CS 487 Second Homework Assignment

Due Tuesday September 12 in class.

1. W & V Problem 2.1

2. W & V Problem 2.5

3. W & V Problem 2.6

4. W & V Problem 2.7

5. In general, packets are generated asynchronously and thus the
beginning of a packet can arrive at a node at any time. Suppose
that a computer wants to communicate with a terminal by sending
a group of one byte extended ASCI characters that comprise 
a frame or packet. One character is reserved to denote the 
beginning of the frame and another for the end of the frame, thus
allowing variable length frames.

(a) How can we handle occurrences of these special characters
in the data part of the frame?

(b) Extend this idea to bit rather than byte oriented frames.


6. One way to deal with burst errors (errors that affect a group
of consectutive bits in a packet) is to arrange successive
groups of k bits into n rows and then add a parity bit for
each row and column. If the data bits are entered by rows,
they are transmitted by columns. If an error burst can be r bits long,
how would you choose k and n? What are the principle advantages and
disadvantages of this method?

7. In a positive acknowledgement and retransmission (PAR) scheme, a node
sends a packet to the next node and waits for an acknowledgement
that the packet has been received. Suppose there is a probability
p that the packet does not reach the next node correctly and a 
probability q that an acknowlegment is not received correctly.

(a) What is the probability that a packet will be sent correctly
and correctly acknowledged the first time it is transmitted?

(b) What is the probability the the sender will time out and
resend the packet successfully on a second try?

(c) How many times on the average must the sender send a 
packet before it is successfully acknowledged?

(d) If a data packet contains is n bits long and contains
k data bits and the acknowledgement packet is r bits long,
what is the efficiency of this scheme in terms of the ratio
of bits of data/bits sent. Is this ratio a fair measure of the
efficiency of the method? Explain.

8. Suppose that we wish the use a 4 bit CRC check for each 8 bit
ASCII character using the generator polynomial X^4+X^3+1. What
is transmitted for the character 'A'? What does the CRC check
yield if the first bit is incorrect? the first and last? the
first four?

9. Two companies plan to offer networking services to the
public. The first offers only a connection-oriented virtual
circuit service while the second only offers a connectionless
packet switching service. How should each charge for its
services? What kind of business should each try to attract?