computer networks module

8/10/2019 Computer Networks Module

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MODULE IV


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COMPUTER NETWORKS

A system consists of two or more autonomous computers/equipment

interconnected by transmission media (e.g., coaxial cable, fiber, twisted pair,

and microwave).NETWORK GOALS

!e main goal of networ"ing is "Resource sharing" , and it is to

ma"e all programs, data and equipment available to anyone on t!e

networ" wit! out t!e regard to t!e p!ysical location of t!e resource

and t!e user.

A second goal is to provide high re ia! i # by !aving alternative

sources of supply. #or example, all files could be replicatied on two or

t!ree mac!ines, so if one of t!em is unavailable, t!e ot!er copies

could be available.

Anot!er goal is sa$ing %one#& $mall computers !ave a muc! better

price/performance ratio t!an larger ones. %ainframes are roug!ly a

factor of ten times faster t!an t!e fastest single c!ip microprocessors,

but t!ey cost t!ousand times more. !is imbalance !as caused many

system designers to build systems consisting of powerful personal

computers, one per user, wit! data "ept on one or more s!ared 'i e

ser$er mac!ines. !is goal leads to networ"s wit! many computers

located in t!e same building. $uc! a networ" is called a LAN( oca

area ne )or*+&

Anot!er closely related goal is to increase t!e systems perfomance ast!e wor" load increases by &ust adding more processors. 'it! central

mainframes, w!en t!e system is full, it must be replaced by a larger

one, usually at great expense and wit! even greater disruption to t!e

users.


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Ring Ne )or* To-o og#

!is diagram illustrates t!e ring networ" topology. A ring topology

suc! as #00 or $ 3 sends messages cloc"wise or countercloc"wise

t!roug! t!e s!ared lin".

S ar Ne )or* To-o og#

!is diagram illustrates t!e star networ" topology. A star topology

typically uses a networ" !ub or switc! and is common in !ome networ"s.


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Mesh Ne )or* To-o og#

!is diagram illustrates t!e mes! networ" topology. A mes! topology

provides redundant communication pat!s between some or all devices

(partial or full mes!).

Tree Ne )or* To-o og#

!is diagram illustrates t!e tree networ" topology. A tree topology

integrates t!e star and bus topologies in a !ybrid approac! to improve

networ" scalability


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!e $ession 7ayer

!e 6resentation 7ayer

!e Application 7ayer

The Ph#sica La#er

responsible for transmitting a series of bit or byte streams, can use

electric or optical signal to carry 9*: and 91:

open and close a p!ysical connection

transmitting a series of data bits

management of p!ysical layer

The Da a Lin* La#er

consists of two sub layers, %A (%edium Access ontrol) and77 (7ogical 7in" ontrol)

data lin" establis!ment and release

brea" upper layer data unit, 6ac"et, into

smaller data unit, #rame

$equence ontrol, #low ontrol

rror 0etection, rror ontrol, rror ;ecovery

The Ne )or* La#er

;outing and ;elay


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ation management

7ogin %anagement

one to one mapping for $ession connection and ransport connection

The Presen a ion La#er

responsible for selection of syntax and semantics

0ata ompression, ncoding and 0ecoding

0ata ncryption and 0ecryption

The A-- ica ion La#er

#ile ransfer

?irtual erminal @andling

lectronic %ail


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;emote 7ogin

T/E TCP0IP MODEL

6/ 6 is based on a four layer reference model. All protocols t!at belong to t!e 6/ 6 protocol suite are located in t!e top t!ree layers of t!is

model.

As s!own in t!e following illustration, eac! layer of t!e 6/ 6 model

corresponds to one or more layers of t!e seven layer pen $ystems

nterconnection ( $ ) reference model proposed by t!e nternational

$tandards rgani>ation ( $ ).

!e types of services performed and protocols used at eac! layer

wit!in t!e 6/ 6 model are described in more detail in t!e following table.

La#er Descri- ion Pro oco s

Application 0efines 6/ 6 application protocols

and !ow !ost programs interface wit!

transport layer services to use t!e

networ".

@ 6, elnet, # 6,

# 6, $3%6, 03$,

$% 6, B 'indows,

ot!er application

protocols


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ransport 6rovides communication session

management between !ost computers.

