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ATM : Congestion Control
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Congestion

When a large number of packets rush to a node of a part of load, the performance of the network degrades.

This situation is called as congestion. It is shown in Fig.1 .

In this situation, the network can not cope up with the excessive traffic and its performance collapses to a very low level.

Almost all the packets are blocked and no packets are delivered.

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Congestion Control

There is no congestion upto point A in Fig.1. So if the network is operated between points O and A then the congestion will not take place.

So the process which maintains the offered load i.e. number of packets in a network below point A , is called as congestion control process.

With the help of congestion control it is possible for the network to carry the offered load.

Effects of Congestion

Refer Fig.1 . As the offered load i.e. number of packets sent is increased, the network throughput also increases.

This happens from "0" to "A" and there is no congestion.

But as the offered load is further increased, the network throughput starts decreasing.

This happens because, the buffers at each node are full and start discarding the packets. Here the mild congestion begins.

The transmitting station now has to retransmit the packets.

This will increase the offered load further and the severe congestion will take place degrading the performance completely.

Causes of Congestion

1) The rate of packet arrival at a particular node is higher than the rate at which the packets are being transmitted.

2) Due to use of slow processor.

3) The bandwidths of the links is not sufficient.

4)When the arrival of packets is not at a uniform rate i.e. if the traffic is bursty.

5)Any mismatch between the network parts.

Traffic rate management

In order to control the congestion, the frame relay network has to discard excess frames.

For this a rate called CIR i.e. committed information rate is defined. It is bits per seconds.

CIR is such that if the actual information rate is less than CIR then the congestion is definitely avoided.

Under the severe congestion conditions, the network is forced to provide a service at less than CIR.

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