

large amounts of data.

SONET

S+NET , or S(nchronous +tical NETwork , is an ANSI standard for the transmission of different

types of information/data, voice, video/over the optical (fiberoptic) cables widely used by

long-distance carriers. +esigned to provide communications carriers with a standard interface for

connecting optical networks, SON4" was formulated by an organi0ation known as the 4'change

%arriers Standards Assocation (4%SA) and later incorporated into an I" recommendation

known as S(nchronous Digital /ierarch(, or SD/ .

"oday, apart from relatively small differences, SON4" and S+E are e2uivalent/SON4" in

North America and Japan, and S+E in 4urope. "ogether, they represent a global standard for

digital networks that enables transmission systems around the world to connect through optical

media. SON4" is comparable to a standard that ensures that train tracks, regardless of

manufacturer, follow the same design specifications and therefore can interconnect to allow

trains to pass over them freely and without problem.

Originally designed in the mid-<B>s, SON4" works at the physical layer and is concerned with

the details related to framing, multiple'ing, managing, and transmitting information

synchronously over optical media. In essence, SON4" specifies a standard means for

multiple'ing a number of slower signals onto a larger, faster one for transmission.

In relation to this multiple'ing capability, two signal definitions lie at the heart of the SON4"

standard

• Optical carrier (O%) levels, which are used by fiberoptic media and which translate

roughly to speed and carrying capacity

• Synchronous transfer signals (S"S), which are the electrical e2uivalents of O% levels and

are used by non-fiber media

So what does that mean6 Well, let1s back up a little. SON4" is an optical transport, true. *ut

remember that it is a long-distance transport. Although transmissions flow through the SON4"

system in optical form, they do not begin and end that way. "ransmissions are multiple'ed onto

the SON4" optical medium, but they come from/and go to/other, electrically based, types of

digital transport such as ". In this, it helps to think of SON4" as something like the $ississippi

5iver, and of the channels it connects to as tributaries that flow into and out of it. (In SON4"

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terminology, those channels actually are called tributaries, so the analogy is reasonably

accurate.) "he following illustration shows basically what happens during a SON4"

transmission

*ecause SON4" is a synchronous transport, the signals it works with are tied to timing, and the

various transmission speeds it handles are based on multiples of a single base signal rate known

as S"S- (Synchronous "ransport Signal level-) and the e2uivalent, optical, O%-. "his base

rate operates at ;.B $bps. "hat sounds really fast, and it begins to show why SON4" is seen

as a desirable transport method, but remember/;.B $bps is base signal. SON4" rates geteven better. Eigher-level SON4" rates really fly. "he ne't step up, for instance, is S"S-?

(e2uivalent to O%-?), which multiple'es three S"S- signals onto a single stream and operates at

three times the base signal rate/;;.;:> $bps. And there1s more. S"S-: (O%-:) operates at

: times the base signal, which works out to C::.>B $bps. And at the top end, there1s S"S-B

(O%-B), with a defined transmission speed of :.BB 8bps (that1s gigabits per second).

+ow it wor#s

As you can see from the preceding illustration, SON4" converts electrical (S"S) signals to

optical (O%) levels for transport. It also &unconverts& them (O% to S"S) at the point where thetransmissions leave the SON4" media for further travel on whatever carrier takes them the rest

of the way to their destination. Eow this all happens is both impressive and intriguing.

"o start off with, SON4" is not a single, very long piece of optical fiber. (Of course not/that

would mean one piece of cable stretching around the world.) Along the way from source to

destination, a transmission can pass through more than one intermediate multiple'er, as well as

through switches, routers, and repeaters for boosting the signal. %ifferent parts of this route are

given different SON&T na)es:

A section is a single length of fi.eroptic ca.le

A line is an* seg)ent of the path that runs .etween two )ultiple/ers

A path is the co)plete route .etween the source )ultiple/er 1where signals fro)

tri.utaries are co).ined8 and the destination )ultiple/er 1where the signals are

de)ultiple/ed so the* can .e sent on their wa*8

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"he transmissions themselves are made up of B>-byte frames that are sent out at the rate of

B>>> per second. "hese frames contain not only data but also a number of bytes related to

overhead/monitoring, management, and so on. "o an interested bystander, there are two

especially remarkable aspects to the way these frames are managed

• !irst, they pour out in a steady stream, whether or not they contain any information. In

other words, they are like freight cars on an endless train. If some data happens to arrive

at the time SON4" is putting a frame together, that data gets popped into the frame/the

freight car is loaded. If no data arrives, the frame leaves the &station& empty.

