Ch02 (1) (User-PC's Conflicted Copy 2014-10-19) (Imam030821321's Conflicted Copy 2014-10-22) (User-PC's Conflicted Copy 2015-02-11)

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McGraw-Hill © The McGraw-Hill Companies, Inc., 2004

Chapter 2

Network Models


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2-1 LAYERED TASKS2-1 LAYERED TASKS

We use the concept ofWe use the concept of layerslayers in our daily life. As anin our daily life. As anexample, let us consider two friends who communicateexample, let us consider two friends who communicate

through postal mail. The process of sending a letter to athrough postal mail. The process of sending a letter to a

friend would be complex if there were no services friend would be complex if there were no services

available from the post office.available from the post office.

Sender, Receiver, and Carrier

Hierarchy

Topics discussed in this sectionTo

pics discussed in this section


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Figure 2.1 Tasks involved in sending a letter


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2-2 THE OSI MODEL2-2 THE OSI MODEL

!stablished in "#$%, the &nternational 'tandards !stablished in "#$%, the &nternational 'tandards

(rgani)ation * (rgani)ation * &'( &'( + is a multinational body dedicated to + is a multinational body dedicated toworldwide agreement on international standards. An &'(worldwide agreement on international standards. An &'(

standard that covers all aspects of networkstandard that covers all aspects of network

communications is the (pen 'ystems &nterconnectioncommunications is the (pen 'ystems &nterconnection

* * ('& ('& + model. &t was first introduced in the late "#%s. + model. &t was first introduced in the late "#%s.

Layered Architecture

PeertoPeer Processes

!nca"sulation

Topics discussed in this sectionTo

pics discussed in this section


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#S$ is the organi%ation.$S# is the &odel.

Note


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Figure 2.2 'even layers of the ('& model


7/58McGraw-Hill © The McGraw-Hill Companies, Inc., 2004

!ach layer de'ines a 'a&ily o' 'unctions distinct 'ro& those o' the other layers.

(ithin a single &achine, each layer calls u"on the services o' the layer )ust

*elo+ it

et+een &achines, layer x on one machine communicates with layer x on

another &achine.

This co&&unication is governed *y an agreedu"on series o' rules and conventionscalled "rotocols.

The "rocesses on each &achine that co&&unicate at a given layer are called

"eerto"eer "rocesses.

Co&&unication *et+een &achines is there'ore a "eerto"eer "rocess using the

"rotocols a""ro"riate to a given layer.

Layer -, 'or ea&"le, uses the services "rovided *y layer 2 and"rovides services 'or layer /.



Figure 2.- The interaction between layers in the ('& model

device A sends a strea& o' *its to device

0through inter&ediate nodes.



•At the higher layers, co&&unication &ust &ove do+n through the layers on device

A, over to device , and then *ack u" through the layers.

•!ach layer in the sending device adds its o+n in'or&ation to the &essage it receives

'ro& the layer )ust a*ove it and "asses the +hole "ackage to the layer )ust *elo+ it.

•At layer # the entire "ackage is converted to a 'or& that can *e trans&itted to the

receiving device.

• At the receiving &achine, the &essage is un+ra""ed layer *y layer, +ith each "rocess

receiving and re&oving the data &eant 'or it.

•For ea&"le, layer 2 re&oves the data &eant 'or it, then "asses the rest to layer -.

Layer - then re&oves the data &eant 'or it and "asses the rest to layer /, and so on.

PeertoPeer Processes

&nterfaces -etween ayers•The "assing o' the data and net+ork in'or&ation do+n through the layers o' the

sending

device and *ack u" through the layers o' the receiving device is &ade "ossi*le *y an

inter'ace *et+een each "air o' ad)acent layers.

•!ach inter'ace de'ines the in'or&ation and services a layer &ust "rovide 'or the

layer a*ove it



(rgani)ation of the ayers

The seven layers can *e thought o' as *elonging to three su*grou"s

• Layers #, 2, and -, are the net+ork su""ort layers they deal +ith the "hysical

as"ects o' &oving data 'ro& one device to another.

