Unit Iiword

8/13/2019 Unit Iiword

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SRI VENKATESWARAA COLLEGE OF TECHNOLOGY

VADAKAL

UNIT II

Medium access CSMA Ethernet Token ring FDDI - Wireless LAN ridges andS!itches

ETHERNET (802.3):

The Ethernet is de"elo#ed in the mid-$%&' () researches at the *ero+ ,alo Alto esearch

Center .,AC/0 the Ethernet is a !orking e+am#le o1 the more general carrier sense2 multi#le

accesses !ith collision detect .CSMA3CD/ local area net!ork technolog)4

The 5carrier sense6 in CSMA3CD means that all the nodes can distinguish (et!een an idle

and a (us) link2 and 5collision detect6 means that all the nodes listens as it transmits and can

there1ore detect !hen a 1rame it is transmitting has inter1ered .collided/ !ith a 1rame transmitted ()

another node4

PHYSICAL PROPERTIES:

An Ethernet segment is im#lemented on a coa+ial ca(le o1 u# to 7''m4 this ca(le is similar to

the t)#e used 1or ca(le T82 e+ce#t that it t)#icall) has an im#edance o1 7' ohms instead o1 ca(leT8s &7 ohms4 9osts connect to an Ethernet segment () ta##ing into it0 ta#s must (e at least :47 m

a#art4

A transcei"er a small de"ice directl) attached to the ta# detects !hen the line is idle and

dri"es the signal !hen the host is transmitting4 It also recei"es incoming signals4 The transcei"er is2

in turn2 connected to an Ethernet ada#tor2 !hich is #lugged into the host4

Multi#le Ethernet segments can (e ;oined together () re#eater4 A re#eater is a de"ice that

1or!ards digital signals2 much like an am#li1ier 1or!ards analog signals4 9o!e"er2 no more than 1ourre#eaters ma) (e #ositioned (et!een an) #air o1 hosts2 meaning that an Ethernet has a total reach o1

onl) :27''m4An Ethernet is limited to su##orting a ma+imum o1 $2':< hosts4 Terminators attached to the

end o1 each segment a(sor( the signal and kee# it 1rom (ouncing (ack and inter1ering !ith trailing

signals4


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STANDARDS:

There are "arious standards o1 Ethernet are2

10Ba!":

The 1irst o1 the #h)sical standards de1ined in the IEEE =':4> model is called $'ase74It is also

kno!n as thick net or thick Ethernet4 A segment o1 the original $'ase7 ca(le can (e u# to 7''m

long4

10Ba!2:

The second im#lementation de1ined () the IEEE=%: series is called $'ase:4It also kno!n as thin-

net2 chea#net2 chea#ernet2 thin!ire Ethernet or thin Ethernet4 In this 5$'6 means the net!ork o#eratesat $' M(#s2 5ase6 re1ers to the 1act that the ca(le is used in a (ase (and s)stem and the 5:6 means

that a gi"en segment can (e no longer than :''m

10Ba!T:

The most #o#ular standard de1ined in the IEEE =':4> series is $'aseT4 It is also kno!n as t!isted#air Ethernet4 The 5T6 stands 1or t!isted #air4 A $'aseT segment is usuall) limited to less than

$''m in length4

ACCESS CONTROL:

This algorithm is commonl) called the Ethernets media access control .MAC/4 It is t)#icall)

im#lemented in hard!are on the net!ork ada#tor4FRA#E FORMAT:

,ream(le allo!s the recei"er to s)nchroni?e !ith the signal4 oth the source and destination hostsare identi1ied !ith a


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gi"en num(er o1 tries and re#orts a transmit error to the host4 Ada#tor t)#icall) retr) u# to $@ times2although the (ack o11 algorithm ca#s n in the a(o"e 1ormula at $'4

TOKEN RINGS (802."):

IB#% Token ing has a nearl) identical IEEE standard2 kno!n as =':47 and the later Fi(er

Distri(uted Data Inter1ace .FDDI/4 esilient ,acket ing .,/is a relati"el) recent technolog) 2 andits corres#onding IEEE standard is kno!n as =':4$&4