• Second, because SON4" is a synchronous transport, each frame contains a device called

a pointer that indicates where the actual data in the frame begins. "his pointer is

necessary because timing is such an important part of SON4" transmission, but the

network itself cannot assume that the arriving data streams are synchroni0ed to the same

clock. ("hat would, in fact, be impossible.) Instead, SON4" allows for a certain amount

of variation in timing and uses a pointer to ensure that the beginning of the data payload

is clearly marked for retrieval at its destination.

(rotocol la*ers in the SON&T standard

In doing all of the work of organi0ing, multiple'ing, transmitting, and routing frames, SON4"

relies on four protocol layers, each of which handles one aspect of the entire transmission. "hese

layers and what they do are

• The photonic la*er7 which converts signals between electrical and optical form

• The section la*er7 which creates the frames and takes care of monitoring for errors in

transmission

• The line la*er7 which is in charge of multiple'ing, synchroni0ing, and demultiple'ing

• The path la*er7 which is concerned with getting the frame from source to destination

"here are many more technical details involved in the definition of a SON4" network, but these

are the basics, and they should help you understand at least roughly how SON4" works. 3erhaps

the most important lesson to carry away from this is the reali0ation that SON4" represents a fast,

reliable transport for developing or future WAN technologies, including *IS+N (and, by

e'tension, A"$).

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SDS

And finally, you come to S'DS , more lengthily known as Switched Multimegabit Data Service"

S$+S is a broadband public networking service offered by communications carriers as a means

for businesses to connect #ANs in separate locations. It is a connectionless, packet-switched

technology designed to provide business with a less e'pensive means of linking networks thanthrough the use of dedicated leased lines. *esides reducing cost, S$+S is notable for being well-

suited to the type of &bursty& traffic characteristic of #AN (or #AN-to-#AN) communications. In

other words, it does the 7ob when it1s needed.

9ecause SM%S is connectionless7 it is availa.le when and as needed7 rather than .eing

2on2 at all ti)es It is also a fast technolog*7 trans)itting at speeds of - M.ps to 1in the

nited States8 ;" M.ps. "he basis of an S$+S connection is a network address designed as a

telephone number that includes country code and area code, as well as the local number. "his

address is assigned by the carrier and is used to connect #AN with #AN. A group address can

also be used to broadcast information to a number of different #ANs at the same time.

sers who need to transfer information to one or more #ANs simply select the appropriate

addresses in order to indicate where the information is to be delivered. S$+S takes it from there

and makes a &best effort& to deliver the packets to their destinations. It does not check for errorsin transmission, nor does it make an attempt at flow control. "hose tasks are left to the

communicating #ANs.

"he packets transferred through S$+S are simple, variable-length affairs containing the source

and destination addresses and up to <BB bytes of data. "hese packets are routed individually and

can contain data in whatever form the sending #AN works with/4thernet packet, "oken 5ing

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packet, and so on. SM%S essentiall* 6ust passes the infor)ation fro) one place to the other

and doesn4t deal with the for) or for)at of the data In other words7 SM%S acts so)ewhat

li#e a courier service3it pic#s up and delivers .ut does not concern itself with the contents

of its pac#ages

%IST<I9T&% =&& %AL 9S

%istri.uted =ueue %ual 9us 1%=%98 is a %ata$lin# la*er co))unication protocol for

Metropolitan Area Networ#s 1MANs87 specified in the I&&& ,>!? standard and designed

for use in MANs %=%9 is designed for data as well as voice and video trans)ission and is

.ased on cell switching technolog* 1si)ilar to ATM). +K+*, which permits multiple systems

to interconnect using two unidirectional logical .uses, is an open standard that is designed for

compatibility with carrier transmission standards such as S$+S.

