ECE/CS 438  Communication Networks
Fall 2008

Homework 3
Assigned on September 26
Due by class time on October 3, 2008

------------------------------------------------------------------------

(1) Suppose that Ethernet addresses are chosen uniformly at random 
	(that is, the probability that a particular address is chosen
		by a host is identical for all addresses).

	(a) What is the probability that on a 1024-host network,
		two hosts will choose the same address ?

	(b) If we consider 1000 networks each containing 1024 hosts,
		what is the probability that two hosts on at least
		one network will choose the same address ?

    An Ethernet address contains 48 bits.

(2) Consider the following two possibilities:

    (i) We put 20 hosts on an Ethernet operating at 10 Mbps.

    (ii) We create two separate Ethernets, with 10 hosts operating
         on each of the Ethernets, and each of the Ethernets
	 operating at 5 Mbps.

    Suggest one potential advantage and one potential disadvantage 
    of approach (i) compared to (ii).

(3) The use of RTS-CTS exchange prior to data transmission
    with IEEE 802.11 does not eliminate all packet losses.

    Suggest two reasons for this.

(4) Explain how the choice of the contention window affects the
    performance of IEEE 802.11 (particularly, how does it affect
    collisions and idle time).