A ring net!ork consists o1 a set o1 nodes connected in a ring4 Data al!a)s 1lo!s in a#articular direction around the ring2 !ith each node recei"ing 1rames 1rom its u#stream neigh(or and

then 1or!arding them to its do!nstream neigh(or4 This ring-(ased to#olog) is in contrast to the

Ethernets (us to#olog)4 Like the Ethernet2 ho!e"er2 the ring-(ased to#olog) is "ie!ed as a single

shared medium0 it does not (eha"e as a collection o1 inde#endent #oint to #oint links that ;ust ha##ento (e con1igured in a loo#4 Thus2 a ring net!orks shares t!o ke) 1eatures !ith an Ethernet

First2 it in"ol"es a distri(uted algorithm that controls !hen each node is allo!ed to transmit

and second all nodes t)#icall) see all 1rames2 !ith the node identi1ied in the 1rame header as thedestination sa"ing a co#) o1 the 1rame as it 1lo!s #ast4

The most common earl) 1orms o1 ring net!ork !ere all token rings4 The !ord 5token6 comes

1rom the !a) access to the shared ring is managed4 The idea is that a token2 !hich is reall) ;ust a

s#ecial seBuence o1 (its2 circulates around the ring0 each node recei"es and then 1or!ards the token4

When a node that has a 1rame to transmit sees the token2 it takes the token o11 the ring .i4e it

does not 1or!ard the s#ecial (it #attern / and instead inserts its 1rame into the ring4 Each node along


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the !a) sim#l) 1or!ards the 1rame2 !ith the destination node sa"ing a co#) and 1or!arding themessage onto the ne+t node on the ring4 When the 1rame makes its !a) (ack around to the sender2

this node stri#s its 1rame o11 the ring .rather than continuing to 1or!ard it/ and reinserts the token4In this !a)2 some node do!nstream !ill ha"e the o##ortunit) to transmit a 1rame4 The media

access algorithm is 1air in the sense that as the token circulates around the ring2 each node gets a

chance to transmit4 Nodes are ser"iced in a round-ro(in 1ashion4

The #ro(lem o1 node 1ailure ma) (e addressed () connecting each station into the ring using

an electromechanical rela)4 As long as the station is health)2 the rela) is o#en and the station is

included in the ring4 I1 the station sto#s #ro"iding the #o!er2 the rela) closed and the ring

automaticall) () #asses the station4

Se"eral o1 these rela)s are usuall) #acked into a single (o+2 kno!n as multistation access unit

.MSAG/4 This has the interesting e11ect o1 making a token ring actuall) look more like a star

to#olog)4

TOKEN RING #EDIA ACCESS CONTROL:

The net!ork ada#tor 1or a token ring contains a recei"er and a transmitter4 When a node isneither the source not the destination o1 the data on the ring2 its ada#tor is sim#l) retransmitting the

data that its recei"er recei"es4 When none o1 the stations connected to the ring has an)thing to send2


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the token circulates around the ring4 As it does so2 an) station that has data to send ma) 5sei?e6 thetoken2 that is not retransmit it and (egin sending data4 nce a station has the token2 it is allo!ed to

send one or more #ackets49o! long a gi"en node is allo!ed to hold the token the token holding time .T9T/ The #riorit) o1

the token changes o"er time due to the use o1 three reser"ation (its in the 1rame header4

The =':47 #rotocol #ro"ides a 1orm o1 relia(le deli"er) using : (its in the #acket trailer2 the

A and C (its4 These are (oth ' initiall)4 When a station sees a 1rame 1or !hich it is the intendedreci#ient2 is not 1unctioning or a(sent4 I1 the A (it is set (ut not the C (it2 this im#lies that 1or some

reason2 the destination could not acce#t the 1rame4 Thus2 the 1rame might reasona(l) (e retransmittedlater in the ho#e that (u11er s#ace had (ecome a"aila(le4