0efines t!e level of service and status

of t!e connection used w!en

transporting data.

6, =06, ; 6

nternet 6ac"ages data into 6 datagrams,

w!ic! contain source and destination

address information t!at is used to

forward t!e datagrams between !osts

and across networ"s. 6erforms routingof 6 datagrams.

6, %6, A;6,

;A;6

3etwor"

interface

$pecifies details of !ow data is

p!ysically sent t!roug! t!e networ",

including !ow bits are electrically

signaled by !ardware devices t!at

interface directly wit! a networ"

medium, suc! as coaxial cable, optical

fiber, or twisted pair copper wire.

t!ernet, o"en

;ing, #00 , B.2-,

#rame ;elay, ;$

2 2, v. -

#or more information about A;6, 6, %6, C%6, =06, and 6

S1NC/RONOUS AND AS1NC/RONOUS TRANSMISSION

$ync!ronous transmission

$ender D receiver !ave identical cloc"s

$eparate cloc" line to carry cloc" signal from sender

9$elf cloc"ing: transmission sc!eme

E +ip!ase encoding (e.g. %anc!ester) F digital sig.

E =sing carrier signal F Analog sig.


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+loc" syncG

$pecial preamble D postamble bit patterns used to

indicate start/end of a bloc" (frame)

Async!ronous transmission

E +its are sent on a c!aracter by c!aracter basis

E ndependent cloc"s at sender D receiver (but matc!ing reasonably)

E loc" syncG receiver resyncHs its cloc" every c!aracter

E +loc" syncG eac! c!aracter is 9brac"eted: by start D stop bits

E AdvantageG simple D c!eap, usable up to 2* "bps

E 0isadvantageG not very efficient

T1PES O. NETWORKS

LANs 2 Loca Area Ne )or*s

ypically connects computer in a single building or campus.

0eveloped in 1 8*s.

%edium G optical fibres, coaxial cables, twisted pair, wireless. 7ow latency (except in !ig! traffic periods).

@ig! speed networ"s (*.2 to 1** %b/sec).

$peeds adequate for most distributed systems

6roblems G %ulti media based applications

ypically buses or rings.

t!ernet, o"en ;ing

MANs 2 Me ro-o i an Area Ne )or*s

Cenerally covers towns and cities (-* "ms)

0eveloped in 1


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,roa3!an3 Signa ing2

1)=ses analog signalling

2)=nidirectional transmission

)#requency division multiplexing is possible)$ignal can travel over long distances before being attenuated

Si%- e4 rans%ission

$implex transmission allows data to travel only in a single, pre specified

direction. An example from everyday life is doorbell t!e signal can go only

from t!e button to t!e c!ime. wo ot!er examples are television and radio

broadcasting. !e simplex standard is relatively uncommon for most types

of computer based telecommunications applicationsL even devices t!at are

designed primarily to receive information, suc! as printers must be able to

communicate ac"nowledgement signals bac" to t!e sender devices.

/a ' 3u- e4 rans%ission

n !alf duplex transmission messages can move in eit!er direction , but only

one way at a time. !e press to tal" radio p!ones used in police cars employt!e !alf duplex standardL only one person can tal" at a time. ften t!e line

between a des"top wor"station and a remote 6= conforms to t!e !alf

duplex patterns as well. f anot!er computer is transmitting to a wor"station,

t!e operator cannot send new messages until t!e ot!er computer finis!es its

message to ac"nowledge an interruption.

.u 3u- e4 rans%ission

#ull duplex transmission wor"s li"e traffic on a busy two way street t!e flow

moves in two directions at t!e same time. #ull duplexing is ideal for

!ardware units t!at need to pass large amounts of data between eac! ot!er as

in mainframe to mainframe communications.

Wire ess A-- ica ion Pro oco
http://www.blurtit.com/q365697.htmlhttp://www.blurtit.com/q365697.htmlhttp://www.blurtit.com/q365697.htmlhttp://www.blurtit.com/q365697.html


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Wire ess A-- ica ion Pro oco (commonly referred to as WAP ) is an

open international standard for application layer networ" communications in

a wireless communication environment. ts main use is to enable access to

t!e %obile 'eb from a mobile p!one or 60A .

