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CHAPTER 12
Multiple Access
Solutions to Odd-Numbered Review Questions and Exercises
Review Questions
1. The three categories of multiple access protocols discussed in this chapter are ran-
dom access, controlled access, and channelization.
3. In controlled access methods, the stations consult one another to find which sta-
tion has the right to send. A station cannot send unless it has been authorized by
other stations. We discuss three popular controlled-access methods: reservation,
polling, and token passing.
5. In random access methods, there is no access control (as there is in controlled
access methods) and there is no predefined channels (as in channelization). Each
station can transmit when it desires. This liberty may create collision.
7. In a random access method, the whole available bandwidth belongs to the station
that wins the contention; the other stations needs to wait. In a channelization
method, the available bandwidth is divided between the stations. If a station does
not have data to send, the allocated channel remains idle.
9. We do not need a multiple access method in this case. The local loop provides a
dedicated point-to-point connection to the telephone office.
Exercises
11. To achieve the maximum efficiency in pure ALOHA, G = 1/2. If we let ns to be the
number of stations and nfs to be the number of frames a station can send per sec-
ond.
G = ns × nfs × Tfr = 100 × nfs × 1 μs = 1/2 → nfs = 5000 frames/s
The reader may have noticed that the Tfr is very small in this problem. This means
that either the data rate must be very high or the frames must be very small.
13. We can first calculate Tfr and G, and then the throughput.
Tfr = (1000 bits) / 1 Mbps = 1 ms
G = ns × nfs × Tfr = 100 × 10 × 1 ms = 1
For pure ALOHA → S = G × e−2G ≈ 13.53 percent
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