The sender can insert the token (ack on to the ring immediatel) 1ollo!ing its 1rame this is

called earl) release or a1ter the 1rame it transmits has gone all the !a) around the ring and (een

remo"ed this is called dela)ed release4

TOKEN RING #AINTENANCE:Each =':47 token ring has one station designated as a monitor4 The monitors ;o( is to ensure

the health o1 the ring ()2 1or e+am#le2 making sure that the token is not lost4 An) station on the ringcan (ecome the monitor2 and there are de1ined #rocedures () !hich the monitor is elected !hen the

ring is 1irst connected or on the 1ailure o1 the current monitor4 A health) monitor #eriodicall)

announces its #resence !ith a s#ecial control message0 i1 a station 1ails to see such a message 1or

some #eriod o1 time2 it !ill assume that the monitor has 1ailed and !ill tr) to (ecome the monitor4The #rocedures 1or electing a monitor are the same !hether the ring has ;ust come u# or the acti"e

monitor has ;ust 1ailed4When a station decides that a ne! monitor is needed2 it transmits a 5claim token6 1rame2

announcing its intent to (ecome the ne! monitor4 I1 that token circulates (ack to the sender2 it can

assume that it is oka) 1or it to (ecome the monitor4 I1 some other station is also tr)ing to (ecome themonitor at the same instant2 the sender might see a claim token message 1rom that other station 1irst4

In this case2 it !ill (e necessar) to (reak the tie using some !ell de1ined rule like 5highest address

!ins64ne res#onsi(ilit) o1 the monitor is to make sure that there is al!a)s a token some!here in

the ring2 either circulating or currentl) held () a station4 It should (e clear that a token ma) "anish

1or se"eral reasons2 such as a (it error2 or a crash on the #art o1 a station that !as holding it4 To detect


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a missing token2 the monitor !atches 1or a #assing token and maintains a timer eBual to the

ma+imum #ossi(le token rotation time4 This inter"al eBuals

Numstation + T9T ingLatenc)Where Numstation is the num(er o1 stations on the ring2 and ingLatenc) is the total #ro#agation

dela) o1 the ring4 I1 the timer e+#ires !ithout the monitor seeing a token2 it creates a ne! one4The monitor also checks 1or corru#ted or or#haned 1rames4 The 1ormer ha"e checksum errors

or in"alid 1ormats2 and !ithout monitor inter"ention2 the) could circulate 1ore"er on the ring4

The monitor drains them o11 the ring (e1ore reinserting the token4 An or#haned 1rame is one

that !as transmitted correctl) onto the ring (ut !hose6 #arent6 died2 that is 2 the sending station !entdo!n (e1ore it could remo"e the 1rame 1rom the ring4 These are detected using another header (it2 the

5monitor6 (it4 This is ' on transmission and set to $ the 1irst time the #acket #asses the monitor4 I1the monitor sees a #acket !ith this (its set2 it kno!s the #acket is going () 1or the second time and it

drains the #acket o11 the ring4

I1 an) station sus#ects a 1ailure on the ring2 it can send a (eacon 1rame to the sus#ect

destination4 ased on ho! 1ar this 1rame gets2 the status o1 the ring can (e esta(lished2 and

mal1unctioning stations can (e ()#assed () the rela)s in the MASG4

FRAME FORMAT:

=':47 use di11erential Manchester encoding4 This 1act is used () the 1rame 1ormat2 !hich uses

5illegal6 Manchester codes in the start and end delimiters4 A1ter the start delimiter comes the accesscontrol ()te2 !hich includes the 1rame #riorit) and the reser"ation #riorit) mentioned a(o"e4 The

1rame control ()te is a demu+ ke) that identi1ies the higher-la)er #rotocol4Similar to the Ethernet2 =':47 addresses are :-(it

CC4 This is 1ollo!ed () the 1rame status ()te2 !hich includes the A and C (its 1or relia(le deli"er)4

FDDI:

Although FDDI is similar to =':47 in man) res#ects2 there are signi1icant di11erences4 For one

FDDI runs on 1i(er2 not co##er4 A more interesting di11erence is that an FDDI net!ork consists o1 a

dual ring t!o de#endent rings that transmit data in o##osite directions2 the second ring is not used

during normal o#eration (ut instead comes into #la) onl) i1 the #rimar) ring 1ails4 That is the ring

loo#s (ack on the secondar) 1i(er to 1orm a com#lete ring2 and as a conseBuence2 an FDDI net!ork

is a(le to tolerate a single (reak in the ca(le or the 1ailure o1 one station4


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Another interesting di11erence is that instead o1 designating one node as a monitor2 all thenodes #artici#ate eBuall) in maintaining the FDDI ring4 Each node maintains an estimate o1 the token

rotation time .TT/ the e+#ected ma+imum time 1or the token to make one com#lete tri# around the

ring4 A node then measures the time (et!een successi"e arri"als o1 the token4 I1 too much time

ela#ses2 suggesting that the token has (een lost2 the node transmits a 5claim 1rame6 in !hich it

includes its current TT estimate4 The claim ser"es t!o 1unctions4 First2 the claim 1rame is "ote 1or a#articular "alue o1 the TT4 An) node that !ants to "ote 1or a shorter TT !ill re#lace that claim1rame !ith its o!n claim 1rame0 other!ise it !ill acce#t the ne! time and 1or!ard the claim 1rame4

