Midterm Exam
CS 638 - Computer Networks
Fall 1990


1. Define or explain each of the following and 
why they are used
a) sliding window 
b) selective repeat
c) character stuffing
d) bit stuffing 
e) negative acknowldgement or NAK

2. The MBI and CED corporations have announced that their network
protocol offerings comply with the OSI model. 
a) What does this mean?
b) What does OSI model compliance tell 
you about the interoperability of their products?
Be sure to explain your answers.

3. a) Define "virtual circuit" and explain what it means for a subnet
to use virtual circuits. 
b) What advantages does use of virtual circuits have over use of 
datagrams?
c) Explain, the relationship (similarities/differences) 
between the circuits used in circuit switching  and 
virtual circuits.

4. a) Briefly explain how a distance vector routing algorithm works.
b) Explain "split horizon" and "hold down" and why they are
used in distance vector routing algorithms (illustration by example
will be helpful).
c) Are similar mechanisms used by link state algorithms - explain 
your answer.

5. The IP/ICMP protocols form the basis for the IP Internet.
In the context of the IP Internet
a) What is the function of IP?
b) What is "subnetting" and why is it used?
c) What are "fragmentation" and "reassembly" and why are they 
needed?
d) Explain one of: ICMP Rediret or ICMP Time Exceeded.

6. Consider the following network where link costs are given by the 
labels on each link. Consider use of SPF at node A, assuming A
knows the complete topology of the network (i.e., all links and their
costs). Give the order in which the nodes are added to the set S by the
SPF algorithm, assuming that routes from A are to be computed.