switching & ieee standard
TRANSCRIPT
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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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Comparison of 802.3 Ethernet and 802.5
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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Comparison of 802.3 Ethernet and 802.5
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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Comparison of 802.3 Ethernet and 802.5
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