Second2 the claim 1rame is a reBuest 1or authori?ation to regenerate the token4 I1 a claim 1rame makes

it all the !a) (ack around to the original sender2 that node kno!s not onl) that the TT it "oted 1or

!as the shortest and has (een acce#ted () all the other nodes2 (ut also that it has (een authori?ed toregenerate the token4

When the token arri"es at a node !ith time to s#are the node can transmit data so long as it

does not make the token 1all (ehind schedule2 other!ise the node cannot transmit data4 A

shortcoming o1 this (asic scheme is that it cannot guarantee an) #articular node the o##ortunit) to

transmit regularl)2 e"en i1 that node has data that is sensiti"e to ;itter2 (ecause an u#stream node

could consume all the a&a'a! time4 To account 1or this #ossi(ilit) FDDI de1ines t!o classes o1tra11ic

S)nchronous

As)nchronous

When a node recei"es a token2 it is al!a)s allo!ed to send s)nchronous data2 !ithout regard

1or !hether the token is earl) or late4 In contrast2 a node can send as)nchronous tra11ic onl) !hen the

token is earl)4WIRELESS:

Wireless technologies di11er in "ariet) o1 dimensions2 most nota(l) in ho! much (and!idththe) #ro"ide and ho! 1ar a#art communicating nodes can (e4 ther im#ortant di11erences include

!hich #art o1 the electromagnetic s#ectrum the) use .including !hether it reBuires a license/ andho! much #o!er the) consume4 Four #rominent !ireless technologies

J lue toothJ Wi-Fi.more 1ormall) kno!n as =':4$$/

J WiMA*.=':4$@/J Third generation or >Kcellular !ireless4


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The most !idel) used !ireless links toda) are usuall) as)mmetric2 that is2 the t!o end#oints areusuall) di11erent kinds o1 nodes4

ASE STATIN2 usuall) has no mo(ilit)2 (ut has a !ired .or at least high (and!idth/connection to the internet or other net!orks4

A 5client node6 is o1ten mo(ile2 and relies on its link to the (ase station 1or all its

communication !ith other nodes4 Wireless communication naturall) su##orts #oint to multi#oint

communication2 (ecause radio !a"es sent () one de"ice can (e simultaneousl) recei"ed () man)de"ices4 9o!e"er2 it is o1ten use1ul to create a #oint to #oint link a(straction 1or higher la)er

#rotocols4This to#olog) im#lies three Bualitati"el) di11erent le"els o1 mo(ilit)4 The 1irst le"el is no

mo(ilit)2 such as !hen a recei"er must (e in a 1i+ed location to recei"e a directional transmission1rom the (ase station2 as is the case !ith the initial "ersion o1 WiMA*4 The second le"el is mo(ilit)

!ithin the range o1 a (ase2 as is the case !ith luetooth4 The third le"el is mo(ilit) (et!een (ases2 asis the case !ith cell #hones and Wi-Fi4

WI*FI (802.11):

This section takes a closer look at a s#eci1ic technolog) centered on the emerging IEEE

=':4$$ standard2 also kno!n as Wi-Fi4 Wi-Fi is technicall) a trademark2 o!ned () a trade grou#

called the Wi-Fi alliance that certi1ies #roduct com#liance !ith =':4$$4 =':4$$ is designed 1or use ina limited geogra#hical area .homes2 o11ice (uildings2 cam#uses/ and its #rimaril) challenge is to

mediate access to a shared communication medium in this case2 signals #ro#agating through s#ace4

PHYSICAL PROPERTIES:

