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Data Transmission and Computer Networks

The Switching Networks

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Switching Networks

Two different technologies are used in the wide-area switched networks:– Circuit switching.– Packet switching.

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Circuit Switching

Communication via circuit switching implies that there is a dedicated communication path between two station.

The path is a connected sequence of links between network nodes

The most common example of circuit switching is the telephone network.

Communication via circuit switching involves three phases:

– Circuit establishment.– Data transfer.– Circuit disconnect.

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1. Circuit Switching

I. Circuit establishment: Before any signals can be transmitted, an end-to-end must be

established. To establish a connection from station A to station F:

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1. Circuit Switching

II. Data Transfer: Information can be transmitted from A

through the network to station F.

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1. Circuit Switching

III. Circuit Disconnect: During the circuit establishment, channel

capacity must be reserved between each pair of nodes in the path. Therefore, during circuit disconnect, signals must be propagated to node 4, 7, 6 to de allocate the dedicated resources .

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2. Packet Switching

The Concept of Packet Switching: In packet switching, data are transmitted in short packets. A longer message is broken up into series of packets.

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2. Packet Switching

The Concept of Packet Switching: Consider a packet to be send from A to F. The packet will include the

address of the destination (F). A packet is sent from A to node 4. Node 4 stores the packet, determine the next node in the route (say 7),

and queues the packet to go out on that link. The packet is then transmitted to 7, which will forward the packet to node

6, and finally to F. C

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2. Packet Switching

Advantages of Packet Switching: Line efficiency is greater, since:

– A single node-to-node link is shared by many packets over time.

– With circuit switching node-to-node link is preallocated using synchronous TDM, Much of time, A link may be idle.

Two stations of different data rate can exchange packets, since each connects to its node at its proper data rate.

For heavy traffic on circuit switching, a connection may be blocked. On packet switching network, packet is still accepted, but delivery delay increases.

Properties can be used in the queues.

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2. Packet switching

There are two approaches in packet switching:– Datagram.– Virtual circuit.

I. Datagram Approach:

Each packet is treated independently. Suppose station A has three packets to send to F. On each packet,

node 4 must make a routing decision. Node 4 could possibly forward a packet to either node 5 or node 7. Node 4 determines that its queue of packets for node 7 is shorter

than for node 5, so it queues the packet for node 7.

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2. Packet switching

II. Virtual Circuit Approach: A route is established before any packet is sent. Suppose A has one or more messages to send to F. It first sends a

special control packet, CALL REQUEST packet, to node 4, requesting a logical connection to F.

Node 4 decide to route the request and all subsequent packets to 7, which decides to route the requests and all subsequent packets to 6, which finally delivers the CALL REQUEST packet to F. C

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2. Packet switching

II. Virtual Circuit Approach: If F is prepared to accept the connection, it sends a CALL ACCEPT packet

to 6. This packet is passed back through nodes 7 and 4 to A. Stations A and F may now exchange data over the route that has been

established Note that the established route in virtual circuit is not a dedicated path, as

in circuit switching. A packet is still buffered at each node and queued for output over a line.

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2. Packet switching

Advantages of the virtual circuit approach:– Packets arrives to destination in order.– Not necessary to make routing decision for each packet at

each node.Advantages of the datagram approach:

– Call setup phase is avoided.– Flexible, If congestion develops in one part of the network,

incoming datagrams can be routed away from the congestion.– If a node fails, all virtual circuits that pass through that node

are terminated. However, with datagram, all subsequent packets may find an alternate route.

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2. Packet switching

Packet Length

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Comparison of Circuit Switching and Packet Switching

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Comparison of Circuit Switching and Packet Switching