Department of Computer Science
Always explain your answers concisely and be sure to be to-the-point.
This part covers the same material as the midterm exam.
1a What is the difference between a network service and a network protocol? Is it possible to have
different implementations of the same protocol?
A service describes what the network offers to its clients by means of an interface. A protocol is a
set of rules describing exactly how that service is to be realized through the exchange of messages
between computers. These rules may be implemented differently (e.g., using different programming
languages), as long as those implementations coincide with the rules.
1b While considering traditional POTS modems, Bob claims that “...it is unusual to use a 2400 baud
modem. Modems usually support 56 kbps.” What is wrong with this statement. Motivate your
Traditional POTS modems still support 2400 baud, which tells how many times a signal may change
per second. The higher data rate is obtained through modulation schemes.
1c What type of multiplexing is used for ADSL? Explain your answer.
Frequency division multiplexing: the bandwidth of the cable is partitioned into separate 4 kHz chan-
nels. This is typical for FDM.
2a Explain why bit stuffing is needed, and how it works.
Bit stuffing is needed when using frame delimiters such as 01111110. In order to send the delimeter
as data in a frame, a zero is inserted after five consecutive 1s have been transmitted. The receiver
simply removes the zero following 5 1s in row.
2b The physical layer sends data as a “continuous” stream of bits. The data link layer uses chunks of
bits, called frames. Why switch to frames?
Continuous bit streams make it impossible to detect errors, to synchronize transmission rates, etc.
2c Data link protocols almost always put the checksum in a trailer, rather than in a header. Why?
Yes, it’s a question from the problems section, and it has been asked before. The CR