Model Paper – 3 SECTION-A Q1. Attempt all questions. All questions carry equal marks. (10x2=20) A. What is congestion? Why congestion occurs? Ans : In a packet switching network, packets are introduced in the nodes (i.e. offered load ), and the nodes in-turn forward the packets (i.e. throughput ) into the network. When the “offered load” crosses certain limit, then there is a sharp fall in the throughput. This phenomenon is known as congestion . In every node of a packet switching network, queues (or buffers) are maintained to receive and transmit packets (store/forward network). Due to busty nature of the network traffic there may be situations where there is overflow of the queues. As a result there will be re-transmission of several packets, which further increases the network traffic. This finally leads to congestion . B. What are the two basic mechanisms of congestion control? Ans : The two basic mechanisms of congestion control are: One is preventive, where precautions are taken so that congestion can not occur. Another is recovery from congestion, when congestion has already taken place C. In what way token bucket algorithm is superior to leaky bucket algorithm? Ans : The leaky bucket algorithm controls the rate at which the packets are introduced in the network, but it is very conservative in nature. Some flexibility is introduced in token bucket algorithm. In token bucket algorithm tokens are generated at each tick (up to certain limit). For an incoming packet to be transmitted, it must capture a token and the transmission takes place at the same rate. Hence some of the busty packets are transmitted at the same rate if tokens are available and thus introduces some amount of flexibility in the system. This also improves the performance. D. What is choke packet? How is it used for congestion control? Ans : Choke packet scheme is a close loop mechanism where each link is monitored to examine how much utilization is taking place. If the utilization goes beyond a certain threshold limit, the link goes to a warning and a special packet, called choke packet is sent to the source. On receiving the choke packet, the source reduced the traffic in order to avoid congestion. The congestion control in the choke packet scheme can be monitored in the following manner.
Each link is monitored to estimate the level of utilization. If the utilization crosses a certain threshold limit, the link goes to a warning state and a choke packet is send to the source.On receiving the choke packet, the source reduces the transmitting limit to a certain level (say, by 50%). If still warning state persists, more choke packets are sent further reducing the traffic. This continues until the link recovers from the warning state. If no further choke packet is received by the so urce within a time interval, the traffic is increased gradually so that the system doesn’t go to congestion state again.
You've reached the end of your free preview.
Want to read all 25 pages?
- Summer '14
- Computer network, Sita, choke packet, choke packet scheme