•Layers , 3, and 4session, "resentation, and a""licationcan *e thought o' as the

user su""ort layers5 they allo+ intero"era*ility a&ong unrelated so't+are syste&s

•Layer /, the trans"ort layer, links the t+o su*grou"s and ensures that +hat the

lo+er layers have trans&itted is in a 'or& that the u""er layers can use

6ote The u""er $S# layers are al&ost al+ays i&"le&ented in so't+are5 lo+er layers

are a co&*ination o' hard+are and so't+are, ece"t 'or the "hysical layer, +hich

is &ostly hard+are.

74 &eans the data unit at layer 4, 73 &eans the data unit at layer 3, and so on.At each layer, a header, or "ossi*ly a trailer, can *e added to the data unitthe trailer is added only at layer 2. (hen the 'or&atted data unit "asses through

the "hysical layer 0layer 1, it is changed into an electro&agnetic signal and

trans"orted along a "hysical link.the signal "asses into layer 1 and is trans'or&ed *ack into digital 'or&



Figure 2./ An exchange using the ('& model



!nca"sulation

The conce"t is called encapsulation/ level N 0 " is not aware of which

part of the encapsulated packet is data and which part is the header

or trailer. For level N 0 ", the whole packet coming from level N is

treated as one integral unit



2-3 LAYERS IN THE OSI MODEL2-3 LAYERS IN THE OSI MODEL

&n this section we briefly describe the functions of each &n this section we briefly describe the functions of each

layer in the ('& model.layer in the ('& model.

Physical Layer

7ata Link Layer

6et+ork Layer

Trans"ort Layer

Session Layer

Presentation Layer

A""lication Layer

Topics discussed in this sectionTo pics discussed in this section


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The "hysical layer is also concerned +ith the 'ollo+ing

1. Physical characteristics o' inter'aces and &ediu&. The "hysical layer de'ines the

characteristics o' the inter'ace *et+een the devices and the trans&ission &ediu&.

#t also de'ines the ty"e o' trans&ission &ediu&.

2. Re"resentation o' *its. The "hysical layer data consists o' a strea& o' *its

0se8uence o' $s or 1s +ith no inter"retation. To *e trans&itted, *its &ust *e

encoded into signalselectrical or o"tical. The "hysical layer de'ines the ty"e o'

encoding 0ho+ $s and # s are changed to signals.

-. 7ata rate. The trans&ission ratethe nu&*er o' *its sent each secondis alsode'ined *y the "hysical layer. #n other +ords, the "hysical layer de'ines the

duration o' a *it, +hich is ho+ long it lasts.

/. Synchroni%ation o' *its. The sender and receiver not only &ust use the sa&e *it

rate *ut also &ust *e synchroni%ed at the *it level. #n other +ords, the sender and

the receiver clocks &ust *e synchroni%ed.

. Line con'iguration. The "hysical layer is concerned +ith the connection o' devices

to the &edia. #n a "ointto"oint con'iguration, t+o devices are connected through

a dedicated link. #n a &ulti"oint con'iguration, a link is shared a&ong several

devices.



3. Physical to"ology. The "hysical to"ology de'ines ho+ devices are connected to

&ake a net+ork. 7evices can *e connected *y using a &esh to"ology 0every

device is connected to every other device, a star to"ology 0devices are connected

through a central device, a ring to"ology 0each device is connected to the net,

'or&ing a ring, a *us to"ology 0every device is on a co&&on link, or a hy*ridto"ology 0this is a co&*ination o' t+o or &ore to"ologies.