!or a $AN to be effective it re2uires a system that can function across long, Lcity-wideM

distances of several miles, have a low susceptibility to error, adapt to the number of nodes

attached and have variable bandwidth distribution. sing +K+*, networks can be thirty miles

long and function in the range of ? $bps to ;; $bps. "he data rate fluctuates due to many

hosts sharing a dual bus, as well as to the location of a single host in relation to the frame

generator, but there are schemes to compensate for this problem making +K+* function reliably

and fairly for all hosts.

"he +K+* is composed of two bus lines with stations attached to both and a frame generator at

the end of each bus. "he buses run in parallel in such a fashion as to allow the frames generated

to travel across the stations in opposite directions. *elow is a picture of the basic +K+*architecture

.

.

Figure: DQDB Architecture

+K+* Architecture

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4ach bus supports traffic in only one direction

*eginning of bus is denoted by a s2uare and end by a triangle

*us * traffic moves from right to left and *us A traffic from left to right

4ach bus connects to stations directly through input and output ports

"he +K+* is composed of a two bus lines with stations attached to both and a cell (4mpty

slots) generator at the start of each bus.

"he buses run in parallel in such a fashion as to allow the cells generated to travel across the

stations in opposite directions.

"he cell generator (head-end) is constantly producing empty cells consisting of fifty-three bytes

(a five byte header and a forty-eight byte payload).

pstream +ownstream

As *us A is configured

PStations : ? are considered to be upstream w.r.t station

PStations : are considered to be downstream w.r.t. station ?

As *us * is configured

PStation : ? are considered to be downstream w.r.t. station

PStations : are considered to be upstream w.r.t. station ?

+K+* Working

Eead-ends generate fi'ed si0e cells in both directions (cell generators)

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"o transmit, a host must know whether the destination is to its right or its left

PIf right, the host must send on left bus

PIf left, the host must send on the right bus

A L+istributed KueueM is used to make sure that cells are transmitted on a first-come first-serve

basis

"echnical !acts Of +K+*

+istance up to :>> F$

$edium %opper or !iber

At distance up to C> F$ appro' speed is .=? $bps (%opper)

At distance up to >> F$ appro' speed is ;> $bps (!iber)

"ransmission 5ate ? $bps to ;> $bps

+K+* !eatures

+K+* is a +## communication protocol for $AN

nlike !++I, +K+* is an I444 standard B>:.C

+esigned for both voice video

"opology used +ual *us - uses : unidirectional logical buses

4'tend up to ?> miles at ?-;; $bps

ses optical fibre links

Kueued-packet distributed switch (K3S9) algorithm

Works on +ata-link layer (especially in $A% sub-layer)

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sed in data, voice and video transmissions

sed in data over cable services

*ased on %ell 5elay "echnology (like A"$)

3rovides connection-oriented, connection less services asynchronous services

S*nchronous %igital +ierarch* 1S%+8 are standardi0ed protocols that transfer multiple digital

bit streams synchronously over optical fiber using lasers or highly coherent light from light-

emitting diodes (#4+).

"he basic unit of framing in S+E is a S"$- (Synchronous "ransport $odule, level ), which

operates at ;;.;:> megabits per second ($bitGs). the S"$-GS"S-?c frame is :,?> octets insi0e.

S%+ fra)e

"he S"$- (Synchronous "ransport $odule, level ) frame is the basic transmission format for

S+E/the first level of the synchronous digital hierarchy. "he S"$- frame is transmitted in

e'actly :; Qs, therefore, there are B,>>> frames per second on a ;;.;: $bitGs O%-? fiber-optic

circuit. "he S"$- frame consists of overhead and pointers plus information payload. "he firstnine columns of each frame make up the Section Overhead and Administrative nit 3ointers,

and the last :C columns make up the Information 3ayload. "he pointers (E, E:, E? bytes)

identify administrative units (A) within the information payload. "hus, an O%-? circuit can

carry ;>.??C $bitGs of payload, after accounting for the overhead.

