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Tuesday, August 20, 2013

When is a situation called as congestion?

- 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. 

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