4. Trans&ission &ode. The "hysical layer also de'ines the direction o' trans&ission

*et+een t+o devices9 si&"le, hal'du"le, or 'ulldu"le. #n si&"le &ode, only

one device can send5 the other can only receive. The si&"le &ode is a one+ayco&&unication. #n the hal'du"le &ode, t+o devices can send and receive, *ut

not at the sa&e ti&e. #n a 'ulldu"le 0or si&"ly du"le &ode, t+o devices can

send and receive at the sa&e ti&e.



The "hysical layer is res"onsi*le 'or &ove&ents o' individual *its 'ro& one ho" 0node to the net.

Note



Figure 2.3 2ata link layer

The data link layer trans'or&s the "hysical layer, a ra+ trans&ission 'acility, to a

relia*le link. #t &akes the "hysical layer a""ear error'ree to the u""er layer0net+ork layer



res"onsi*ilities o' the data link layer include the 'ollo+ing

1. Fra&ing. The data link layer divides the strea& o' *its received 'ro& the net+ork

layer into &anagea*le data units called 'ra&es.

2. Physical addressing. #' 'ra&es are to *e distri*uted to di''erent syste&s on the

net+ork, the data link layer adds a header to the 'ra&e to de'ine the sender and:or

receiver o' the 'ra&e. #' the 'ra&e is intended 'or a syste& outside the sender;s

net+ork, the receiver address is the address o' the device that connects the net+ork to

the net one.

-. Flo+ control. #' the rate at +hich the data are a*sor*ed *y the receiver is less than the

rate at +hich data are "roduced in the sender, the data link layer i&"oses a 'lo+

control &echanis& to avoid over+hel&ing the receiver.

/. !rror control. The data link layer adds relia*ility to the "hysical layer *y adding

&echanis&s to detect and retrans&it da&aged or lost 'ra&es. #t also uses a &echanis&

to recogni%e du"licate 'ra&es. !rror control is nor&ally achieved through a trailer

added to the end o' the 'ra&e.

. Access control. (hen t+o or &ore devices are connected to the sa&e link, datalink

layer "rotocols are necessary to deter&ine +hich device has control over the link at

any given ti&e.



The data link layer is res"onsi*le 'or &oving'ra&es 'ro& one ho" 0node to the net.

Note



Figure 2.4 3op0to0hop deliveryho"toho" 0nodetonode delivery

*y the data link layer.


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As the 'igure sho+s, co&&unication at the data link layer occurs *et+een t+o

ad)acent nodes. To send data 'ro& A to F, three "artial deliveries are &ade. First,

the data link layer at A sends a 'ra&e to the data link layer at 0a router.

Second, the data link layer at sends a ne+ 'ra&e to the data link layer at !.

Finally, the data link layer at ! sends a ne+ 'ra&e to the data link layer at F.

6ote that the 'ra&es that are echanged *et+een the three nodes have di''erent

values in the headers. The 'ra&e 'ro& A to has as the destination address and

A as the source address. The 'ra&e 'ro& to ! has ! as the destination address

and as the source address. The 'ra&e 'ro& ! to F has F as the destination

address and ! as the source address. The values o' the trailers can also *e

di''erent i' error checking includes the header o' the 'ra&e.


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Figure 2.< Network layer

The net+ork layer is res"onsi*le 'or the sourcetodestination delivery o'

a "acket, "ossi*ly across &ulti"le net+orks 0links.

#' t+o syste&s are connected to the sa&e link, there is usually

no need 'or a net+ork layer


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res"onsi*ilities o' the net+ork layer include the 'ollo+ing9

1. Logical addressing. The "hysical addressing i&"le&ented *y the data link layer

handles the addressing "ro*le& locally. #' a "acket "asses the net+ork *oundary,

+e need another addressing syste& to hel" distinguish the source and destination

syste&s. The net+ork layer adds a header to the "acket co&ing 'ro& the u""er

layer that, a&ong other things, includes the logical addresses o' the sender and

receiver.

