小魚缸

1. An upper-layer packet is split into 10 frames, each of which has an 80 percent chance of arriving
undamaged. If no error control is done by the data link protocol, how many times must the message be
sent on average to get the entire thing through?

4. One of your classmates, Scrooge, has pointed out that it is wasteful to end each frame with a flag byte
and then begin the next one with a second flag byte. One flag byte could do the job as well, and a byte
saved is a byte earned. Do you agree?

14. What is the remainder obtained by dividing x7 + x5 + 1 by the generator polynomial x3 + 1?

16. Data link protocols almost always put the CRC in a trailer rather than in a header. Why?

19. In protocol 3, is it possible that the sender starts the timer when it is already running? If so, how might
this occur? If not, why is it impossible?

20. Imagine a sliding window protocol using so many bits for sequence numbers that wraparound never
occurs. What relations must hold among the four window edges and the window size, which is constant
and the same for both the sender and the receiver.

22. In protocol 6, when a data frame arrives, a check is made to see if the sequence number differs from the
one expected and no_nak is true. If both conditions hold, a NAK is sent. Otherwise, the auxiliary timer is
started. Suppose that the else clause were omitted. Would this change affect the protocol's correctness?

29. Frames of 1000 bits are sent over a 1-Mbps channel using a geostationary satellite whose propagation
time from the earth is 270 msec. Acknowledgements are always piggybacked onto data frames. The
headers are very short. Three-bit sequence numbers are used. What is the maximum achievable
channel utilization for
a. (a) Stop-and-wait.
b. (b) Protocol 5.
c. (c) Protocol 6.