A 'A6 browser provides all of t!e basic services of a computer based

web browser but simplified to operate wit!in t!e restrictions of a mobile

p!one, suc! as its smaller view screen. 'A6 sites are websites written in, or

dynamically converted to, '%7 ('ireless %ar"up 7anguage) and accessed

via t!e 'A6 browser.

+efore t!e introduction of 'A6, service providers !ad extremely limited

opportunities to offer interactive data services. nteractive data applications

are required to support now commonplace activities suc! asG

mail by mobile p!one

rac"ing of stoc" mar"et prices

$ports results 3ews !eadlines

%usic downloads
http://en.wikipedia.org/wiki/Open_standardhttp://en.wikipedia.org/wiki/International_standardhttp://en.wikipedia.org/wiki/Standardhttp://en.wikipedia.org/wiki/Application_layerhttp://en.wikipedia.org/wiki/Wireless_communicationhttp://en.wikipedia.org/wiki/Mobile_Webhttp://en.wikipedia.org/wiki/Mobile_phonehttp://en.wikipedia.org/wiki/Personal_digital_assistanthttp://en.wikipedia.org/wiki/Mobile_browserhttp://en.wikipedia.org/wiki/Web_browserhttp://en.wikipedia.org/wiki/Websitehttp://en.wikipedia.org/wiki/Wireless_Markup_Languagehttp://en.wikipedia.org/wiki/Emailhttp://en.wikipedia.org/wiki/Open_standardhttp://en.wikipedia.org/wiki/International_standardhttp://en.wikipedia.org/wiki/Standardhttp://en.wikipedia.org/wiki/Application_layerhttp://en.wikipedia.org/wiki/Wireless_communicationhttp://en.wikipedia.org/wiki/Mobile_Webhttp://en.wikipedia.org/wiki/Mobile_phonehttp://en.wikipedia.org/wiki/Personal_digital_assistanthttp://en.wikipedia.org/wiki/Mobile_browserhttp://en.wikipedia.org/wiki/Web_browserhttp://en.wikipedia.org/wiki/Websitehttp://en.wikipedia.org/wiki/Wireless_Markup_Languagehttp://en.wikipedia.org/wiki/Email


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LOCAL AREA NETWORK (LAN+

!e 7ocal Area 3etwor" (7A3) is by far t!e most common type of

data networ". As t!e name suggests, a 7A3 serves a local area (typically t!e

area of a floor of a building, but in some cases spanning a distance of several

"ilometers). ypical installations are in industrial plants, office buildings,

college or university campuses, or similar locations. ypical data

transmission speeds are one to 1** megabits per second.

A wide variety of 7A3s !ave been built and installed, but a few types

!ave more recently become dominant. !e most widely used 7A3 system ist!e t!ernet system developed by t!e Berox orporation.

ntermediate nodes (i.e. repeaters , bridges and switc!es ) allow 7A3s

to be connected toget!er to form larger 7A3s. A 7A3 may also be

connected to anot!er 7A3 or to 'A3s and %A3s using a Mrouter M.

n summary, a 7A3 is a communications networ", w!ic! isG

local (i.e. one building or group of buildings)

controlled by one administrative aut!ority

assumes ot!er users of t!e 7A3 are trusted

usually !ig! speed and is always s!ared

7A3s allow users to s!are resources on computers wit!in anorganisation, and may be used to provide a (s!ared) access to remote

organisations t!roug! a router connected to a %etropolitan Area 3etwor"

(%A3) or a 'ide Area 3etwor" ('A3) .
http://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/enet.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/repeater.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/bridge.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/wan.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/man.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/router.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/router.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/man.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/man.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/wan.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/enet.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/repeater.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/bridge.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/wan.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/man.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/router.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/router.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/man.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/man.htmlhttp://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/wan.html


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!e components in a 7ocal Area 3etwor" can be connected in a few

ways, w!ic! is call 7A3 topologies. !ere exit basic 7A3 topologiesG

S ar2 All stations are connected by cable (or wireless) to a central point,

suc! as !ub or a switc!. f t!e central node is operating in a broadcast

fas!ion suc! as a @ub, transmission of a frame from one station to t!e node

is retransmitted on all of t!e outgoing lin"s. n t!is case, alt!oug! t!e

arrangement is p!ysically a star, it is logically a bus. n t!e case of t!e

central node acting as switc!, an incoming frame is processed in t!e node

and t!en retransmitted on an outgoing lin" to t!e destination station.

t!ernet protocols (


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t!at developed for cable television. ypically, t!e bandwidt! of a

broadband sys tem is ** to ** %@>. $uc! 7A3s are intended to

support low speed data, video and voice on a single p!ysical medium.