2. Routing. (hen inde"endent net+orks or links are connected to create

intemetworks 0net+ork o' net+orks or a large net+ork, the connecting devices

0called routers or switches+ route or switch the packets to their final destination. (ne

of the functions o' the net+ork layer is to "rovide this &echanis&.

As the 'igure sho+s, no+ +e need a sourcetodestination delivery. The net+ork layer at

A sends the "acket to the net+ork layer at . (hen the "acket arrives at router , the

router &akes a decision *ased on the 'inal destination 0F o' the "acket. As +e +ill see inlater cha"ters, router uses its routing ta*le to 'ind that the net ho" is router !. The

net+ork layer at , there'ore, sends the "acket to the net+ork layer at !. The net+ork

layer at !, in tu&, sends the "acket to the net+ork layer at F.


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The net+ork layer is res"onsi*le 'or thedelivery o' individual "ackets 'ro&

the source host to the destination host.

Note


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Figure 2.= 'ource0to0destination delivery


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Figure 2.1> Transport layer

The trans"ort layer is res"onsi*le 'or "rocessto"rocess delivery o'

the entire &essage.

The trans"ort layer, on the other hand, ensures that the +hole

&essage arrives intact and in order


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res"onsi*ilities o' the trans"ort layer include the 'ollo+ing9

1. Service"oint addressing. Co&"uters o'ten run several "rogra&s at the sa&e ti&e.

For this reason, sourcetodestination delivery &eans delivery not only 'ro& one

co&"uter to the net *ut also 'ro& a s"eci'ic "rocess 0running "rogra& on one

co&"uter to a s"eci'ic "rocess 0running "rogra& on the other. The trans"ort layerheader &ust there'ore include a ty"e o' address called a service0point address *or port

address+. The network layer gets each packet to the correct co&"uter5 the trans"ort

layer gets the entire &essage to the correct "rocess on that co&"uter.

2. Seg&entation and reasse&*ly. A &essage is divided into trans&itta*le seg&ents,

+ith each seg&ent containing a se8uence nu&*er. These nu&*ers ena*le the

trans"ort layer to reasse&*le the &essage correctly u"on arriving at the destination

and to identi'y and re"lace "ackets that +ere lost in trans&ission.

-. Connection control. The trans"ort layer can *e either connectionless or connection

oriented. A connectionless trans"ort layer treats each seg&ent as an inde"endent

"acket and delivers it to the trans"ort layer at the destination &achine. A connection

oriented trans"ort layer &akes a connection +ith the trans"ort layer at the

destination &achine 'irst *e'ore delivering the "ackets. A'ter all the data are

trans'erred, the connection is ter&inated.

/. Flo+ control. Like the data link layer, the trans"ort layer is res"onsi*le 'or 'lo+

control. Ho+ever, 'lo+ control at this layer is "er'or&ed end to end rather than

across a single link.


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. !rror control. Like the data link layer, the trans"ort layer is res"onsi*le 'or error

control. Ho+ever, error control at this layer is "er'or&ed "rocessto "rocess rather

than across a single link. The sending trans"ort layer &akes sure that the entire

&essage arrives at the receiving trans"ort layer +ithout error 0da&age, loss, or

du"lication. !rror correction is usually achieved through retrans&ission.


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The trans"ort layer is res"onsi*le 'or the deliveryo' a &essage 'ro& one "rocess to another.

Note


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Figure 2.11 4eliable process0to0process delivery of a message


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The trans"ort layer is res"onsi*le 'or the deliveryo' a &essage 'ro& one "rocess to another.

Note


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Figure 2.12 'ession layer is the net+ork dialog controller

#t esta*lishes, &aintains, and synchroni%es the interaction a&ong co&&unicating syste&s.


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S"eci'ic res"onsi*ilities o' the session layer include the 'ollo+ing

1. 7ialog control. The session layer allo+s t+o syste&s to enter into a dialog. #t

allo+s the co&&unication *et+een t+o "rocesses to take "lace in either

hal'du"le 0one +ay at a ti&e or 'ulldu"le 0t+o +ays at a ti&e &ode.

