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Saturday, August 24, 2013

How can the problem of congestion be controlled?

Networks often get trapped in the situation of what we call network congestion. For avoiding such collapses, congestion avoidance and congestion control techniques are often used by the networks nowadays. 

In this article, we discuss about how we can control the problem of network congestion using these techniques. Few very common techniques are:
  1. Exponential back off (used in CSMA/ CA protocols and Ethernet.)
  2. Window reduction (used in TCP)
  3. Fair queuing (used in devices such as routers)
  4. The implementation of the priority schemes is another way of avoiding the negative effects of this very common problem. Priority schemes let the network transmit the packets having higher priority over the others. This way only the effects of the network congestion can be alleviated for some important transmissions. Priority schemes alone cannot solve this problem.
  5. Another method is the explicit allocation of the resources of the network to certain flows. This is commonly used in CFTXOPs (contention – free transmission opportunities) providing very high speed for LAN (local area networks) over the coaxial cables and phone lines that already exist.
- The main cause of the problem of network congestion is the limited capacity of the network. 
- This is to say that the network has limited. 
- The resources also include the link throughput and the router processing time. 
- Congestion control is concerned with curbing the entry of the traffic in to the telecommunications network so that the problem of congestive collapse can be avoided. 
- The over-subscription of the link capabilities is avoided and steps are taken to reduce the resources. 
- One such step is reducing the packet transmission rate. 
- Even though if it sounds similar to flow control, it is not the same thing. 
- Frank Kelly is known as the pioneer of the theory of congestion control. 
- For describing the way in which the network wide rate allocation can be optimized by the individuals by controlling their rates, he used two theories namely the convex optimization theory and the micro economics theory. 

Some optimal rate allocation methods are:
Ø  Max – min fair allocation
Ø  Kelly’s proportional fair allocation

Ways to Classify Congestion Control Algorithm

There are 4 major ways for classifying the congestion control algorithms:
  1. Amount as well as type of feedback: This classification involves judging the algorithm on the basis of multi-bit or single bit explicit signals, delay, loss and so on.
  2. The performance aspect taken for improvement: Includes variable rate links, short flow advantage, fairness, links that can cause loss etc.
  3. Incremental deployability: Modification is the need of sender only, modification is required by receiver and the sender, modification is needed only by the router, and modification is required by all three i.e., the sender, receiver and the router.
  4. Fairness criterion being used: It includes minimum potential delay, max – min, proportional and so on.
Two major components are required for preventing network congestive collapse:
  1. End to end flow control mechanism: This mechanism has been designed such that it can respond well to the congestive collapse and thus behave accordingly.
  2. Mechanism in routers: This mechanism is used for dropping or reordering packets under the condition of overload.

- For repeating the dropped information correct behavior of the end point is required. 
- This indeed slows down the information transmission rate. 
- If all the end points exhibit this kind of behavior, the congestion would be lifted from the network. 
- Also, all the end points would be able to share the available bandwidth fairly. - Slow start is another strategy using which it can be ensured that the router is not overwhelmed by the new connections before congestion can be detected. 

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