2 PROBLEM SOLUTIONS FOR CHAPTER 1
effects of advertising campaigns by special interest groups of one kind or an-
other also have to be considered. Another major issue is security. A lot of
people might worry about some 14-year kid hacking the system and falsifying
the results.
8. Call the routers A, B, C, D, and E. There are ten potential lines: AB, AC,
AD, AE, BC, BD, BE, CD, CE, and DE. Each of these has four possibilities
(three speeds or no line), so the total number of topologies is 4
10
= 1,048,576.
At 100 ms each, it takes 104,857.6 sec, or slightly more than 29 hours to
inspect them all.
9. Distinguish n + 2 events. Events 1 through n consist of the corresponding
host successfully attempting to use the channel, i.e., without a collision. The
probability of each of these events is p(1 − p)
n − 1
. Event n + 1 is an idle
channel, with probability (1 − p)
n
. Event n + 2 is a collision. Since these
n + 2 events are exhaustive, their probabilities must sum to unity. The
probability of a collision, which is equal to the fraction of slots wasted, is
then just 1 − np(1 − p)
n − 1
− (1 − p)
n
.
10. Among other reasons for using layered protocols, using them leads to break-
ing up the design problem into smaller, more manageable pieces, and layering
means that protocols can be changed without affecting higher or lower ones.
One possible disadvantage is the performance of a layered system is likely to
be worse than the performance of a monolithic system, although it is
extremely difficult to implement and manage a monolithic system.
11. In the ISO protocol model, physical communication takes place only in the
lowest layer, not in every layer.
12. Message and byte streams are different. In a message stream, the network
keeps track of message boundaries. In a byte stream, it does not. For ex-
ample, suppose a process writes 1024 bytes to a connection and then a little
later writes another 1024 bytes. The receiver then does a read for 2048 bytes.
With a message stream, the receiver will get two messages, of 1024 bytes
each. With a byte stream, the message boundaries do not count and the re-
ceiver will get the full 2048 bytes as a single unit. The fact that there were
originally two distinct messages is lost.
13. Negotiation has to do with getting both sides to agree on some parameters or
values to be used during the communication. Maximum packet size is one
example, but there are many others.
14. The service shown is the service offered by layer k to layer k + 1. Another
service that must be present is below layer k, namely, the service offered to
layer k by the underlying layer k − 1.
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