Thursday, June 9, 2011

Congestion


An important issue in a packet-switched network is congestion. Congestion in a network may occur if the lead on the network – the number of packets sent to the network-is greater than capacity of the network-the number of packets a network can handle. Congestion cintrol refers to the mechanism and techniques to control the congestion and keep the load below the capacity.

Queue in router

1. The packet is a put at the end of the input queue while waiting to be checked.
2. The processing module of the router removes the packet from the input queue once it reaches the front of the queue and uses its routing table and the destination address to find the route.
3. The packet is put in the appropriate otuput queue and waits its turn to be sent.
Congestion control refers to techniques and mechanisms that can either prevent congestion, before it happens, or remove congestion, after it has happened.

Open Loop
In open loop congestion control, policies are applied to prevent congestion before it happens. In these mechanisms, congestion cintrol is handled by either the source or the destination.

1. Retransmision policy
Retransmission is sometimes unavoidable. If the sender feels that a sent packet is lost or corrupted, the packet nedds to be retransmitted.

2. Window policy
The type of window at the sender may alsi affect congestion. The Selective Repeat window is better than the Go-Back-N window for congestion control.

3. Acknowledgment policy
The acknowledgment policy imposed by the receiver may also affect congestion. If the receiver does not acknowledge every packet it receives, it may slow down the sender and help prevent congestion.

4. Admission policy
An admission policy, which is a quality of  service mechanism, can also prevent congestion in virtual-circuit networks. Switches in a flow first check the resource requirement of a flow before admitting it to the network. A router can deny establishing a virtual circuit cennection if there is congestion if there is congestion in the network or if there is apossibility of future congestion.

5. Discarding policy
A good discarding policy by the routers may prevent congestion at the same time may not harm the integrity of the transmission. For example, in audio transmission, if the policy is to discard less sensitive packets when congestion is likely to happen, the quality of sound is still preserved and congestion is prevented or alleviated.



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