Switching and Bridging In Computer Network | Media Access And Internetworking

Switching:

A switch is a mechanism that allows us to interconnect links to form a larger network. A switch is a multi-input, multi-output device that transfers packets from an input to one or more outputs.

A Switch's primary job is to receive incoming packets on one of its links and to transmit them on some other link. This function is sometimes referred as switching or forwarding.

Types of Switching

• Data-gram or connection less approach.
• Virtual circuit or connection oriented approach.
• Source Routing.

All networks need to have a way to identify the end nodes. such identifiers are usually called addresses.

Data-gram:

We include in every packet enough information to enable any switch to decide how to get it to its destination. That is every packet contains the complete destination address. To decide how to forward a packet, a switch consults a forwarding table sometimes called routing table.

Characteristics of Data-gram networks:

A host can send a packet anywhere at any time. When a host sends a packet, it has no way of knowing if the network is capable of delivering it.

Each packet is forwarded independently of previous packets that might have been sent to the same destination.

A switch or link failure might not have any serious effect on communication.

Virtual circuit switching:

This approach is also referred as a connection oriented model. It requires setting up a virtual connection from the source host to the destination host before any data is sent.

Two stages in virtual circuit switching:

• Connection Setup  
• Data Transfer

Connection Setup: In this phase it is necessary to establish a connection state in each of the switches between the source host and destination host. The connection state for a single connection consists of an entry in a virtual circuit table in each switch through which the connection passes.

One entry in the virtual circuit table on a single switch contains:

Virtual circuit Identifier that uniquely identifies the connection at this switch. An incoming interface in which packets for this virtual circuit leave the switch. Virtual Circuit Identifier that will be used for outgoing packets.

If a packet arrives on the designated incoming interface and that packet contains the designated virtual circuit identifier value in its header then that packet should be sent out the specified outgoing interface with the specified outgoing virtual circuit identifier value having been first placed in its header.

A network administrator wants to manually create a new virtual connection from host A to host B the administrator needs to identify the path through the network from A to B.

Then administrator then picks a Virtual circuit identifier value that is currently unused on each link for the connection suppose that the Virtual circuit identifier value 5 is chosen for the link from host A to switch 1, and 11 is chosen for the link from switch 1 to switch 2. Then Virtual circuit identifier value 7 is chosen to link from switch 2 to switch 3 and virtual circuit identifier value 4 is chosen for link switch 3 to host B.

Once the virtual circuit tables have been set up, the data transfer phase can proceed for any packet that it wants to send to host B, A puts the Virtual circuit identifier value 5 in the header and sends it to switch 1. Switch 1 receives packet on interface 2 puts virtual circuit identifier 11 to its header and send it to switch 2. Switch 2 puts the virtual circuit identifier value 7 to its header and send it to switch 3. Switch3 puts the virtual circuit identifier value 4 to its header and send it to host B.

Strategies used in Virtual Circuit Switching.

Buffers are allocated to each virtual circuit when the circuit is initialized. The sliding window protocol is run between each pair of nodes.

The circuit is rejected by a given node if not enough buffers are available.

Asynchronous Transfer Model:

ATM is the most well known virtual circuit based networking technology.

G.F.C - Generic Flow Control
V.P.I - Virtual Path Identifier
V.C.I -Virtual Circuit Identifier
C.R.C - Cyclic Redundancy Check

Source Routing:

In source routing assign a number to each output of each switch and to place that number in the header of the packet. For each packet that arrives on an input the switch would read the port number in the header and transmit the packet on that output.

In this example the packet needs to traverse three switches to get from host A to host B. At switch 1, it needs to exist on port 1, at switch 2 it needs to exist on port 0 at switch 3.

Which the packet leaves host A, the original header contains the list of port (3,0,1) where each switch reads the rightmost element of the list. To make sure that the next switch gets the appropriate information each switch rotates the list after it has read its own entry.

Bridges:

Class of switch that is used to forward packet between LAN is called bridge. Collection of LAN connected by one or more bridges is usually said to be extended LAN.

Each bridge inspect the source address in all the frame it receives. When host A sends a frame to the bridge the bridge receives the frame and records the fact that a frame from host A was received on port 1.

Spanning Tree Algorithm:

The idea of spanning tree algorithm is a subset of the actual network topology that has no loops and that has no loops and that reaches all the LAN in the extended LAN.

The idea of spanning tree algorithm is a subset of the actual network topology that has no loops and that reaches all the LAN in the extended LAN.

The spanning tree algorithm was developed at the digital Equipment corporation. Each bridge has a unique identifier. Here we use B1, B2...etc. The algorithm first elects the bridge with the smallest 1D as the root. Each bridge computes the shortest path to the root. Finally all the bridges connected to a given LAN elect a single designated bridge that will be responsible for forwarding frames toward root.

 

Each LAN designated bridge is the one that is closest to the root. If two or more bridges are equally close to the root then smallest 1D wins.