• A circuit-switching scenario in which Ncs users, each requiring a bandwidth of
ID: 3876390 • Letter: #
Question
• A circuit-switching scenario in which Ncs users, each requiring a bandwidth of 25 Mbps, must share a link of capacity 100 Mbps.
• A packet-switching scenario with Nps users sharing a 100 Mbps link, where each user again requires 25 Mbps when transmitting, but only needs to transmit 10 percent of the time.
What is the probability that any 4 users (of the total 7 users) are transmitting and the remaining users are not transmitting? (Hint: you will need to use the binomial distribution [1, 2]).
What is the probability that more than 4 users are transmitting? Comment on what this implies about the number of users supportable under circuit switching and packet switching.
Explanation / Answer
(30%) Consider an application that transmits data at a steady rate (for example, the sender generates an N-bit unit of data every k time units, where k is small and fixed). Also, when such an application starts, it will continue running for a relatively long period of time. Answer the following questions, briefly justifying your answer: a. (15%) Would a packet-switched network or a circuit-switched network be more appropriate for this application? Why? A circuit-switched network would be well suited to the application described, because the application involves long sessions with predictable smooth bandwidth requirements. Since the transmission rate is known and not bursty, bandwidth can be reserved for each application session circuit with no significant waste. In addition, we need not worry greatly about the overhead costs of setting up and tearing down a circuit connection, which are amortized over the lengthy duration of a typical application session. b. (15%) Suppose that a packet-switched network is used and the only traffic in this network comes from such applications as described above. Furthermore, assume that the sum of the application data rates is less than the capacities of each and every link. Is some form of congestion control needed? Why? Given such generous link capacities, the network needs no congestion control mechanism. In the worst (most potentially congested) case, all the applications simultaneously transmit over one or more particular network links. However, since each link offers sufficient bandwidth to handle the sum of all of the applications' data rates, no congestion (very little queuing) will occur.
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