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# Data Communications Essay

1198 words - 5 pages

COMP2330 Data Communications and Networking
Written Assignment 1

Sample Solutions
1. Which of the OSI layers handles each of the following: (a) Dividing the transmitted bit stream into frames. (b) Determining which route through the subnet to use.
(10 marks)
Ans:
(a) Data Link Layer
(b) Network Layer
2. An image is 1024x768 pixels with 3 bytes/pixel. Assume the image is uncompressed. How long does it take to transmit it over a 56-kbps modem channel? Over a 1-Mbps cable modem? Over a 10-Mbps Ethernet? Over 100-Mbps Ethernet? ...view middle of the document...

5. We have a message D = 1010001101 (10 bits). Using a predetermined divisor P = 110011 (6 bits), please find the FCS (frame check sequence) using CRC technique. (15 marks)
Ans:
The FCS is 01110.
Remark: if your answer is “1110”, you will be wrong because the length of FCS is exactly 5 bits, i.e., one bit less than the length of the divisor P.

6. Isn Sliding-Window flow control, suppose k bits are used to specify the sequence
numbers of frames. It can support the range of sequence numbers from 0 to 2k-1.
Explain, by an example, why the maximum window size can not exceed 2k-1.

Ans: We consider the Go-back-N scheme which is based on sliding window flow control. Suppose k=3, i.e., a 3-bit sequence number is used to denote the frames. Suppose a sender sends frame 0 and gets back an RR1 from the receiver. Suppose window size is 23=8. The sender continues to send frames 1, 2, 3, 4, 5, 6, 7, 0 in its window and get another RR1. However, the sender does not know the meaning of this RR1. It could mean that all eight frames are received correctly and the RR1 is a cumulative acknowledgement. It could also mean that all eight frames are damaged or lost in transmit, and the receiver is repeating its previous RR1. Therefore, window size should be less than 23.

7. A channel has a data rate of R bps and a propagation delay of t s/km. The distance between the sending and receiving nodes is L km. Nodes exchange fixed-size frames of B bits.
What is the propagation time?
What is the frame transmission time?
How to set the window size to achieve the maximum channel utilization in the Sliding-Window flow control? (15 marks)
Ans:
Propagation time (in seconds): t*L
Frame transmission time (in seconds): B/R
In order to achieve the maximum channel utilization, the window size should be at least . The notation means the smallest integer greater than or equal to x.

8. Suppose Selective-Reject ARQ is employed in error control. Explain, by example, how to deal with the case that RR message is damaged.
Ans:
Suppose receiver gets frame i and replies sender RR-(i+1) which is lost or damaged. If sender receives the next acknowledgement, i.e., RR-(i+n), before its timer expires, transmission resumes without errors. If sender’s timer expires, it sends a request to receiver asking for instructions. The receiver receiving this request will send sender a new RR which indicates the next frame it expects.

9. Explain the basic idea of the Statistical Time Division Multiplexing.
...

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