FALLSEM2013 14 CP0180 07 Aug 2013 RM01 Circuit and Packet Swicthing1

7/27/2019 FALLSEM2013 14 CP0180 07 Aug 2013 RM01 Circuit and Packet Swicthing1

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Switching

Unit -I


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Switching Networks

In the switched network methodology, the network consists ofa set of interconnected nodes, among which information is

transmitted from source to destination via different routes,

which is controlled by the switching mechanism.

Nodes not concerned with content of data End devices that wish to communicate with each other are

called stations-

Computer, terminal, phone, etc.

The switching devices are called nodes. Some nodes connect to other nodes and some are to connected

to some stations.


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Simple Switched Network


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Nodes

Nodes may connect to other nodes only, or to stationsand other nodes

Node to node links usually multiplexed.

Node-station links are generally dedicated point-to-point links.

Network is usually not fully connected.

Some redundant connections are desirable for reliability


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Simple Switched Network


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Taxonomy of Switched Networks


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Circuit Switching

A switching technology that establishes an electricalconnection between stations using a dedicated path.

Dedicated communication path between two stations

Three phasesEstablish

Transfer

Disconnect

Must have switching capacity and channel capacityto establish connection

Must have intelligence to work out routing


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Circuit Switching


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Circuit Switching

InefficientChannel capacity dedicated for duration of connection

If no data, capacity wasted

Set up (connection) takes time Once connected, transfer is transparent

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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Public Circuit Switched Network


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Telecommunication Components

SubscriberDevices attached to network

Subscriber lineThe link between the subscriber and the network

Known as Local Loop or Subscriber loop

Few km up to few tens of km Exchange

Switching centers in the network.

A switching center that directly supports subscribers is known as anend office.

TrunksBranches between exchanges

Trunks carry multiple voice frequency circuits using either FDM orsynchronous TDM


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Circuit Establishment


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Circuit Switch Elements


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Circuit Switching Concepts

Digital SwitchProvide transparent signal path between devices

Network Interface Element

- represents the functions and hardware needed to connect digitaldevices, such as data processing devices and digital telephones.

Control Unit

Establish connections

Generally on demand

Handle and acknowledge requests

Determine if destination is free construct path

Maintain connection

Disconnect


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Characteristics of Circuit Switching

BlockingA network is unable to connect stations because all paths

are in use

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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Packet Switching Principles

In a packet-switched network, there is no resourcereservation; resources are allocated on demand.

Data transmitted in small packets

Typically 1000 octets

Longer messages split into series of packetsEach packet contains a portion of user data plus some

control info

Control info

Routing (addressing) info

Packets are received, stored briefly (buffered) andpast on to the next node


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Use of Packets


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Advantages

Line efficiencySingle node to node link can be shared by many packets

over time

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 busyDelivery may slow down

Priorities can be used


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Packet Switching Technique

Station breaks long message into packets Packets sent one at a time to the network

Packets handled in two ways

Datagram ApproachVirtual circuit Approach


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Datagram Approach

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.

A switch in a datagram network uses a routing table

that is based on the destination address.


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A datagram network with four switches


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Datagram

Diagram


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Virtual Circuit Approach

Preplanned route established before any packets sent Call request and call accept packets establish

connection (handshake)

Each packet contains a virtual circuit identifierinstead 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 circuitsNetwork 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 packetsMore flexible

Routing can be used to avoid congested parts of the network


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Cell Switching

Cell Switching is a connection oriented switching technique in

which message is transmitted in the form of packets of same size.

Cell switching is similar to packet switching as it operates in a

similar way to packet switching but uses small fixed length cells

for data transport.

Used in Asynchronous Transfer Mode networks.

Cell switching can handle multiple data types, i.e. voice, video and

data. Moreover, cell switching is typically a high bandwidth and

high speed technology.

Cell switching is essentially an attempt to combine the best of

circuit switching and packet switching

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FALLSEM2013 14 CP0180 07 Aug 2013 RM01 Circuit and Packet Swicthing1