Switching & IEEE Standard

8/14/2019 Switching & IEEE Standard

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SWITCHING & IEEE

STANDARD

By:

Er.Amit Mahajan


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SWITCHING

A switched network consists of a series of interlinked

nodes,called switches.

They are capable of creating temporary connections between

two or more devices linked to the switch but not to each other.


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Three methods of switching:

Circuit switching

Packet switchingMessage switching


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Cont

In this fig the network has switches that

allow traffic from sources to destinations.

A source & destination can be a

computer,router,bridge,or any other

device that connects other networks.


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Packet switching

For data communication,packet switchingnetworks were designed; data arepacketized and sent packet by packet.

The main difference between a circuitswitched & packet switched network isthat the links are shared,channelizedbetween different communication paths.

A link between switch 1 & 2 may carryseveral packets at the the same time,eachsent by a different source & going todifferent destinations.


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Cont

Packet switching uses two approaches:

1. The datagram approach

2. The virtual circuit approach

The datagram approach is mostly used in the

network layer.

The virtual circuit approach is a data linktechnology.


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Message switching

In this no physical copper path is established inadvance between sender & receiver.

Instead, when the sender has a block of data to

be sent,it is stored in the first switchingoffice(i.e,router) & then forwarded later.

Each block is received in its entirety,inspectedfor errors,& then transmitted.

With message switching, there is no limit onblock size,which means router must have disksto buffer long blocks.


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IEEE STANDARD

The standards are divided into parts.The 802.1 standard gives an introduction

to the set of standards & defines theinterface primitives.

The 802.2 standard describes the upperpart of the data link layer.

The parts 802.3 through 802.5 describes

the 3 lan standards,the CSMA/CD,tokenbus & token ring standards respectively.

Each standard covers the physical layer &

MAC sublayer protocol.


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IEEE Standard 802.3

This standard is for a 1-persistant

CSMA/CD lan.

E.g: when a station wants to transmit,it

listen to the cable.if the cable is busy,the

station waits until it goes idle;otherwise it

transmits immediately. If two or more

stations simultaneously begin transmitting

on an idle cable,they will collide. Allcolliding stations then terminate their

transmission,wait a random time,& repeat

the whole process all over again.


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Cont

The real beginning of 802.3 was the

ALOHA system constructed to allow radio

communication.

Later,carrier sensing was added,& Xerox

PARC built a 2.94 Mbps CSMA/CD

system to connect over 100 personal

working stations on a 1-km cable. This

system was called ETHERNET.THE Xerox ethernet was so successful

that Xerox,DEC,& intel drew up a standard

for a 10-Mbps ethernet.


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Cont

This standard formed the basis for 802.3.

The published 802.3 standards differs from

the ethernet specification in that it describes a

whole family of 1-persistant CSMA/CDsystems,running at speeds from 1 to 10 Mbps

on various media.


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802.3 Cabling

Four types of cabling are used.10Base5 cabling,popularly called thick

ethernet. It resembles a yellow garden

hose,with markings every 2.5 metres toshow where the taps go.

Connections to it are generally made usingvampire taps in which a pin is carefully

forced halfway into the coaxials core.The notation 10Base5 means that it

operates at 10 Mbps.


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Cont

10Base2 or thin ethernet,which,incontrast to the garden-hose-like thickethernet, bends easily.

Connections to it are made using industrystandards BNC connectors to form Tjunctions,rather than using vampire taps.

Thin ethernet is much cheaper & easier to

install,but it can run for only 200 metres &can handle only 30 machines per cablesegment.


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Problems

Detecting cable breaks,bad taps,or loose

connectors can be a major problem.For this reason,a pulse of known shape is

injected into the cable. If the pulse hits an

obstacle or the end of the cable,an echowill be generated & sent back. By carefullytiming the intervel between sending thepulse & receiving the echo,it is possible to

localize the origin of the echo.This technique is called time domain

reflectometry.


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Cont

The problem associated with finding cablebreaks have driven systems toward a different

kind of wiring pattern,in which all stations have

a cable running to a central hub.

Usuallly,these wires are telephony company

twisted pairs,mostly used in buildings.

This scheme is called 10Base-T.


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Cont

The 10Base-F cabling for 802.3 uses fiber

optics.

This is expensive due to the cost of the

connectors & terminators,but it has

excellent noise immunity.


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NAME CABLE Max.

segment

Nodes/

Seg.

Advantages

10Base5 Thickcoax

500 m 100 Good for backbone

10Base2 Thin coax 200 m 30 Cheapestsystem

10Base-T Twistedpair

100 m 1024 Easymaintenance

10Base-F Fiber

optics

2000 m 1024 Best b/w

buildings


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802.3 MAC Sublayer Protocol

Each frame starts with a preamble of 7bytes,each contatining the bit pattern of10101010.

preamble Destinationaddress

Sourceaddress

Start of framedelimiter

Bytes7 1 2 or 6

data pad checksum

2 or 62 0-1500 0-46 4

Lengthof datafield


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Cont

The frame contains two addresses,one for

the destinatin & one for the source.

