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(ELEC315)00_spring_final.pdf
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ELEC315 Final Exam Spring 2000 (A)
NAME:_____________________ Student ID:____________________

(1) Suppose we use 5 bits to represent letters and the mapping is shown below.
a: 0 (00000)
b: 1(00001)
c: 2 (00010)
d: 3 (00011)
.....

z: 25(11001) _: 26(11010) ! : 27(11011) , : 28(11100)
We use the RAS public key cypher to send messages and we group 5 bits and encrypt it. The public key (e,n) is (7,33) and the private key (d,n) is (3,33). Assume we receive the following ciphertext:
11111110000011110000
What is the message? (a) dave (b) john (c) lucy (d) july (e) none of the above.
(2)
Assume an application program has a binary stream of 5600 bytes to send. It already has set up a TCP channel with the destination. The TCP layer program breaks the binary streams into three TCP segments (datagrams) and the lengths of the data portions are 2000, 2000, and 1600 bytes. Assume the TCP header is 20 bytes and the sequence field of the first datagram is 2500. What is the sequence field of the TCP header of the second datagram?

(a)
4500 (b) 4480 (c) 2480 (d) 2520 (e) none of the above.

(3)
Before transmission, each packet in question 2 will be processed by the IP (network) layer protocol. Assume the last packet is too long and the IP layer program will break it into two packets. The length field of the first packet is 2000. What is the length field of the second packet? (Assume the IP header is 20 bytes)

(a)
500 (b) 570 (c) 590 (d) 610 (e) none of the above.

(4)
In a data link layer protocol, the sending window size = 4, the receiving window size = 4. The initial state of the window is shown as (I) below. After a while, the senders has sent three packets 0, 1, 2, 3 and receiver has receives three packets correctly 0, 1, and 3 correctly. Which of the following states are possible states describing the current status of the channel ?


0


7 0
1
61

Send
2 5 2 5
2
(I)
(II)
(III)
(IV)


1
1

1
2 2
2
Rcv
2
(a)
(I) and (IV) are possible, but no (II) and (III) (b) only III is possible, but not the rest

(c)
only (II) is possible, but not the rest (d) only (III) is possible, but not the rest

(e)
none of the previous statements are correct.

(5)
The initial sequence number in TCP is tied to the clock value. To make it simple, assume it is the same as the clock value. At t=5 sec, a TCP connection was set up; at t=20 sec it crashed; at t=26 sec, the connection was re-established. Assume the latency of the network is 5 sec. Will there ever be two packets of the same sequence number be produced in the network?

(6)
yes, if the data rate after re-connection is much faster than before (b) yes, if the data rate after

re-connection is much slower than before (c) No. it is impossible to produce two packets of the same sequence number in this case, regardless of the rates before and after the crash. (d) none of the statements are correct

(7)
A C