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Communication Switching

Techniques

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Switched NetworkA Switched Network consists of interlinked nodes called Switches.

Switches are hardware and software that are capable of creating temporary connections between two or more devices linked to the switch.

In a Switched network, some of nodes are connected to the end system or communication devices like computer or telephone.

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Switched Network

A,B,C,D,E and so on are end systems or communication devices.

Switches are labeled as I,II,III,IV,V and so on.

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Switching

Circuit Switching

Packet Switching

Message Switching

Virtual Circuit

SwitchingDatagram Switching

Types of Switching Techniques

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Circuit SwitchingDedicated physical connection or path is established between the sending and receiving device.

Dedicated path made of one or more links.

Routing is made when the path or link is set up across the network. After the has been set between the sender and receiver, the information is forwarded continuously over the link.

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Data transferred between the two stations are not in the form of packets. The data are continuous flow sent by the source station and received station.

No addressing involved in data transfer.

Communication takes place in three phases:--Circuit establishment-Data transfer phase-Circuit disconnect

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Path is release only when data transmission between sender and receiver is over.

In circuit switching, the resources need to be reserved during the setup phase, the resources remain dedicated for the entire duration of data transfer until the teardown phase.

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The four switches are connected these computers thus provide dedicated links by reducing the line cost. I,II,III and IV are circuit switches .Switch I,III,IV are connected to computers while II is only routing node.

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Advantages of Circuit SwitchingDedicated path establish provides a guaranteed data rate.

Data is transmitted without any delay, as there is no waiting time at each switch.

Suitable for long continuous transmission.

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Disadvantages of Circuit Switching It cannot be used to transmit any other data even if the channel is free.

Inefficient in terms of utilization of resources.

Long time is required to establish a physical link.

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Packet SwitchingMessage is broken up into packets of fixed and variable size.

Each packet includes header that contains source address , destination address and other control information.

Size of packets depend upon type of network and protocol used.

Resources are allocated on demand on a first come first basis.

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Packet are sent over network node to node.

At each node, packet is stored briefly and then routed according to information in its header. These types of networks are called store and forward networks.

Individual packets may follow different routes to reach destination.

Two types of Packet Switching:--Datagram Packet Switching-Virtual Packet Switching

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Datagram Packet SwitchingEach packet is separately addressed that is contains destination address in its header and is treated independently.

Packets in this approach are called datagram.

Datagram Switching is done at network layer.

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Datagram Packet Switching

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Datagram approach is used to deliver four packets from station A to X. All the four packets belong to same message but they travel via different paths to reach the destination.

It can cause the datagram to arrive out of order at their destination with different delays between the packets.

Packets may also be lost because of lack of resources.

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Virtual Packet SwitchingCircuit connection called virtual circuit is set up between sender and receive before any data transfer takes place.

Virtual circuit approach has a set up, data transfer and tear down phase.

Resources can be allocated during set up phase or on demand.

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All the packets of a message follow the same path establish during the connection.

It is implemented in the data link layer.

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Advantages of Packet Switching

Switching node can route the packet as and when required.

Routing is done on packet basis, the intermediate nodes do not have to wait for entire message, hence transmission is very.

It increases the bandwidth of network by allowing many devices to communicate through the same network channel.

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Disadvantages of Packet SwitchingPackets may be lost on their route, so sequence numbers are required to identify the missing packets.

Large amount RAM is required to handle the large quantities of packets.

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Message SwitchingMessage is a logical unit of information and can be of any length.

If a station wishes to send a message to another station, it first adds destination address to the message.

No direct link is established between sender and receiver.

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Each message is treated as an independent unit.

Each complete message is then transmitted from device to device through internetwork i.e. message is transmitted from source node to intermediate node.

Intermediate node stores complete message temporarily, inspect it for errors and transmits message to next node based on available channel and its routing information. It is called Store and Forward network.

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When message reaches a node, the channel on which it came is released for use by another message.

Message M1 is transmitted from A to D and M2 is transmitted from C to B. Message M1 follows route A->I->II->D and M2 follows route C->IV->II->B depending upon availability of free path at that particular moment.

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Message Switching

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Advantages of Message SwitchingNo physical connection is required between source and destination.

Reduces traffic congestion on network because of store and forward facility.

Support message length of unlimited size.

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Disadvantages of Message Switching

As message length is unlimited, each switching node must have sufficient storage to buffer message.

Storing and forwarding facility introduces delay thus unsuitable for real time applications like voice and video.

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Comparison between Switching Techniques

Parameter Circuit Switching

Packet Switching

Message Switching

Path or Route Path is created between two points by setting switches.

Virtual circuit is created between two points.

No need to create a link or path.

Dedicated or sharing

Links that make a path are dedicated and cannot be used other connections.

In virtual circuit , links that make a route can be shared by other connections.

Message reaches a node the channel on which it came is released by another message.

Store and forward transmission

No use Each node may store incoming packet temporarily & forward them later.

Store and forward transmission is used.

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Parameter Circuit Switching

Packet Switching

Message Switching

Path followed by packets

It is always same. May or may not be different

May or may not be same.

Call setup It is required. Not required. Not required.Application Real time transfer

of voice signal.Internet for datagram and reliable stream service between computers.

Telegraph network for transmission of telegrams.

Layer used for implementation

It is implemented at physical layer.

Virtual circuit network is implemented at data link layer and datagram network at network layer.

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