6. physical layer-switching

7/23/2019 6. Physical Layer-Switching

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PhysicalLayer SwitchingNetwork

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SwitchingNetwork

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AnetworkisasetofconnecteddevicesWehavemultipledevicesowtoconnectthemtomakeone!to!one

communicationpossi"leSolution#$singtopology

%ut it is impractical and wasteful whenapplied to very large

networkA"ettersolutionisswitchingA switched networkconsistsof a series of

interlinked nodes&

calledswitches

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SimpleSwitchedNetwork

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(

The devices attached to the network may be referred to as

stations or end nodes. The stations may be computers,

terminals, telephones, or other communicating devices

The switching devices whose purpose is to provide

communication refer as nodes or switch nodes.

The switching nodes are not concerned with the content of

the data

They provide a switching facility that will move the data

from node to node until they reach their destination.


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SwitchingNetwork

(

)hreedi*erentswitchingtechnologies+ircuitswitching,essageswitching

Packetswitching


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+ircuitSwitching

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.edicatedcommunicationpath"etweentwostationsaconnectioncalledacircuitissetup"etween

twodevices&whichisusedforthewholecommunication/

That path is a connected se!uence of links between network nodes"n each physical link a logical channel is dedicated to connection

)hreephasesSetupPhase0sta"lishdedicatedchannel

"etweenthedevices.ata)ransferPhase)eardownPhase0.isconnect


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+ircuitSwitching

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+ircuitSwitching

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5i6ed "andwidth7uaranteed capacity8ne9cient+hannelcapacitydedicatedfor

durationofconnection8fnodata&capacitywasted

Setup0connectiontakestime:nceconnected&transferis

transparent.evelopedforvoicetra9c0phone

also used for digital dataNot suita"le for "rusty tra9c


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6ample

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#ircuit switching was developed to handle voice traffic but is now also used for

data traffic. The best$known example of a circuit$switching network is the

public telephone network

Subscribers:The devices that attach to the network, typically telephones, but

the percentage of data traffic increases year by year.

Subscriber line:The link between the subscriber and the network, also

referred to as thesubscriber loopor local loop, mostly using twisted$pair wire.

Exchanges:The switching centers in the network. % switching center that

directly supports subscribers is known as an end office.

Trunks:The branches between exchanges. Trunks carry multiple voice$

fre!uency circuits using either FD or synchronous TD

&ubscribers connect directly to an end office, which switches traffic between

subscribers and between a subscriber and other exchanges

The other exchanges are responsible for routing and switching traffic between

end offices

If two subscribers connect to different end offices, a circuit between themconsists of a chain of circuits through one or more intermediate offices.


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Public CircuitSwitched

Network

Circuit Establishment


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,essageSwitching

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8ntelecommunications&messageswitchingwastheprecursorofpacketswitching&wheremessageswereroutedintheirentirety&onehopatatime/

Nophysicalpathisesta"lishedinadvance"etweensenderandreceiver/

Whenthesenderhasa"lockofdatato"esent&itisstoredinthe;rstswitchingo9ce0i/e/&

routerandthenforwardedlater&onehopatatime

ach"lockisreceivedinitsentirety&inspected

forerrors&andthenretransmitted

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,essageSwitching

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8f message is large it monopoli=es the link%etter way to divide the message into

smaller parts

1>


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Packet Switching


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Advantages

line e9ciency single link shared "y many packets over time

packets


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.atagram Swithcing

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In the datagramapproach, each packet is treatedindependently, with no reference to packets that have

gone before&o the packets, each with the same destination address,

do not all follow the same route, and they may arriveout of se!uence at the exit point

The exit node restores the packets to their original

order before delivering them to the destination. In some

datagram networks, it is up to the destination ratherthan the exit node to do the reordering.

If a packet$switching node crashes momentarily, all of

its !ueued packets may be lost. %gain, it is up to either

the exit node or the destination to detect the loss of a


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.atagram

.iagram


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?irtual circuit approach

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In the virtual circuitapproach, a preplanned route isestablished before any packets are sent

% transmitted packet is buffered at each node, and

!ueued for output over a line, while other packets on

other virtual circuits may share the use of the lineThe difference from the datagram approach is that,

with virtual circuits, the node need not make a routing

decision for each packet. It is made only once for all

packets using that virtual circuit.