+ands of frequencies are establis!ed for eac! type of service. #or instance,a band mig!t be establis!ed between 1* to 2- %@>. !is band could be

furt!er divided into *** @> subc!annels.

+roadband, t!en, s!ould !e viewed as any ot!er transmission

medium if eac! subc!annel is used by only one user. ransmission

between t!e !ost and user would be encrypted and modulated, t!us t!e

medium is transparent to bot!. f multiple users s!are a subc!annel, t!en t!e

entire sy s tem mus t be ; 0 if processing classified information.

,ASE,AND LA N

+aseband 7A3s use baseband signaling on a single p!ysical

transmission medium. 0at a rates of 1* %bps are ac!ieved between

nodes. =p to 1*** nodes may exist on a 7 A3. $ome nodes may exist as

terminal servers, eac! supporting multiple terminals. $uc! 7A3s also u se

multiple levels of protocol or function layers.

At t!e present time, baseband 7A3s present significant c!allenges

and ris"s in secure applications. !erefore, in order for a baseband 7A3 to

be secure, it mus t !e installed in a 60$ . All users on t!e baseband 7A3

must operate at t!e same security level.

TA6ONOM1 O. MULTIPLE7ACCESS PROTOCOLS


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Ran3o% Access Pro oco s

n random access or contention met!ods, no station is superior to

anot!er station and none is assigned t!e control over anot!er. 3o

station permits, or does not permit, anot!er station to send. At eac!

instance, a station t!at !as data to send uses a procedure defined by

t!e protocol to ma"e a decision on w!et!er or not to send.

random access %A protocol specifiesG

!ow to detect collisions

!ow to recover from collisions (e.g., via delayed

retransmissions)

xamples of random access %A protocolsG

slotted A7 @A

unslotted A7 @A

$%A, $%A/ 0, $%A/ A

A7 @A networ"


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Pure (uns o e3+ ALO/A

unslotted Alo!aG simpler, no sync!roni>ation

w!en frame first arrives

transmit immediately

collision probability increasesG

frame sent at t* collides wit! ot!er frames sent in Nt* 1,t*O1P


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6rocedure for pure A7 @A protocol

CSMA (Carrier Sense Mu i- e Access+

$%AG listen before transmit (basic idea)G


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f c!annel sensed idleG transmit entire frame

f c!annel sensed busy, defer transmission

+e!avior of t!ree persistence met!ods


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#low diagram for t!ree persistence met!ods

Carrier Sense Mu i- e Access Wi h Co ision De ec ion (CSMA0CD+

a carrier sensing sc!eme is used.

a transmitting data station t!at detects anot!er signal w!ile

transmitting a frame , stops transmitting t!at frame, transmits a &am

signal , and t!en waits for a random time interval ("nown as Mbac"off
http://en.wikipedia.org/wiki/Carrier_wavehttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Transmission_(telecommunications)http://en.wikipedia.org/wiki/Frame_(telecommunications)http://en.wikipedia.org/wiki/Jam_signalhttp://en.wikipedia.org/wiki/Jam_signalhttp://en.wikipedia.org/wiki/Timehttp://en.wikipedia.org/wiki/Carrier_wavehttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Transmission_(telecommunications)http://en.wikipedia.org/wiki/Frame_(telecommunications)http://en.wikipedia.org/wiki/Jam_signalhttp://en.wikipedia.org/wiki/Jam_signalhttp://en.wikipedia.org/wiki/Time


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delayM and determined using t!e truncated binary exponential bac"off

algorit!m) before trying to send t!at frame again.

CSMA0CD is a modification of pure arrier $ense %ultiple Access

(CSMA ).

