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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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2.2
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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2.4
#SO is the organi$ation.OS# is the model.
Note
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2.5
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$ation o! its%ine Con!igurations'hysical To*ologyTransmission 4ode
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The data link layer is res*onsile !or moving!rames !rom one ho* 0node to the net.
Note
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Figure 2.8 -ata lin* layer
Res*onsiilities6Framing'hysical &ddressing
Flo" Control)rror Control&ccess Control 04ulti*leing etc
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Figure 2.9 op/to/hop delivery
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The net"ork layer is res*onsile !or thedelivery o! individual *ackets !rom
the source host to the destination host.
Note
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Figure 2.: Networ* layer
Res*onsiilities6%ogical &ddressingRouting
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Figure 2.; %ource/to/destination delivery
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The trans*ort layer is res*onsile !or the deliveryo! a message !rom one *rocess to another.
Note
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2.20
Figure 2.1< Transport layer
Res*onsile !or6
Service 'oint &ddressing
Segmentation 7 ReassemlyConnection Control
Flo" Control
)rror Control
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2.21
Figure 2.11 0eliable process/to/process delivery of a message
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2.22
The session layer is res*onsile !or dialogcontrol and synchroni$ation.
Note
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2.23
Figure 2.12 %ession layer
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2.24
The *resentation layer is res*onsile !or translation,com*ression, and encry*tion.
Note
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2.25
Figure 2.1+ resentation layer
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2.26
The a**lication layer is res*onsile !or*roviding services to the user.
Note
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2.27
Figure 2.1 Application layer
Services6
et"ork =irtual TerminalFile Trans!er &ccess 7 4anagement
4ail Services
/irectory Services
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2.28
Figure 2.13 %ummary of layers
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2.29
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
Topics discussed in this section:Topics discussed in this section:
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2.30
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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2.31
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)
2.32
2 5 ADDRESSING
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2.33
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
Topics discussed in this section:Topics discussed in this section:
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2.34
Figure 2.19 Addresses in T12$
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2.35
Figure 2.1: 0elationship of layers and addresses in T12$
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2.36
$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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2.37
Figure 2.1; hysical addresses
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2.38
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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2.39
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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2.40
Figure 2.2< $ addresses
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2.41
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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2.42
Figure 2.21 ort addresses
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2.43
The *hysical addresses "ill change !rom ho* to ho*,ut the logical addresses usually remain the same.
Note
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2.44
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