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SWITCHING CONCEPTS

BY

N.V.D.MURALIDHAR

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SWITCHES

Problem

-Each user can potentially call any other user

-Cant have direct line

-Switches establish temporary circuits

-Switching systems come in two partsSwitch and switch controller

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Routing and Switching

Many connections will need path through more

Than one switch

Static routing uses same approach all the time

Dynamic routing allow for changes in routing

Depending on traffic

Switching is local connectivity between input

And output line

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BasicConcepts and Terminology

-Transfer medium- buses that deliver cells

-Queues-Memory locetions that temporarilyStore cells

-Switch element -building block for switch

-Switch fabric -the shape of the switches

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BasicConcepts and Terminology

Blocking:-A network is unable to connect stations

because all paths are in use

-A blocking network allows this-Used on voice systems

- short duration calls

Non Blocking-Permits all stations to connect (in pairs)At once

-Used for some data connections

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SWITCHING

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-When there are many devices, it is necessary to develop suitablemechanism for communication between any two devices.

- One alternative is to establish point-to-point communicationbetween each pair of devices using mesh topology.

-However, mesh topology is impractical f or large number of device

because the number of links increases exponentially (n(n-1)/2 ) with the number of devices.

-A better alternative is to use switching techniques leading to

switched communication network.

-In the switched network methodology, the network consists

of a set of interconnected nodes, among which inf ormation is

transmitted from source to destination via different routes,

which is controlled by the switching mechanism.

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Switching Networks

Long distance transmission is typically doneover a network of switched nodes

Nodes not concerned with content of data

End devices are stations Computer, terminal, phone, etc.

A collection of nodes and connections is a

communications network Data routed by being switched from node to

node

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Nodes

Nodes may connect to other nodes only, or to

stations and other nodes

Node to node links usually multiplexed Network is usually partially connected

Some redundant connections are desirable forreliability

Two different switching technologies

Circuit switching

Packet switching

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Simple Switched Network

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The switching perf ormed by different

nodes can be categorized into theFollowing three types:

*CircuitSwitching

*PacketSwitching

*MessageSwitching

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Circuit-Switching

Circuit switching creates a direct physicalconnection between two devices such as

phones or computers.

We can use switches to reduce the number

and length of links.

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A circuit switch

A circuit switch is a device with n inputs and m outputsthat creates a temporary connection between an inputlink and an output link.

The number of inputs does not have to match the

number of outputs.

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A folded switch

An n-by-n folded switch can connect n lines in full-duplex

mode. For example, it can connect n telephones in such away that each phone can be connected to every otherphone.

Circuit switching uses space-division switch [paths in the

circuit are separated from each other spatially] or time-division switch.

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Circuit switching uses threetechnologies:

Space-division switching,

Time-division switching

or

combination of both

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Space division switching

-Developed for analog environment

-Separate physical paths

*Cross bar switch- Number of cross points grows as square of number of stations-Loss of cross point prevents connection

Inefficient use of -All stations connected only a few cross pointsin use

-Non Blocking

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Crossbar matrix

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Crossbar switches

-In the simplest form n input lines and n output lines feed to form n*nelectronic switches

-If all the lines are full duplex , half the number of switches are required

-A part from the number of number of I/O pins are also a problem

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Multistage switch

Multistage switch combines crossbar switches in several stages. Design of a multistage switch depends on the number of stages

and the number of switches required (or desired) in each stage.

Normally, the middle stages have fewer switches than do the

first and last stages.

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MULTIPLE Switching paths

Multiple paths are available in multistage switches.

Blocking refers to times when two inputs are looking forthe same output. The output port is blocked.

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Figure 8.7 Time-division multiplexing, without and with a time-slot interchange

Time-division switching uses time-division multiplexing to

achieve switching. Two methods used are:

Time-slot interchange (TSI) changes the order of the slots based onthe desired connection.

TDMbus

Time-Division Switch

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Time-slot interchange

TSI consists of random access memory (RAM) with several

memory locations. The size of each location is the same asthe size of a single time slot.

The number of locations is the same as the number of inputs.

The RAM fills up with incoming data from time slots in the

order received. Slots are then sent out in an order based onthe decisions of a control unit.

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Packet Switching Principles

Circuit switching designed for voice

Resources dedicated to a particular call

Much of the time a data connection is idle Data rate is fixed

Both ends must operate at the same rate

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Basic Operation

Data transmitted in small packets Typically 1000 octets

Longer messages split into series of packets

Each packet contains a portion of user data plussome control info

Control info

Routing (addressing) info Packets are received, stored briefly (buffered)

and past on to the next node Store and forward

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Use of Packets

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Advantages

Line efficiency

Single node to node link can be shared by many packets overtime

Packets queued and transmitted as fast as possible Data rate conversion

Each station connects to the local node at its own speed

Nodes buffer data if required to equalize rates

Packets are accepted even when network is busy Delivery may slow down

Priorities can be used

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Switching Technique

Station breaks long message into packets

Packets sent one at a time to the network

Packets handled in two ways Datagram

Virtual circuit

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Datagram

Each packet treated independently

Packets can take any practical route

Packets may arrive out of order Packets may go missing

Up to receiver to re-order packets and

recover from missing packets

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Datagram

Diagram

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Virtual Circuit

Preplanned route established before anypackets sent

Call request and call accept packets establishconnection (handshake)

Each packet contains a virtual circuitidentifier instead of destination address

No routing decisions required for each packet Clear request to drop circuit

Not a dedicated path

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Virtual

Circuit

Diagram

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Virtual Circuits v Datagram

Virtual circuits Network can provide sequencing and error control Packets are forwarded more quickly

No routing decisions to make Less reliable

Loss of a node looses all circuits through that node

Datagram No call setup phase

Better if few packets More flexible

Routing can be used to avoid congested parts of thenetwork

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