$%A/ 0G carrier sensing, deferral as in $%A

collisions detected wit!in s!ort time

colliding transmissions aborted, reducing c!annel wastage

ollision detectionG

easy in wired 7A3sG measure signal strengt!s, compare transmitted,

received signals

difficult in wireless 7A3sG receiver s!ut off w!ile transmitting

ollision detection is used to improve CSMA performance by

terminating transmission as soon as a collision is detected, and reducing t!e

probability of a second collision on retry.
http://en.wikipedia.org/wiki/Truncated_binary_exponential_backoffhttp://en.wikipedia.org/wiki/Carrier_sense_multiple_accesshttp://en.wikipedia.org/wiki/Collision_(telecommunications)http://en.wikipedia.org/wiki/Truncated_binary_exponential_backoffhttp://en.wikipedia.org/wiki/Carrier_sense_multiple_accesshttp://en.wikipedia.org/wiki/Collision_(telecommunications)


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%et!ods for collision detection are media dependent, but on an

electrical bus suc! as t!ernet, collisions can be detected by comparing

transmitted data wit! received data. f t!ey differ, anot!er transmitter is

overlaying t!e first transmitter5s signal (a collision), and transmissionterminates immediately. A &am signal is sent w!ic! will cause all transmitters

to bac" off by random intervals, reducing t!e probability of a collision w!en

t!e first retry is attempted. $%A/ 0 is a layer 2 protocol in t!e $

model .

ven w!en it !as not!ing to transmit, t!e $%A/ 0 %A sublayer

monitors t!e p!ysical medium for traffic by watc!ing t!e carrier$ense signal

provided by t!e 67$ (6!ysical layer signals to %A layer). '!enever t!e

medium is busy,t!e $%A/ 0 %A defers to t!e passing frame by

delaying any pending transmission of its own. After t!e last bit of t!e
http://en.wikipedia.org/wiki/OSI_seven-layer_modelhttp://en.wikipedia.org/wiki/OSI_seven-layer_modelhttp://en.wikipedia.org/wiki/OSI_seven-layer_modelhttp://en.wikipedia.org/wiki/OSI_seven-layer_model


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2ing and brea"ing secure communication. lassical

cryptanalysis involves an interesting combination of analytical reasoning,

application of mat!ematical tools, pattern finding, patience, determination,

and luc". ryptanalysts are also called attac"ers.

ryptology embraces bot! cryptograp!y and cryptanalysis.

'e can !ave two "inds of encryption G

2. S#%%e ric Ke# Encr#- ion2 !ere is a single "ey w!ic! is s!ared

between t!e two users and t!e same "ey is used for encrypting and

decrypting t!e message.

. Pu! ic Ke# Encr#- ion2 !ere are two "eys wit! eac! user G a public

"ey and a private "ey. !e public "ey of a user is "nown to all but t!e

private "ey is not "nown to anyone except t!e owner of t!e "ey. f a

user encrypts a message in !is private "ey t!en it can be decrypted by

anyone by using t!e sender5s public "ey. o send a message securely,

we encrypt t!e message in t!e public "ey of t!e receiver w!ic! can

only be decrypted by t!e user wit! !is private "ey.


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4*


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MODULE ;


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A-- ica ion La#er is a term used in categori>ing protocols and

met!ods in arc!itectural models of computer networ"ing . +ot! t!e $

model and t!e nternet 6rotocol $uite ( 6/ 6) contain an application layer.

n 6/ 6, t!e Application 7ayer contains all protocols and met!ods t!at fall

into t!e realm of process to process communications via an nternet 6rotocol

( 6) networ" using t!e ransport 7ayer protocols to establis! underlying

!ost to !ost connections.

n t!e $ model, t!e definition of its Application 7ayer is narrower in

scope, distinguis!ing explicitly additional functionality above t!e ransport

7ayer at two additional levelsG $ession 7ayer and 6resentation 7ayer . $

specifies strict modular separation of functionality at t!ese layers and

provides protocol implementations for eac! layer.

Vir ua er%ina

n open systems , a $ir ua er%ina (VT ) is an application service t!atG

1. Allows !ost terminals on a multi user networ" to interact wit! ot!er

!osts regardless of terminal type and c!aracteristics,

2. Allows remote log on by local area networ" managers for t!e purpose

of management,

. Allows users to access information from anot!er !ost processor for

transaction processing,

. $erves as a bac"up facility .