2. Synchroni%ation. The session layer allo+s a "rocess to add check"oints, or

synChroni%ation "oints, to a strea& o' data.

For ea&"le, i' a syste& is sending a 'ile o' 2>>> "ages, it is advisa*le to insert

check"oints a'ter every 1>> "ages to ensure that each 1>>"age unit is received

and ackno+ledged inde"endently. #n this case, i' a crash ha""ens during the

trans&ission o' "age 2-, the only "ages that need to *e resent a'ter syste&

recovery are "ages >1 to 2-. Pages "revious to >1 need not *e resent.

Figure 2.12 illustrates the relationshi" o' the session layer to the trans"ort and

"resentation layers.


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The session layer is res"onsi*le 'or dialogcontrol and synchroni%ation.

Note


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Figure 2.1- 1resentation layer

The "resentation layer is concerned +ith the synta and se&antics o' the

in'or&ation echanged *et+een t+o syste&s.

S"eci'ic res"onsi*ilities o' the "resentation layer include the 'ollo+ing9


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" " " y g

1. Translation. The "rocesses 0running "rogra&s in t+o syste&s are usually

echanging in'or&ation in the 'or& o' character strings, nu&*ers, and so on. The

in'onnation &ust *e changed to *it strea&s *e'ore *eing trans&itted. ecause

di''erent co&"uters use di''erent encoding syste&s, the "resentation layer isres"onsi*le 'or intero"era*ility *et+een these di''erent encoding &ethods. The

"resentation layer at the sender changes the in'or&ation 'ro& its senderde"endent

'or&at into a co&&on 'or&at. The "resentation layer at the receiving &achine

changes the co&&on 'or&at into its receiverde"endent 'or&at.

2. !ncry"tion. To carry sensitive in'or&ation, a syste& &ust *e a*le to ensure "rivacy.!ncry"tion &eans that the sender trans'or&s the original in'or&ation to another

'or& and sends the resulting &essage out over the net+ork. 7ecry"tion reverses the

original "rocess to trans'or& the &essage *ack to its original 'or&.

-. Co&"ression. 7ata co&"ression reduces the nu&*er o' *its contained in the

in'or&ation. 7ata co&"ression *eco&es "articularly i&"ortant in the trans&issiono' &ulti&edia such as tet, audio, and video.


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The "resentation layer is res"onsi*le 'or translation,co&"ression, and encry"tion.

Note


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Figure 2.1/ Application layer The a""lication layer ena*les the user, +hether

hu&an or so't+are, to access the net+ork.

#t "rovides user inter'aces and su""ort 'or services such as electronic &ail, re&ote 'ile access and trans'er, shared

data*ase &anage&ent, and other ty"es o' distri*uted in'or&ation services.

$' the &any a""lication services availa*le, the 'igure sho+s only three9

".5A(( *message0handling services+,

6.5.7 *directory services+, and file transfer, access, and

-. &anage&ent 0FTA?. The user in this ea&"le e&"loys 5A(( to send an e&ail &essage.

S"eci'ic services "rovided *y the a""lication layer include the 'ollo+ing9


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S"eci'ic services "rovided *y the a""lication layer include the 'ollo+ing9

1. 6et+ork virtual ter&inal. A net+ork virtual ter&inal is a so't+are version o' a

"hysical ter&inal, and it allo+s a user to log on to a re&ote host. To do so, the

a""lication creates a so't+are e&ulation o' a ter&inal at the re&ote host. The user;s

co&"uter talks to the so't+are ter&inal +hich, in turn, talks to the host, and vice

versa. The re&ote host *elieves it is co&&unicating +ith one o' its o+n ter&inals and

allo+s the user to log on.