=':4$$ run o"er si+ di11erent #h)sical la)er #rotocols4 Fi"e are (ased on s#read s#ectrum

radio2 and one on di11used in1rared .and is o1 historical interest onl) at this #oint/4 The 1astest runs ata ma+imum o1 7< M(#s4

The original =':4$$ standard de1ined t!o radio (ased #h)sical la)ers standards2 one using1reBuenc) ho##ing and the other using direct seBuence4 oth #ro"ide u# to : M(#s4 Then #h)sical

la)er standard =':4$$ ( !as added4 Gsing a "ariant o1 direct e+em#t :4


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A related #ro(lem called the e+#osed node #ro(lem !here each o1 the 1our nodes is a(le to

send and recei"e signals that reach ;ust the nodes to its immediate le1t and right4 For E* a can

e+change 1rames !ith A and C (ut it cannot reach D 2 !hile C can reach and D (ut not A4 Su##ose

is sending to A4 Node C is a!are o1 this communication (ecause it hear s transmission4 It !ould

(e a mistake2 ho!e"er2 1or C to conclude that it cannot transmit to an)one ;ust (ecause it can hear

s transmission4 For e+am#le2 su##ose C !ants to transmit to node D4 This is not a #ro(lem since

Cs transmission to D !ill not inter1ere !ith As a(ilit) to recei"e 1rom 4

=':4$$ addresses these t!o #ro(lems !ith an algorithm called multi#le access !ith collision

a"oidance .MACA/4 The idea is 1or the sender and recei"er ot e+change control 1rames !ith each

other (e1ore the sender actuall) transmits an) data4 This e+change in1orms all near() nodes that a

transmission is a(out to (egin4 S#eci1icall)2 the sender transmits a Request to send .TS/ 1rame tothe recei"er0 the TS 1rame includes a 1ield that indicates ho! long the sender !ants to hold the

medium4 The recei"er then re#lies !ith a clear to send (CTS) 1rame4 This 1rame echoes this length1ield (ack to the sender4 An) node that sees the TS 1rame !ill collide !ith each other4

=':4$$ does not su##ort collision detection2 (ut instead the senders reali?e the in !hich casethe) each !ait a random amount o1 time (e1ore tr)ing again4 The amount o1 time a gi"en node dela)

is de1ined () the same e+#onential (acko11 algorithm used on the Ethernet4

DISTRIB$TION SYSTE#

Instead o1 all nodes created eBual2 some nodes are allo!ed to roam and some are connected

to a !ired net!ork in1rastructure4 =':4$$ calls these (ase stations access points (AP)2 and the) are

connected to each other () a so-called distribution system. A distri(ution s)stem that connects three


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access #oints2 each o1 !hich ser"ices the nodes in some region4 Although t!o nodes can

communicate directl) !ith each other i1 the) are !ithin reach o1 each other2 the idea (ehind this

con1iguration is that each node associates itsel1 !ith one access #oint4 For node A to communicate

!ith node E2 1or e+am#le2 A 1irst sends a 1rame to its access #oint .A,-$/2 !hich 1or!ards the 1rame

across the distri(ution s)stem to A,-> 2 !hich 1inall) transmits the 1rame to E4

The techniBue 1or selecting an A, is called scanning and in"ol"es the 1ollo!ing 1our ste#s

$4 The node sends a #ro(e 1rame0:4 All A,s !ithin reach re#l) !ith a #ro(e es#onse 1rames0

>4 The node selects one o1 the access #oints2 and sends that A, an Association eBuest 1rames0

:-(it CC4 The Control 1ield contains three su(1ields o1 interest a @-


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(it T)#e 1ield that indicates !hether the 1rame carries data2 is an TS or CTS 1rame2 or is (eing used

() the scanning algorithm0 and a #air o1 $-(it 1ields-called ToDS and 4

The =':4$$ 1rame 1ormat is that it contains 1our2 rather than t!o2 address4 ho! these addressare inter#reted de#ends on the settings o1 the ToDS and FromDS (its in the 1rames Control 1ield4

This is to account 1or the #ossi(ilit) that the 1rame had to (e 1or!arded across the distri(utions)stems2 !hich !ould mean that the original sender is not necessaril) the same as the most recent

transmitting node4Similar reasoning a##lies to the destination address4 In the sim#lest case2 !hen one node is

sending directl) to another2 the DS (its are '2 Addr$ identi1ies the target node2 and Addr: identi1ies

the source node4

In the most com#le+ case2 (oth DS (its are set to $2 indicating that the message !ent 1rom a