= defines a virtual terminal protocol based on t!e $ application layer

protocols. @owever, t!e virtual terminal protocol is not widely used on t!e

nternet.
http://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/OSI_modelhttp://en.wikipedia.org/wiki/OSI_modelhttp://en.wikipedia.org/wiki/Internet_Protocol_Suitehttp://en.wikipedia.org/wiki/Transport_Layerhttp://en.wikipedia.org/wiki/Session_Layerhttp://en.wikipedia.org/wiki/Presentation_Layerhttp://en.wikipedia.org/wiki/OSI_protocolshttp://en.wikipedia.org/wiki/Open_system_(computing)http://en.wikipedia.org/wiki/Application_servicehttp://en.wikipedia.org/wiki/Server_(computing)http://en.wikipedia.org/wiki/User_(computing)http://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/Computer_terminalhttp://en.wikipedia.org/wiki/Accesshttp://en.wikipedia.org/wiki/Informationhttp://en.wikipedia.org/wiki/Central_processing_unithttp://en.wikipedia.org/wiki/Facilityhttp://en.wikipedia.org/wiki/ITU-Thttp://en.wikipedia.org/wiki/Open_Systems_Interconnectionhttp://en.wikipedia.org/wiki/Application_layerhttp://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/OSI_modelhttp://en.wikipedia.org/wiki/OSI_modelhttp://en.wikipedia.org/wiki/Internet_Protocol_Suitehttp://en.wikipedia.org/wiki/Transport_Layerhttp://en.wikipedia.org/wiki/Session_Layerhttp://en.wikipedia.org/wiki/Presentation_Layerhttp://en.wikipedia.org/wiki/OSI_protocolshttp://en.wikipedia.org/wiki/Open_system_(computing)http://en.wikipedia.org/wiki/Application_servicehttp://en.wikipedia.org/wiki/Server_(computing)http://en.wikipedia.org/wiki/User_(computing)http://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/Computer_terminalhttp://en.wikipedia.org/wiki/Accesshttp://en.wikipedia.org/wiki/Informationhttp://en.wikipedia.org/wiki/Central_processing_unithttp://en.wikipedia.org/wiki/Facilityhttp://en.wikipedia.org/wiki/ITU-Thttp://en.wikipedia.org/wiki/Open_Systems_Interconnectionhttp://en.wikipedia.org/wiki/Application_layer


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0ata transfer connections and t!e data transfer processes t!at use t!em

can be created dynamically w!en needed, but t!e control connection persists

t!roug!out a session. nce t!e control connection disappears, t!e session is

terminated and t!e software at bot! ends terminates all data transfer

processes.

EMAIL (e ec ronic %ai 7 SMTP > MIME > ESMTP +

mail is t!e most widely used application service w!ic! is used by

computer users. t differs from ot!er uses of t!e networ"s as networ"

protocols send pac"ets directly to destinations using timeout and

retransmission for individual segments if no ac" returns. @owever in t!e

case of email t!e system must provide for instances w!en t!e remote

mac!ine or t!e networ" connection !as failed and ta"e some special

action. mail applications involve two aspects

=ser agent( pine, elm etc.)

ransfer agent( sendmail daemon etc.)


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ven a $% 6 based receiver can reply to it. Also if t!ere is an error message

or t!ere is no answer t!en t!e sender uses $% 6.

DELIVER1 PROTOCOLS

!e delivery protocols determine !ow t!e mail is transferred by t!e mail

transfer agent to t!e user agent w!ic! provides an interface for reading

mails.

!ere are "inds

1. 6 6 (6ost ffice 6rotocol)

2. 2. %A6( ntermediate %ail Access 6rotocol)

. .0%$6(0istributive %ail $ystem 6rotocol)

DNS (Do%ain Na%e Ser$ice+

!e internet primarily uses 6 addresses for locating nodes. @owever,

its !umanly not possible for us to "eep trac" of t!e many important nodes as

numbers. Alp!abetical names as we see would be more convenient to

remember t!an t!e numbers as we are more familiar wit! words. @ence, in

t!e c!aotic organi>ation of numbers ( 6 addresses) we would be muc!

relieved if we can use familiar sounding names for nodes on t!e networ".

!ere is also anot!er motivation for 03$. All t!e related information

about a particular networ" (generally maintained by an organi>ation, firm or

university) s!ould be available at one place. !e organi>ation s!ould !ave

complete control over w!at it includes in its networ" and !ow does it

Morgani>eM its networ". %eanw!ile, all t!is information s!ould be available

transparently to t!e outside world.


