- Congestion is an important issue that can arise in packet switched network.
- Congestion is a situation in Communication Networks in which too many packets are present in a part of the subnet.
- Congestion in a network may occur when the load on the network is greater than the capacity of the network.
- Due to Congestion the performance degrades.
Factors that Causes the Congestion:
Packet arrival rate exceeds the outgoing link capacity.
Insufficient memory to store arriving packets.
- Congestion Control is the techniques and mechanisms which can either prevent congestion from happening or remove congestion after it has taken place.
- Congestion control mechanisms are divided into two categories, one category prevents the congestion from happening and the other category removes congestion after it has taken place.
I) Open Loop Congestion Control:
In Open Loop Congestion Control, policies are used to prevent the congestion before it happens.
Congestion control is handled either by the source or by the destination.
II) Closed Loop Congestion Control:
Closed loop congestion control mechanisms try to remove the congestion after it happens.
It uses some kind of feedback.
NEED of Congestion Control:
- It is not possible to completely avoid the congestion but it is necessary to control it.
- Congestions leads to a large Queue Length.
- It results in Buffer Overflow & Loss of Packets.
- So the congestion control is necessary to ensure that the user gets the negotiated Quality of Services.
Congestion prevention policies:
I) Retransmission Policy:
The sender retransmits a packet, if it feels that the packet it has sent is lost or corrupted.
However retransmission increases the congestion in the network.
But we need to implement good retransmission policy to prevent congestion.
The retransmission policy and the retransmission timers need to be designed to optimize efficiency and at the same time prevent the congestion.
II) Window Policy:
To implement window policy, selective reject window method is used for congestion control.
Selective Reject method is preferred over Go-back-n window as in Go-back-n method, when timer for a packet times out, several packets are resent, although some may have arrived safely at the receiver.
Thus, this duplication may make congestion worse.
Selective reject method sends only the specific lost or damaged packets.
III) Acknowledgement Policy:
The acknowledgement 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.
Acknowledgments also add to the traffic load on the network.
Thus, by sending fewer acknowledgements we can reduce load on the network.
To implement it, several approaches can be used:
A receiver may send an acknowledgement only if it has a packet to be sent.
A receiver may send an acknowledgement when a timer expires.
A receiver may also decide to acknowledge only N packets at a time.
IV) Discarding Policy:
A router may discard less sensitive packets when congestion is likely to happen.
Such a discarding policy may prevent congestion and at the same time may not harm the integrity of the transmission.
V) 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 connection if there is congestion in the network or if there is a possibility of future congestion.