%arried within the information payload, which has its own frame structure of nine rows and :C

columns, are administrative units identified by pointers. Also within the administrative unit are

one or more virtual containers (@%s). @%s contain path overhead and @% payload. "he first

column is for path overheadD it is followed by the payload container, which can itself carry other

containers. Administrative units can have any phase alignment within the S"$ frame, and this

alignment is indicated by the pointer in row four.

"he section overhead (SOE) of a S"$- signal is divided into two parts the regenerator section

overhead (5SOE) and the !ultilex section overhead ($SOE). "he overheads contain

information from the transmission system itself, which is used for a wide range of management

https://en.wikipedia.org/wiki/Microsecond



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functions, such as monitoring transmission 2uality, detecting failures, managing alarms, data

communication channels, service channels, etc.

"he S"$ frame is continuous and is transmitted in a serial fashion byte-by-byte, row-by-row.

"ransport overhead "he transport overhead is used for signaling and measuring transmissionerror rates, and is composed as follows

Section overhead %alled 5SOE (regenerator section overhead) in S+E terminology := octets

containing information about the frame structure re2uired by the terminal e2uipment.

#ine overhead %alled $SOE (multiple' section overhead) in S+E ; octets containing

information about error correction and Automatic 3rotection Switching messages (e.g., alarms

and maintenance messages) as may be re2uired within the network. "he error correction is

included for S"$-C and above.

A 3ointer 3oints to the location of the J byte in the payload (the first byte in the virtual

container).

3ath virtual envelope +ata transmitted from end to end is referred to as path data. It is composed

of two components

3ayload overhead (3OE) Nine octets used for end-to-end signaling and error measurement.

3ayload ser data (== bytes for S"$->GS"S-, or :,?> octets for S"$-GS"S-?c) !or S"S-,

the payload is referred to as the synchronous payload envelope (S34), which in turn has B

stuffing bytes, leading to the S"S- payload capacity of =;C bytes.

<&MOT& MONITO<IN@ T&'+NI=&S 5emote monitoring is one of the method of

remote monitoring which uses SN$3 (simple network management protocol) and a NI%

(network interface card) to broadcast the alarm over an Internet connection to the remote

monitoring provider that can track the data to follow trends in parametric data for diagnostic andtroubleshooting purposes.

. 3O##IN8 3olling works with topologies in which one device is designated as a primary

station and the other devices are secondary stations. All data e'changes must be made

through the primary device even when the ultimate destination is a secondary device. "he

primary device controls the linkD the secondary devices follow its instructions. It is up to the

primary device to determine which device is allowed to use the channel at a given time. "he

primary device, therefore, is always the initiator of a session (see !igure).

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If the primary wants to receive data, it asks the secondary if they have anything to sendD this is

called poll function. If the primary wants to send data, it tells the secondary to get ready to

receiveD this is called select function.

Select "he select function is used whenever the primary device has something to send.

5emember that the primary controls the link. If the primary is neither sending nor receiving data,

it knows the link is available. If it has something to send, the primary device sends it. What it

does not know, however, is whether the target device is prepared to receive. So the primary must

alert the secondary to the upcoming transmission and wait for an acknowledgment of the

secondary1s ready status. *efore sending data, the primary creates and transmits a select (S4#)

frame, one field of which includes the address of the intended secondary.3oll "he poll function is used by the primary device to solicit transmissions from the secondary

devices. When the primary is ready to receive data, it must ask (poll) each device in turn if it has

anything to send. When the first secondary is approached, it responds either with a NAF frame if

it has nothing to send or with data (in the form of a data frame) if it does. If the response is

negative (a NAF frame), then the primary polls the ne't secondary in the same manner until it

finds one with data to send. When the response is positive (a data frame), the primary reads the

frame and returns an acknowledgment (A%F frame), verifying its receipt.

:. "5A3 A management station, called a manager, is a host that runs the SN$3 client program.

A managed station, called an agent, is a router (or a host) that runs the SN$3 server

program. $anagement is achieved through simple interaction between a manager and an

agent. "he agent keeps performance information in a database. "he manager has access to the

values in the database. !or e'ample, a router can store in appropriate variables the number of

packets received and forwarded. "he manager can fetch and compare the values of these two

variables to see if the router is congested or not.