The standard allows 2-bytes & 6-bytes

addresses.

The high order bit of the destination

address is a 0 for ordinary addresses & 1

for group addresses.

Group addresses allow multi stations to

listen to a single address.


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Cont

When a frame is sent to a group address,

all the stations in the group receive it.Sending to a group of stations is calledmulticast.

The length field tells how many bytes arepesent in the datd field,from a min of 0 tomax of 1500.

When a transceiver detects a collision,it

truncates the current frame,which meansthat stray bits & piece of frame appear onthe cable all the time.


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Cont

To make it easier to distinguish valid

frames from garbage,802.3 states that

valid frames must be atleast 64 bytes

long,from destination address tochecksum.

If the data portion of a frame is less than

46 bytes,the pad field is used to fill out theframe to the minimum size.


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IEEE Standard 802.4: Token Bus

In this if there are n stations & it takes T sec tosend a frame,no frame will ever have to waitmore than nT sec to be sent.

Every body liked the conceptual idea of a ringbut did not like physical implementation becausea break in the ring cable would bring the wholenetwork down.

Ring is a poor fit to the linear topology of mostassembly lines.

This standard,802.4 descirbes a LAN called atoken bus.


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Token Bus

13 11 7 19

201

4

17

Logical

ring

Direction of tokenmotion

This stationnot currentlyin the logical

ring

Broad bandcoaxial cable


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Cont

The token propagates around the logical

ring,with only the token holder beingpermitted to transmit frames. Since onlyone station at a time holds the token,

collisions do not occur.When a station passes the token,it sends

a token frame specifically addressed to itslogical neighbor in the ring.

For the physical layer,the token bus usesthe 75-ohms broadband coaxial cableused for cable television.


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The 802.4 frame format

Destination address Sourceaddress

Bytes

2 or6

data Checksum

2 or6

0-8182 4 11 11

FramecontrolStart delimiter

preamble

Enddelimiter


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IEEE Standard 802.5: Token Ring

A ring consists of a collection of ring

interfaces connected by point to point lines.

Each bit arriving at an interface is copied

into a 1-bit buffer & then copied out onto the

ring again.

While in the buffer,the bit can be inspected &

possibly modified before being written out.

This copying step introduces a 1-bit delay ateach interface.


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stationsRingiterface

Undirectionalring

Ring Network


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Cont

In a token ring a special bit pattern, called

the token, circulates around the ring

whenever all stations are idle.

When a station wants to transmit a frame,

it is required to seize the token & remove it

from the ring before transmitting.


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There are two operating modes,listen & transmit.

In listen mode,the input are simply copied tooutput.

In transmit mode,which is entered only after thetoken has been seized,the interface breaks theconnection between input & output,entering its

own data onto the ring.

1 bit delayRing interface

Tostation

Fromstation

Tostation

fromstation


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Comparison of 802.3 Ethernet and 802.5

Token Ring

Generally they have similar technology with similarperformance.

802.3 Ethernet Advantages

Widely used at present. People are experienced in

using this technology.

Simple Protocol. New computers can be added with

having to bring the network down.

Almost zero delay at low load, there is no need to

wait for a token, you can transmit when ready.


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Token Ring

802.3 Ethernet DisadvantagesThe electronics is more complicated for

carrier sense and collision detection.

Ethernet is non-deterministic system(possibility of repeated collisions). This

means that Ethernet is not suitable for

network applications that require guaranted

delivery times.Poor performance at high loads as there can

be lots of collisions reducing the number of

messages that are successfully transmitted.


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Token Ring

802.5 Token Ring AdvantagesToken Ring uses point-to-point connections between ring

interfaces so that the electronic hardware can be fully

digital and simple. There is no need for collision detection.

Can use any medium,twisted pair is cheap and easy to

install but could equally use fiber optic if available.

Throughput excellent at high loads since there is no

possibility of collisions unlike 802.3.


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Token Ring

802.5 Token Ring DisadvantagesComputers must wait for the token to arrive, therefore at

load, a computer is delayed before sending.

Each token ring has a monitor computer, to look after the

ring (i.e. remove damaged frames, handle lost frames

and lost tokens). This introduces a critical point of failure.

If the monitor computer failed, the remaining computers

would have to wait until it is replaced before being able to

continue.


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Conthttp://www.javvin.com/protocolToken.htmlhttp://en.wikipedia.org/wiki/Token_ringhttp://www.#IP/IEEE_802.4_-_Token-

Passing_Bus_Access_Method.htmhttp://www.freesoft.org/CIE/RFC/1042/10.htm

http://en.wikipedia.org/wiki/Network_switchhttp://en.wikipedia.org/wiki/Circuit_switchinghttp://voip.about.com/od/voipbasics/a/switchingtypes.

htm

http://www.thefreedictionary.com/message+switchinghttp://en.wikipedia.org/wiki/Message_switchinghttp://en.wikipedia.org/wiki/Packet_switching


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THANKSTHANKS

Documents

Switching & IEEE Standard