2. File trans'er, access, and &anage&ent. This a""lication allo+s a user to access 'iles in

a re&ote host 0to &ake changes or read data, to retrieve 'iles 'ro& a re&ote

co&"uter 'or use in the local co&"uter, and to &anage or control 'iles in a re&ote

co&"uter locally.

-. ?ail services. This a""lication "rovides the *asis 'or e&ail 'or+arding and storage.

/. 7irectory services. This a""lication "rovides distri*uted data*ase sources and

access 'or glo*al in'or&ation a*out various o*)ects and services.


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The a""lication layer is res"onsi*le 'or"roviding services to the user.

Note


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Figure 2.1 'ummary of layers

2 4 TCP/IP PROTOCOL SUITE2 4 TCP/IP PROTOCOL SUITE


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2-4 TCP/IP PROTOCOL SUITE2-4 TCP/IP PROTOCOL SUITE

The layers in theThe layers in the T819&1 protocol suiteT819&1 protocol suite do not exactlydo not exactly

match those in the ('& model. The original T819&1match those in the ('& model. The original T819&1 protocol suite was defined as having four layers protocol suite was defined as having four layers host0to0host0to0

network network , , internet internet , , transport transport , and , and applicationapplication . 3owever, . 3owever,

when T819&1 is compared to ('&, we can say that thewhen T819&1 is compared to ('&, we can say that the

T819&1 protocol suite is made of five layersT819&1 protocol suite is made of five layers physical physical , ,

data link data link , , network network , , transport transport , and , and applicationapplication . .

Physical and 7ata Link Layers

6et+ork Layer

Trans"ort Layer

A""lication Layer



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Figure 2.13 T819&1 and ('& model

2 5 ADDRESSING2 5 ADDRESSING


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2-5 ADDRESSING2-5 ADDRESSING

:our levels of addresses are used in an internet employing:our levels of addresses are used in an internet employing

the T819&1 protocolsthe T819&1 protocols physical physical , , logical logical , , port port , and , and specificspecific . .

Physical Addresses

Logical Addresses

Port AddressesS"eci'ic Addresses



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Figure 2.14 Addresses in T819&1


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Figure 2.1< 4elationship of layers and addresses in T819&1

! l 6 "


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&n :igure 6."# a node with physical address " sends a frame to a node with physical address ;%. The two nodes

are connected by a link *bus topology AN+. As the

figure shows, the computer with physical address " is

the sender, and the computer with physical address ;% is

the receiver.

!xample 6."


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Figure 2.1= 1hysical addresses

! l 6 6


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As we will see in 8hapter "


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:igure 6.6 shows a part of an internet with two routers

connecting three ANs. !ach device *computer or

router+ has a pair of addresses *logical and physical+ for

each connection. &n this case, each computer is

connected to only one link and therefore has only one

pair of addresses. !ach router, however, is connected to

three networks *only two are shown in the figure+. 'o

each router has three pairs of addresses, one for eachconnection.

!xample 6.<


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Figure 2.2> &1 addresses

! l 6 $


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:igure 6.6" shows two computers communicating viathe &nternet. The sending computer is running three

processes at this time with port addresses a, b, and c. The

receiving computer is running two processes at this time

with port addresses > and k. 1rocess a in the sendingcomputer needs to communicate with process > in the

receiving computer. Note that although physical

addresses change from hop to hop, logical and port

addresses remain the same from the source to

destination.

!xample 6.$


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Figure 2.21 1ort addresses


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The "hysical addresses +ill change 'ro& ho" to ho",*ut the logical addresses usually re&ain the sa&e.

Note

!xample 6 7


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!xample 6.7

As we will see in 8hapter 6


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The "hysical addresses change 'ro& ho" to ho",*ut the logical and "ort addresses usually re&ain the sa&e.

Note

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Ch02 (1) (User-PC's Conflicted Copy 2014-10-19) (Imam030821321's Conflicted Copy 2014-10-22) (User-PC's Conflicted Copy 2015-02-11)