Tuesday, August 20, 2013
- Network congestion is quite a common problem in the queuing theory and data networking.
- Sometimes, the data carried by a node or a link is so much that its QoS (quality of service) starts deteriorating.
- This situation or problem is known as the network congestion or simply congestion.
This problem has the following two typical effects:
Ø Queuing delay
Ø Packet loss and
Ø Blocking of the new connections
- The last two effects lead to two other problems.
- As the offered load increases by the increments, either the throughput of the network is actually reduced or the throughput increases by very small amounts.
- Aggressive re-transmissions are used by the network protocols for compensating for the packet loss.
- The network protocols thus tend to maintain a state of network congestion for the system even if the actual initial load is too less that it cannot cause the problem of network congestion.
- Thus, two stable states are exhibited by the networks that use these protocols under similar load levels.
- The stable state in which the throughput is low is called the congestive collapse.
- Congestive collapse is also called congestion collapse.
- In this condition, the switched computer network that can be reached by a packet when because of congestion there is no or little communication happening.
- In such a situation even if a little communication happens it is of no use.
- There are certain points in the network called the choke points where the congestion usually occurs.
- At these points, the outgoing bandwidth is lesser than the incoming traffic.
- Choke points are usually the points which connect the wide area network and a local area network.
- When a network falls in such a condition, it is said to be in a stable state.
- In this state, the demand for the traffic is high but the useful throughput is quite less.
- Also, the levels of packet delay are quite high.
- The quality of service gets extremely bad and the routers cause the packet loss since their output queues are full and they discard the packets.
- The problem of the network congestion was identified in the year of 1984.
- The problem first came in to the scenario when the backbone of the NSF net phase dropped 3 times of its actual capacity.
- This problem continued to occur until the Van Jacobson’s congestion control method was implemented at the end nodes.
Let us now see what is the cause of this problem?
- When the number of packets being set to a router exceeds its packet handling capacity, many packets are discarded by the routers that are intermediate.
- These routers expect the re-transmission of the discarded information.
- Earlier, the re-transmission behavior of the TCP implementations was very bad.
- Whenever a packet was lost, the extra packets were sent in by the end points, thus repeating the lost information.
- But this doubled the data rate.
- This is just the opposite of what routine should be carried out during the congestion problem.
- The entire network is thus pushed in a state of the congestive collapse resulting in a huge loss of packets and reducing the throughput of the network.
- Congestion control as well as congestion avoidance techniques are used by the networks of modern era for avoiding the congestive collapse problem.
- Various congestion control algorithms are available that can be implemented for avoiding the problem of network congestion.
- There are various criteria based up on which these congestion control algorithms are classified such as amount of feedback, deploy-ability and so on.