CircuitandPacketSwitching Vijay kumar

8/9/2019 CircuitandPacketSwitching Vijay kumar

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Circuit Switching and Packet

Switching

SWITCHING


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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 stationsComputer, terminal, phone, etc.

A collection of nodes and connections is acommunications network

Data routed by being switched from node tonode


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Nodes

Nodes may connect to other nodes only, or tostations and other nodes

Node to node links usually multiplexed

Network is usually partially connectedSome redundant connections are desirable for

reliability

Two different switching technologies

Circuit switchingPacket switching


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Simple Switched Network


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Switching Activities

Some nodes connect only to other nodes(intermediary nodes). Sole purpose is to switchdata

Some nodes have one or more stationsattached. They accept from and deliver data tothe attached station.

Node-to-node links are usually multiplexed

Multiple paths enhance reliability


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Circuit Switching

Originated in public telephone networks

Well suited to analog transmission of voice signal

Dedicated communication path between two stations

Three phases Establish Transfer

Disconnect

Must have switching capacity and channel capacity to

establish connection Must have intelligence to work out routing


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Circuit Switching - Applications

Inefficient

Channel capacity dedicated for duration of connection

If no data, capacity wasted

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

Developed for voice traffic (phone)


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Public Circuit Switched

Network


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Telecom Components

Subscriber Devices attached to network

Subscriber line Link between subscriber and network

Also called Local Loop or Subscriber LoopAlmost all Local Loops are TPW

Range from Few km up to tens of km

Exchange Switching center in the network

End office specific switching center that supports subscribers Trunks

Branches between exchanges

Multiplexed


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Circuit Establishment


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Circuit Switching Concepts

Digital Switch Provide transparent signal path between devices

Typically allows full duplex transmission

Network Interface

Functions and h/w needed to connect digitaldevices to the network

Control Unit Establish connections - Generally on demand, Handle and

acknowledge requests, Determine if destination is free,constructpath

Maintain connection

Disconnect


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Blocking or Non-blocking

Circuit Switching

Blocking

A network may not be able to connect stationsbecause all paths are in use (more stations thanpath)

Used on voice systems

Short duration calls

Non-blocking

Permits all stations to connect (in pairs) at once (atleast as many paths as stations)

Used for some data connections


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Space Division Switching

Developed for analog environment, but carriedover into digital

Signal paths are physically separate (slide 15)

Each connection requires dedicated path(crossbar switch)


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Crossbar switch

Number of crosspoints grows as square ofnumber of stations

Loss of crosspoint prevents connection

Inefficient use of crosspointsIf all stations connected, only a few crosspoints in

use

Non-blocking


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Space Division Switch


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Multistage Switch

Reduced number of crosspoints

More than one path through network

Increased reliability

More complex control May be blocking


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Three Stage Space Division

Switch


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Time Division Switching

Modern digital systems rely on intelligent controlof space and time division elements

Use digital time division techniques to set up

and maintain virtual circuits Partition low speed bit stream into pieces that

share higher speed stream


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Control Signaling Functions

Audible communication with subscriber

Transmission of dialed number

Call can not be completed indication

Call ended indication Signal to ring phone

Billing info

Equipment and trunk status info Diagnostic info

Control of specialist equipment


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Switch to Switch Signaling

Subscribers connected to different switches

Originating switch seizes interswitch trunk

Send off hook signal on trunk, requesting digit

register at target switch (for address) Terminating switch sends off hook followed by

on hook (wink) to show register ready

Originating switch sends address


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In Channel Signaling

Use same channel for signaling control and call Requires no additional transmission facilities

Inband Control signals have same electromagnetic properties

(frequency) as voice signal

Can go anywhere a voice signal can

Impossible to set up a call on a faulty speech path

Out of bandVoice signals do not use full 4kHz bandwidth

Narrow signal band within 4kHz used for control Can be sent whether or not voice signals are present

Need extra electronics

Slower signal rate (narrow bandwidth)


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Drawbacks ofIn Channel

Signaling

Limited transfer rate

Delay between entering address (dialing) andconnection

Overcome by use of common channel signaling


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Softswitch Architecture

Latest trend in circuit-switching technology

General purpose computer running software to make ita smart phone switch

Lower cost, greater functionality

Can packetize digitized voice data, allowing voice overIP

Performs call routing

Separates call processing from hardware function ofswitch


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Traditional Circuit Switching


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Circuit Switching Shortcomings

Inefficient for data because of idle time

Provides for transmission at constant rate must transmit and receive at same data rate.

Limits versatility


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Packet Switching Basic

Operation

Data transmitted in small packets

Typically 1000 octets (8 bit byte)

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 passed 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

DatagramVirtual circuit


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Circuit vs. Packet Switching

Circuit Switched

Bandwidthguaranteed

Circuit capacity notreduced by othernetwork traffic

Circuit costsindependent of

amount of datatransmitted, resultingin wasted bandwidth

Packet Switched Bandwidth

dynamically allocatedon as-needed basis

May have concurrenttransmissions overphysical channel

May have delays andcongestion

More cost-effective,offer betterperformance

CircuitandPacketSwitching Vijay kumar

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