!ireless node onto the distri(ution s)stem and then 1rom the distri(ution s)stem to another !ireless

node4 With (oth (its set2 Addr$ identi1ies the ultimate destination2 Addr: identi1ies the immediate

sender .the one that 1or!arded the 1rame 1rom the distri(ution s)stem to the ultimate destination/2

Addr> identi1ies the intermediate destination .the one that acce#ted the 1rame 1rom a !ireless node

and 1or!arded it across the distri(ution s)stem/2 and Addr< identi1ies the original source4 In terms o1the e+am#le gi"en in 1ig42 Addr$ corres#onds to E2 Addr: identi1ies A,->2 Addr> corres#onds to A,-

$2 and Addr< identi1ies A4

CONNECTING DEVICES:

Net!orking and internet!orking de"ices are classi1ied into 1our categories re#eaters2

(ridges2 routers2 and gate!a)s4


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BRIDGES AND LAN SWITCHES:

It is a node that 1or!ard 1rames 1rom one Ethernet to the other4 This node !ould (e in

#romiscuous mode2 acce#ting all 1rames transmitted on either o1 the Ethernets2 so it could 1or!ard

them to the other4 A (ridge is connected (et!een t!o LANs !ith #ort4 ) using the #ort num(er the

LANs are addressed4 Connected LANs are kno!n as e+tended LAN

LEARNING BRIDGES:ridges maintains a 1or!arding ta(le !hich contains each host !ith their #ort num(er4

9a"ing a human maintain this ta(le is Buite a (urden2 so a (ridge can learn this in1ormation 1or itsel14

The idea is 1or each (ridge to ins#ect the source address in all the 1rames it recei"es4 When a (ridge

1irst (oots2 this ta(le is em#t)0 entries are added o"er time4 Also a timeout is associated !ith each

entr) and the (ridge is cards the entr) a1ter a s#eci1ied #eriod o1 time4

SPANNING TREE ALGORITH#

I1 the e+tended LAN is ha"ing loo#s then the 1rames #otentiall) loo# through the e+tended

LAN 1ore"er4 There are t!o reasons to an e+tended LAN to ha"e a loo# in it4 ne #ossi(ilit) is that

the net!ork is managed () more than one administrator0 no single #erson kno!s the entirecon1iguration o1 the net!ork4 Second2 loo#s are (uilt in to net!ork on #ur#ose to #ro"ide redundanc)

in case o1 1ailure4 ridges must (e a(le to correctl) handle loo#s4 This #ro(lem is addressed ()

ha"ing the (ridges run a distri(uted s#anning tree algorithm4


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The s#anning tree algorithm !ad de"elo#ed () Digital EBui#ment Cor#oration4 The main idea is 1or

the (ridges to select the #orts o"er !hich the) !ill 1or!ard 1rames4 The algorithm selects as 1ollo!s4

Each (ridge has a uniBue identi1ier4 In the a(o"e e+am#le the) are la(eled as $2 :2 > the

algorithm 1irst elects the (ridge !ith smallest ID as the root o1 the s#anning tree4 The root (ridgeal!a)s 1or!ards 1rames out o"er all o1 its #orts4 Then each (ridge com#utes the shortest #ath to root

and notes !hich o1 its #orts is on this #ath4 This #ort is also elected as the (ridges #re1erred #ath tothe root4 Finall)2 all the (ridges connected to a gi"en LAN elect a single designated (ridge that !ill

(e res#onsi(le 1or 1or!arding 1rames to!ard the root (ridge4 Each LANs designated (ridge is the onethat is closest to the root2 and i1 t!o or more (ridges are eBuall) close to the root2 then the (ridge

!hich ha"ing smallest ID !ins4

In the a(o"e e+am#le2 $ is the root (ridge since it ha"ing the smallest ID4 oth > and 7are connected to LAN A2 (ut 7 is the designated (ridge since it is closer to the root4 Similarl) 7

and & are connected to LAN 2 (ut 7 is the designated (ridge e"en the) are eBuall) closer to the

root since 7 ha"ing smallest ID4


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Unit Iiword

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