Ch02 - Network Models

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2.1

Chapter 2Network Models

Copyright The McGraw-Hill Companies, Inc. Permission required or reproduction or display.


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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 servicesfriend 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 section:To

pics discussed in this section:


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2.3

Figure 2.1 Tasks involved in sending a letter

Sender SideReceiver SideOn the Way

Hierarchy must e managed to manage !lo" o! services.


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#SO is the organi$ation.OS# is the model.

Note


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2-2 THE OSI MODEL2-2 THE OSI MODEL

Established in !"#, the $nternational %tandardsEstablished 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 networ*standard that covers all aspects of networ*

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.

%ayered &rchitecture

'eer(to('eer 'rocesses

)nca*sulation

Topics discussed in this section:To

pics discussed in this section:


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2.6

Figure 2.2 %even layers of the &%$ model


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Figure 2.+ The interaction between layers in the &%$ model


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Figure 2.2 %even layers of the &%$ model

et"ork Su**ort

-ser Su**ort


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Figure 2. An exchange using the &%$ model


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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.

'hysical %ayer

/ata %ink %ayer

et"ork %ayer

Trans*ort %ayer

Session %ayer

'resentation %ayer

&**lication %ayer

Topics discussed in this section:Topics discussed in this section:


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The *hysical layer is res*onsile !or movements o!

individual its !rom one ho* 0node to the net.

Note


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Figure 2.3 hysical layer

4a5or Concerns6Co ordination "ith

!unctions to carry a it

stream4echanical 7 )lectrical

S*eci!icationsTransmission 'rocedure

Res*onsiilities6'hysical Characteristics o! medium 7 /eviceRe*resentation o! its

/ata RateSynchroni$ation o! its%ine Con!igurations'hysical To*ologyTransmission 4ode


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The data link layer is res*onsile !or moving!rames !rom one ho* 0node to the net.

Note


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Figure 2.8 -ata lin* layer

Res*onsiilities6Framing'hysical &ddressing

Flo" Control)rror Control&ccess Control 04ulti*leing etc


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Figure 2.9 op/to/hop delivery


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The net"ork layer is res*onsile !or thedelivery o! individual *ackets !rom

the source host to the destination host.

Note


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Figure 2.: Networ* layer

Res*onsiilities6%ogical &ddressingRouting


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Figure 2.; %ource/to/destination delivery


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The trans*ort layer is res*onsile !or the deliveryo! a message !rom one *rocess to another.

Note


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Figure 2.1< Transport layer

Res*onsile !or6

Service 'oint &ddressing

Segmentation 7 ReassemlyConnection Control

Flo" Control

)rror Control


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Figure 2.11 0eliable process/to/process delivery of a message


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The session layer is res*onsile !or dialogcontrol and synchroni$ation.

Note


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Figure 2.12 %ession layer


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The *resentation layer is res*onsile !or translation,com*ression, and encry*tion.

Note


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Figure 2.1+ resentation layer


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The a**lication layer is res*onsile !or*roviding services to the user.

Note


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Figure 2.1 Application layer

Services6

et"ork =irtual TerminalFile Trans!er &ccess 7 4anagement

4ail Services

/irectory Services


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Figure 2.13 %ummary of layers


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

The layers in theThe layers in the T12$ protocol suiteT12$ protocol suite do not exactlydo not exactly

match those in the &%$ model. The original T12$match those in the &%$ model. The original T12$protocol suite was defined as having four layers:protocol suite was defined as having four layers: host/to/host/to/

networ*networ*,, internetinternet,, transporttransport, and, and applicationapplication. owever,. owever,

when T12$ is compared to &%$, we can say that thewhen T12$ is compared to &%$, we can say that the

T12$ protocol suite is made of five layers:T12$ protocol suite is made of five layers: physicalphysical,,

data lin*data lin*,, networ*networ*,, transporttransport, and, and applicationapplication..

'hysical and /ata %ink %ayers

et"ork %ayer

Trans*ort %ayer

&**lication %ayer



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Figure 2.18 T12$ and &%$ model

& net"ork in a T1$$ internetwor* can e a local(area net"ork or a "ide(area net"ork.


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Figure 2.18 T12$ and &%$ model

$nternetwor*ing rotocol ($)

#' *rovides no error checking or tracking

#' does not kee* track o! the routes and has no !acility !or reordering datagrams

once they arrive at their destination


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Internet Protocol

Address Resolution Protocol (ARP)

Reverse Address Resolution Protocol (RARP)

Internet Control Message Protocol (ICMP)

Internet Group Message Protocol (IGMP)

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2 5 ADDRESSING


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2-5 ADDRESSING2-5 ADDRESSING

3our levels of addresses are used in an internet employing3our levels of addresses are used in an internet employing

the T12$ protocols:the T12$ protocols:physicalphysical,, logicallogical,,portport, and, and specificspecific..

'hysical &ddresses

%ogical &ddresses

'ort &ddressesS*eci!ic &ddresses



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Figure 2.19 Addresses in T12$


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Figure 2.1: 0elationship of layers and addresses in T12$


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$n 3igure 4.! a node with physical address + sends aframe to a node with physical address 5#. The two nodes

are connected by a lin* (bus topology 6AN). As the

figure shows, the computer with physical address + is

the sender, and the computer with physical address 5#is

the receiver.

Example 4.


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Figure 2.1; hysical addresses


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As we will see in 1hapter 7, most local/area networ*suse a "5/bit (8/byte) physical address written as 4

hexadecimal digits9 every byte (4 hexadecimal digits) is

separated by a colon, as shown below:

Example 4.4


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3igure 4.4+ shows a part of an internet with two routers

connecting three 6ANs. Each 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 lin* and therefore has only one

pair of addresses. Each router, however, is connected to

three networ*s (only two are shown in the figure). %o

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

Example 4.7


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Figure 2.2< $ addresses


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3igure 4.4 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 *. rocess 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.

Example 4."


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Figure 2.21 ort addresses


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The *hysical addresses "ill change !rom ho* to ho*,ut the logical addresses usually remain the same.

Note


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Example 4.;

As we will see in 1hapter 47, a port address is a 8/bitaddress represented by one decimal number as shown.

93+

& 18(it *ort address re*resented

as one single numer.


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The *hysical addresses change !rom ho* to ho*,ut the logical and *ort addresses usually remain the same.

Note

Ch02 - Network Models

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