COSC 3213: Computer Networks
Winter 2005
Assignment # 3
Instructors: Amir Asif and Natalija Vlajic
Review chapter 3 (section 3.8) and chapter 5 (section 5.2) before attempting the assignment.
Question 1: If the ratio of error detection bits (EDB) to payload bits remain constant, what are the tradeoffs
between:
(a) having the error bits applied to smaller blocks of data, i.e. spread out over the sequence (see Figure 1), vs.
(b) having the error bits applied to bigger blocks of data (see Figure 2).
Figure 1
Figure 2
Question 2: Suppose a header consists of the following bits:
11111111 11111111 11111111 00000000 11110000 11110000 11000000 11000000
Find the 16-bit Internet checksum for this code.
Question 3: In this problem, we will generate the CRC codeword for the information bits 110110 using two
different generator polynomials and then compare the error detection capability of the two codes.
(a) Calculate the CRC codeword corresponding to the information bits 110110 if the generator polynomial
used is given by g 1 ( x)  x  1 .
(b) Repeat (a) for the generator polynomial g 2 ( x)  x 3  x 2  1 .
(c) What is the error detection capability of (b)? In other words, determine if single, double, and triple bits of
errors can be determined in the codeword produced in (b). If not, give an example of an error pattern that
cannot be detected in (b).
(d) Repeat (a) for the generator polynomial g 3 ( x)  g1 ( x) g 2 ( x) . Comment on the error detection capability
of the codeword produced.
Question 4: A telephone modem is used to connect a personal desktop to a host computer. The speed of the
modem is 56 kbps and the one-way propagation delay is 100ms.
(a) Find the efficiency of the stop-and-wait ARQ if the information frame size is 256 bytes with 8 bytes of
overhead. Assume the length of the ACK frame is 8 bytes and a bit error rate of 105 for the
communication channel. Repeat for a frame size of 512 bytes.
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(b) Repeat (a) for the Go-back-N ARQ if three-bit sequence numbering is used with frame size of 256 bytes
assuming a bit error rate of 105.
Repeat (b) for the Selective Repeat ARQ.
Question 5: Two neighboring nodes (A and B) use the selective-repeat ARQ with a 3-bit sequence and a
window size of 4 at the transmitter and receiver. Assuming A is transmitting and B is receiving, show the
content of the window and the positions of the respective pointers at both nodes A and B for the following
succession of events.
(a) Before A sends any frames.
(b) After A sends frames 0, 1, and 2; B acknowledges 0 and 1, and both ACK’s are received by A.
(c) After A sends frames 3, 4, and 5; B acknowledges 4, and the ACK is received by A.
Question 6: a 64-kilobyte message is to be transmitted from the source to the destination, as shown below.
The network limits packets to a maximum size of two kilobytes, and each packet has a 32-byte header. The
transmission lines in the network have a bit error rate of 10-6 and Stop-and-Wait ARQ is used in each
transmission line. How long does it take on the average to get the message from the source to the
destination? Assume that the signal propagates at a speed of 2*105 km/sec.
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