"he manager can also make the router perform certain actions. !or e'ample, a router periodically

checks the value of a reboot counter to see when it should reboot itself. It reboots itself, for

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e'ample, if the value of the counter is >. "he manager can use this feature to reboot the agent

remotely at any time. It simply sends a packet to force a > value in the counter.

Agents can also contribute to the management process. "he server program running on the agent

can check the environment, and if it notices something unusual, it can send a warning message,

called a trap, to the manager.

In other words, management with SN$3 is based on three basic ideas

. A manager checks an agent by re2uesting information that reflects the behavior of the agent.

:. A manager forces an agent to perform a task by resetting values in the agent database.

?. An agent contributes to the management process by warning the manager of an unusual

situation.

Securit* )anage)ent is the identification of an organi0ation1s assets (including information

assets), followed by the development, documentation, and implementation of policies and

procedures for protecting these assets. An organisation uses such security management

procedures as information classification, risk assessment, and risk analysis to identify threats,

categorise assets, and rate system vulnerabilities so that they can implement effective controls.. Firewall: !irewalls are fre2uently used to prevent unauthori0ed Internet users from accessing

private networks connected to the Internet, especially intranets. All messages entering or

leaving the intranet pass through the firewall, which e'amines each message and blocks those

that do not meet the specified security criteria.

!irewalls can be either hardware or software but the ideal firewall configuration will consist

of both. In addition to limiting access to your computer and network, a firewall is also useful

for allowing remote access to a private network through secure authentication certificates and

logins.

%ommon !irewall "echni2ues

!irewalls are used to protect both home and corporate networks. A typical firewall programor hardware device filters all information coming through the Internet to your network or

computer system. "here are several types of firewall techni2ues that will prevent potentially

harmful information from getting through

a. 3acket !ilter #ooks at each packet entering or leaving the network and accepts or re7ects it

based on user-defined rules. 3acket filtering is fairly effective and transparent to users, but it

is difficult to configure. In addition, it is susceptible to I3 spoofing.

b. Application 8ateway Applies security mechanisms to specific applications, such as !"3 and

"elnet servers. "his is very effective, but can impose a performance degradation.

c. %ircuit-level 8ateway Applies security mechanisms when a "%3 or +3 connection is

established. Once the connection has been made, packets can flow between the hosts without

further checking.

d. 3ro'y Server Intercepts all messages entering and leaving the network. "he pro'y server

effectively hides the true network addresses. A pro'y server can act as an intermediary

between the user1s computer and the Internet to prevent from attack and une'pected access.

3ro'y severs can implement Internet access control like authentication for Internet

connection, bandwidth control, online time control, Internet web filter and content filter etc.

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e. Ne't 8eneration !irewall (N8!W) A newer class of firewalls, ne't generation firewall -

N8!W, filters network and Internet traffic based upon the applications or traffic types using

specific ports. Ne't 8eneration !irewalls (N8!Ws) blend the features of a standard firewall

with 2uality of service (KoS) functionalities in order to provide smarter and deeper

inspection.

:. LANs: @#AN is a LvirtualM #AN, consisting of a subset of devices communicating

privately on a larger network. In more technical terms, a @#AN is a uni2ue, broadcast

domain created by smart and managed 4thernet switches. (nmanaged switches cannot be

used to create @#ANs, as they do not have a user interface to facilitate this technology).

*ecause @#ANs segment a network, creating multiple broadcast domains, they effectively

allow traffic from the broadcast domains to remain isolated while increasing the network1s

bandwidth, availability and security.

S4%5IN8 @#AN +4@I%4S "he first principle in securing a @#AN network is physical

security. If an organi0ation does not want its devices tampered with, physical access must be

strictly controlled. %ore switches are usually safely located in a data center with restrictedaccess, but edge switches are often located in e'posed areas.

"he best security practices for @#AN include

. Introducing password-protected console or virtual terminal access with specified timeouts

and restricted access policiesD

:. %reating an access-list to restrict telnetGSEE access from specific networks and hostsD

?. +isabling high-risk protocols on any port that doesn1t re2uire them

. %ontrolling inter-@#AN routing through the use of I3 access lists.

CN -EP Unit 5 and 6

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