CS 640 - Midterm Exam - Fall 1997 - Name ________________________________

1. (10) Two parts; 5 points for each part; no partial credit
Assume a generator polynomial G(x) = x**6 + x**2 + x + 1 and a frame 10110101001010. 
a. Give the CRC for this G(x) and frame. 




b. Give the shift register corresponding to G(x).





2. (16) Provide BRIEF answers 
a.   What is the principal difference between circuit switching and virtual circuit switching?






b. Give an advantage of datagrams over virtual circuits.






c. Give an advantage of virtual circuits over datagrams. 






d. Define "connection-oriented service interface."






3. (16) Describe how LAPB (X.25 Level) deals with (be sure to relate your answers to 
specific protocol mechanisms:
a. error detection 






b. handling of detected errors






c. detection of frame boundaries.



NAME__________________________


d. flow control (include description of relevant commands/responses)








4. (10) Assume the network of Figure 1 with links labeled by costs. Assume use of SPF. 
Show the order in which nodes are added to S where S contains nodes for which the shortest 
path from A has been found.  For each node in S, give its predecessor on the path from A, 
the source node.






5. (10) Assume the network of Figure 2 where all links have cost equal to one. Consider 
the routing tables entries at nodes B, C, D, E, and F  for destination A. Assume use of 
a distance vector algorithm where routing tables are always exchanged at the same time 
by all nodes. 

After the link B <----> A fails, the entries for destination A are:

	at B	infinity
	at C	B 2
	at D	C 3
	at E	B 2
	at F	C 3
a. Assume split horizon and hold-down are not used. Give the values for the routing table 
entries at B, C, D, E, and F for destination A after each of the next two routing table 
exchanges.








b. Repeat a) using split horizon.








c.      Repeat a) using hold-down and split horizon.








 



NAME _______________________________

The remaining questions are True/False (T or F) - 1 point each
6.              7.           8.            9.            10.           11.            12.        
13.          14.          15.
16.          17.          18.          19.           20.           21.            22.    
23.          24.          25.          26.           27.           28.            29.    
30.          31.          32.          33.    
34.          35.          36.          37.           38.           39.    
40.          41.          42.          43.           44.

ATM 
6. Switched virtual circuits differ from permanent virtual circuits in that cells may not 
arrive in the order in which they were sent.
7. AAL5 packets have a total length that is a multiple of 48.
8. The AAL5 SAR layer consists of a single field in the ATM header.
9. When ATM is used over 802.2,  some ATM cells may have total length more than 53 bytes. 
10. AAL5 has mechanisms to provide guaranteed bit rates
11. ATM provides a reliable connection-oriented service
12. ATM uses virtual circuit switching.
BRIDGES 
13. When bridges are used, optimality of path length of routes is not a goal.
14. Bridges are sometimes preferred to routers because they scale better than the latter.
15. A disadvantage of bridges is that they always broadcast an incoming packet on every line 
except the one on which the packet arrived.
802.5 
16. Automatic retransmission occurs when a frame returns to the sender with an incorrect FCS.
17. Tokens are distinguished from data frames by a single bit.
18. Any station that sees a frame with an incorrect FCS removes it from the ring .
19. The active monitor is the only station that can send a "claim_token" frame.
20. The MAC protocol provides a fair service to all stations.
21. If the maximum bandwidth (in the standard) were increased, the minimum frame size would 
also increase.
22. A station that receives a frame can remove it from the ring if the frame has synchronous 
data in it. 
802.3 
23. Every station that senses a collision can send a jam signal.
24. The backoff procedure on collisions does not guarantee that no station will be starved.
25. If the bandwidth (in the standard) were decreased, the minimum frame size would increase.
26. A repeater connects different collision domains.
27. If an ack is not received within a specified time, 802.3 retransmits the frame.
28. A bridge can simultaneously be connected (via different ports) to 10 and 100 Mbps 802.3 
networks. 
29. A sending station retransmits immediately after it detects a collision.
FDDI 
30. More than one token can be on the ring at any time.
31. The receiving station may remove a frame from the ring if it heads an incorrect FCS.
32. At most two frames can be on the ring at a given time.
33. Some synchronous traffic can always be sent when a station gets the token. 
MISCELLANEOUS
34. If the sequence number of frames for a link protocol is given by a four bit field and 
the receive window is five, then the maximum send window is ten.
35. Sliding windows can be used by a link protocol that provides a connectionless service 
interface.
36. LAP B does not use credits to inform a sender of a receiver's current  receive window.
37. Byte-oriented link protocols are preferable to bit-oriented link protocols because they 
depend on a fixed character set representation.
38. Bandwidth reservation is required for virtual circuits but not for datagrams.
39. The bandwidth delay product is a measure of how much pipelining is needed for efficient 
operation of a window oriented link protocol.
ROUTING
40. Routing loops cannot occur with link state protocols.
41. A routing protocol is said to be stable if it never directs a packet to an incorrect 
destination.
42. A distributed routing protocol depends on specially designated nodes to compute and 
then distribute routing tables.
43. Distance vector is preferred to link state in networks with long paths. 
44. The SPF algorithm no longer is used by link state protocols.