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4ed cells ( cell relay ) and provides data lin" layer services t!at run

over $ 7ayer 1 p!ysical lin"s. !is differs from ot!er tec!nologies based

on pac"et switc!ed networ"s (suc! as t!e nternet 6rotocol or t!ernet ), inw!ic! variable si>ed packets ("nown as frames w!en referencing 7ayer 2)

are used. A% exposes properties from bot! circuit switc!ed and small

pac"et switc!ed networ"ing, ma"ing it suitable for wide area data

networ"ing as well as real time media transport. A% uses a connection

oriented model and establis!es a virtual circuit between two endpoints

before t!e actual data exc!ange begins.

A% is a core protocol used in t!e $ 3 /$0@ bac"bone of t!e

public switc!ed telep!one networ" .

ATM Pro oco Archi ec ure

!e asynchronous transfer mode (A%) protocol arc!itecture is

designed to support t!e transfer of data wit! a range of guarantees for quality

of service. !e user data is divided into small, fixed lengt! pac"ets, called

cells, and transported over virtual connections. A% operates over !ig! data

rate p!ysical circuits, and t!e simple structure of A% cells allows switc!ing

to be performed in !ardware, w!ic! improves t!e speed and efficiency of

A% switc!es.

#igure s!ows t!e reference model for A%. !e first t!ing to notice is

t!at, as well as layers, t!e model !as planes. !e functions for transferringuser data are located in t!e user planeL t!e functions associated wit! t!e

control of connections are located in t!e control planeL and t!e co ordination

functions associated wit! t!e layers and planes are located in t!e

management planes.
http://wiki/Packet_switchinghttp://wiki/Cell_relayhttp://wiki/Data_link_layerhttp://wiki/OSI_modelhttp://wiki/Internet_Protocolhttp://wiki/Ethernethttp://wiki/Circuit_switchinghttp://wiki/Connection-orientedhttp://wiki/Connection-orientedhttp://wiki/Virtual_circuithttp://wiki/SONEThttp://wiki/Public_switched_telephone_networkhttp://wiki/Packet_switchinghttp://wiki/Cell_relayhttp://wiki/Data_link_layerhttp://wiki/OSI_modelhttp://wiki/Internet_Protocolhttp://wiki/Ethernethttp://wiki/Circuit_switchinghttp://wiki/Connection-orientedhttp://wiki/Connection-orientedhttp://wiki/Virtual_circuithttp://wiki/SONEThttp://wiki/Public_switched_telephone_network


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82

ATM ne )or*

+efore describing t!e functions of t!e t!ree layers in t!e A%

reference model, s!all briefly describe t!e format of A % cells. #igure

s!ows t!e two basic types of cell.

(a+ ATM ce s a he user7ne )or* in er'ace? (!+ ATM ce s a he

ne )or*7no3e in er'ace


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8

ac! A % cell consists of - bytesG t!e !eader is five bytes long and

t!e remaining < bytes (t!e cell payload) carry information from !ig!er

layers. !e only difference between t!e two types of A % cell is t!at t!e

cells at t!e user networ" interface carry a data field for t!e flow control of data from users. !is means t!at only eig!t bits are available for virtual pat!

identifiers, rat!er t!an 12 bits at t!e networ" node interface.

!e virtual connections set up in A % networ"s are identified by t!e

combination of t!e virtual path identifier and virtual channel identifier fields

s!own in #igure 2-. !ese two fields provide a !ierarc!y in t!e numbering

of virtual connections, w!ereby a virtual pat! contains a number of virtualc!annels as is illustrated in #igure 24. An advantage of t!is !ierarc!y is t!at

in some cases t!e switc!ing of A % cells may be based on t!e virtual pat!

identifier alone.

Vir ua -a h an3 $ir ua channe re a ionshi-

!e payload type field identifies t!e type of cell. do not intend to describe

t!e specific types of A% cell, but t!ere are types for carrying user information, signalling information for controlling virtual connections, and

management information. !ere are two basic types of user information cellG

one in w!ic! congestion !as been identified and one in w!ic! it !as not.