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?irtual+ircuit

.iagram


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?irtual +ircuits v .atagram

virtual circuitsnetwork can provide se


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Packet

Si=e


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+ircuit v Packet Switchingperformance depends on various delayspropagation delaytransmission timenode delay

range of other characteristics& including#transparencyamount of overhead


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,essageSwitching

22 0a +ircuit Switching 0" ,essage Switching 0c Packet14


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2


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%locking or Non!"locking"locking networkmay "e una"le to connect stations "ecause

all paths are in useused on voice systems

non!"locking networkpermits all stations to connect at onceused for some data connections


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2'

+ircuit switching can use either of twotechnologies# the space!division switch orthe time!division switch

8n space!division switching& the paths in the

circuit are separated from one anotherspatially0Physically

A cross"ar switch connects n inputs to moutputs in a grid, using electronic micro

switches 0transistors at each cross point)he maBor limitation of this design is the

num"er of cross points re


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8-4 STRUCTURE OF

S!"TC#

Crossbar switch with three inputs and four outputs


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We divide the N input lines into groups,each of n lines. For each group, we use onecross"ar of si=e n x k, where k is thenumber of crossbars in the middle stage.

8n other words& the ;rst stage has N/ncrossbars of n x k crosspoints.

We use k crossbars, each of size (N/n) x(N/n) in the middle stage.

We use N/n crossbars, each of size k x n atthe third stage.

)he multistage switch has one draw"ack!"locking during periods of heavy tra9c

+los investigated the condition of

Multistage switch


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Multistage switch

"n a three-stage s$itch% the t&tal number &' cr&ss(&ints is

)k* + k,*n.)

$hich is much smaller than the number &' cr&ss(&ints in a

single-stage s$itch ,*)./

#$a%ple


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Design a three-stage, 200 200 switch (N = 200) with

= ! and n = 20"

#$a%ple

#$a%ple


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Design a three-stage, 200 200 switch (N = 200) with

= ! and n = 20"

&olution

'n the first stage we hae Nn or *0 crossbars, each of si+e 20 !"

'n the second stage, we hae ! crossbars, each of si+e *0 *0" 'n

the third stage, we hae *0 crossbars, each of si+e ! 20" he total

nu%ber of crosspoints is 2N (Nn)2, or 2000crosspoints" hisis . percent of the nu%ber of crosspoints in a single-stage switch

(200 200 = !0,000)"

#$a%ple

cc&rding t& the Cl&s criteri&n:n0 ,*).1)

2 )n3 1

Cr&ss(&ints 4* 5,)*.1)

3 16


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/edesign the preious three-stage, 200 200 switch, using the Clos

criteria with a %ini%u% nu%ber of crosspoints"

&olution

e let n = (2002)*2, or n = *0" e calculate = 2n 1 * = *" 'n the

first stage, we hae 200*0, or 20, crossbars, each with *0 *

crosspoints" 'n the second stage, we hae * crossbars, each with

*0 *0 crosspoints" 'n the third stage, we hae 20 crossbars each

with * *0 crosspoints" he total nu%ber of crosspoints is 20(*0

*) *(*0 *0) 20(* *0) = .00"

i%e-slot interchange)i .i i i S it hi


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i%e slot interchange

modern digital systems use intelligent control ofspace C time division elements

use digital time division techni


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)he advantage of space!division switchingis that it is instantaneous/ 8ts disadvantageis the num"er of crosspoints re


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i%e space ti%e switch


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'

A switch used in a packet!switched networkhas a di*erent structure from a switch usedin a circuit!switched network/

A packet switch has four components# input

ports& output ports& the routing processor&and the switching fa"ric/

An input port performs the physical anddata link functions of the packet switch like

"its are constructed from the received signal/packet is decapsulated from the frameerrors are detected and correctedthe packet and sent to the switching fa"ric/


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)he output port performs the samefunctions as the input port& "ut in thereverse order

Switching fa"ric to move the packet from

the input


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3acet switch co%ponents


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'nput port

4utput port


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A "anyan switch is a multistage switch withmicroswitches at each stage that route thepackets "ased on the output portrepresented as a "inary string/

)he pro"lem with the "anyan switch is thepossi"ility of internal collision

)his pro"lem can solve "y sorting thearriving packets "ased on their destination

port/


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5 ban6an switch


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#$a%ples of routing in a ban6an switch


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)hankEou

6. physical layer-switching

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