!e cell loss priority ( 76) field is a single bitL if t!e bit is * t!at cell

!as a !ig! priority, and if t!e bit is 1 t!e cell !as a low priority. !is


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8-

AAL Ser$ice C asses @ AAL T#-es

#igure s!ows t!e internal components of t!e AA7. !e services

provided by eac! of t!ese components is as followsG

!e $ervice $pecific #unctions ($$ $) sublayer provides additional

functions and mec!anisms t!at may be required for t!e a specific

service. !is layer is not required for all AA7 services, and may be

represented as a 3=77 layer.

!e ommon 6art onvergence 7ayer ( 6 $) operates on t!e

complete AA7 frames, providing !eader and trailer record control as

well as ensuring t!e integrity of delivered information.

!e $egmentation and ;eassembly 7ayer ($A;) converts t!e 6 $

frames to and from A % cell payload information.


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84

As t!e information is transferred t!roug! eac! of t!ese layers it ta"es

on a different format. !e $ervice 0ata =nits ($0=) is t!e representation of

data between two service layersL for example, t!e A % $0= is presented to

t!e $A; layer for AA7 processing. !e nterface 0ata =nit ( 0=) is asubcomponent of t!e $0=L one or more 0=s can contribute to a single

$0=. #inally, t!e familiar term 6rotocol 0ata =nit (60=) is used to

represent data between a sublayer and its supporting sublayer. 'it! t!ese

options, t!e way information is named can become confusing.

AAL Su! a#ers

!e following A % Adaptation 7ayer protocols (AA7s) !ave been

defined for Async!ronous ransfer %ode. !ese protocols are loosely

associated wit! t!e various classes of traffic expected to be carriedG

AAL

$upports constant bit rate, connection oriented, sync!ronous traffic

(e.g., uncompressed voice).

AAL :


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88

0efinition never completed, but was envisioned to be assigned for

variable bit rate, connection oriented, sync!ronous traffic.

AAL 50<

$upports variable bit rate, connection oriented, async!ronous traffic(e.g., B.2- data) or connectionless pac"et data (e.g., $%0$ traffic) wit!

an additional byte !eader in t!e information payload of t!e cell.

AAL ;

$imilar to AA7 / wit! a simplified information !eader sc!emeL t!is

AA7 assumes t!at t!e data is sequential from t!e end user and uses t!e

6 bit to indicate t!e last cell in a transmission xamples of services

t!at use AA7 - are lassic 6 over A %, and 7A3 mulation (7A3 ).

AA7 - is t!e most widely used A % Adaptation 7ayer 6rotocol

SD/0SONET

T $0@ ($ync!ronous 0igital @ierarc!y) and $ 3 ($ync!ronous

ptical 3 wor") are standards for interfacing optical networ"s.

T $imple multiplexing processes

T asy access to various signals in a multiplexed !ig! bit rate signal

T A flexible and efficient way of networ"ing

E 3etwor" 0istributionG Add/0rop capability

E 3etwor" survivabilityG A6$ (Automatic 6rotection $witc!ing)

E raffic ross connectionG capacity management, bandwidt!

management and protection route diversity

T $upport advance 3etwor" %anagement $ystem ( A%D6) E

ver!ead bits for #ault, onfiguration, 6erformance %onitoring,

$ecurity and Accounting management


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8ed interface can support multi vendor interwor"ing,

international connection and many different servicesL i.e. A%, 6

SD/ $s& SONET

T oget!er t!ey are a set of global standards t!at interface equipment

from different vendors. $0@ is basically t!e international version of

$ 3 , and $ 3 can be t!oug!t of as t!e 3ort! American version

of $0@.

T !ere are some slig!t differences between $ 3 and $0@.

T !e main differences are in t!e basic $0@ and $ 3 frame

formats, but $0@ and $ 3 are essentially identical beyond t!e

$ $ signal level. !e base signal for $ 3 is $ $ 1 and t!e base

signal for $0@ is $ % 1. $ $ c is equivalent to $ % 1 and t!e

lower tributaries can be mapped interc!angeably between t!e two

formats from t!at point on.

T n $0@, bot! electrical and optical signals are referred to as $ %

signals.

T n $ 3 , !owever, electrical signals are called $ $ and optical

signals are referred to as .

computer networks module

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