Csci B438 - Test 1 - Due October 7, 2003

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Work independently and show your work. Read thoroughly and contact me immediately at rwisman@ius.edu if clarification is required. Do all *’ed questions and an additional 20 points.

1) (5) In a last minute compromise between crusty old people and too tall people for the passage of the Information Highway Bill, Congress approved the bits per second rate each person would be provided to be equal one’s age in years multiplied by their height in millimeters. For example, my Bps is based on 54 years times 1676 millimeters or 90504 bps. Following the organization of the table in Tanenbaum, Figure 2-2, page 89, complete the four columns for your age and height bits per second in one row. Determine for a 100 Khz. line, sending 13 bits using a bi-level encoding. Show work.

2) (5) Using the Hamming error correcting code construction method described in the notes to construct a one-bit error correcting code that uses 4 check bits and as many data bits as possible. Your answer should show the computational expressions for the data bits m, check bits p, and the error position vector.

3a*) (5) Design a coding scheme that uses a combination of amplitude and phase modulation capable of transmitting the alphabet ('A'-'Z') at one character per baud. Illustrate design as in Fig. 2-25, p. 128 of Tanenbaum.

b*) (5) From 3a) list the amplitude in volts and phase in degrees or radians that correspond to all 26 characters.

 
4) (10) Do all.
 
 a) What percent of the 1.544 Mbps of a T1 carrier delivers data to the end user?
 
 b) Suppose that you need to deliver 10 billion bytes of data stored on several 650 million byte CD’s as quickly as possible. You can walk at an average of 6 kilometers per hour. At what distance can you send the data over a T1 carrier more quickly than walk the CD’s? Show your calculations.
 
 c) A 1000 km communications channel operates at the T1 rate of 1.544 Mbps. The propagation speed in the channel is 2/3 the speed of light. How many bits fit in the channel?

5*) (20)

1. void sender3(void) { 2. seq_nr next_frame_to_send; 3. frame r; 4. frame s; 5. packet buffer; 6. event_type event; 7. next_frame_to_send = 0; 8. from_network_layer(buffer); 9. while (true) { 10. s.info = buffer; 11. s.seq = next_frame_to_send; 12. to_physical_layer(s); 13. start_timer(s.seq); 14. wait_for_event(event); 15. if( event == frame_arrival) {  16. from_physical_layer(r); 17. if(r.ack == next_frame_to_send) {  18. from_network_layer(buffer); 19. next_frame_send = 20. (next_frame_to_send + 1) % 2; 21. } 22. } 23. } 24. }

25. void receiver3(void) { 26. seq_nr frame_expected; 27. frame r; 28. frame s; 29. event_type event; 30. frame_expected = 0; 31. while (true) { 32. wait_for_event(event); 33. if( event == frame_arrival) { 34. from_physical_layer(r); 35. if(r.seq == frame_expected) { 36. to_network_layer(r.info);  37. frame_expected = 38. (frame_expected + 1) % 2;  39. } 40. s.ack = 1 - frame_expected; 41. to_physical_layer(s); 42. } 43. } 44. }

Protocol 3 improved Protocol 2 by avoiding deadlock on lost frames and preventing the delivery of duplicates to the network layer.

a. What would be the effect of switching lines 13 and 14?

b. Explain the effect of receiver3 process stopping?

35.  
36.  to_network_layer(r.info); 
37.  frame_expected = 
38.  (frame_expected + 1) % 2; 
39.  
 

Protocol failure occurs when the network layer of the receiver is not delivered exactly the same data as sent from the sender network layer. The protocol now fails by changing lines 35-39 to those above.

c. Give at least one type of failure now possible (e.g. deadlock, lost frames, etc.).

d. Describe a scenario in which the failure occurs to support your answer in c.

e. Assume that no transmission errors, timeouts or other problems occur. Can a receiver running Protocol 2 receive data from a sender running any of the Protocols 1, 3, or 4 as listed in the text? Give a short explanation for each protocol.

6*) (10) The following diagrams the frames sent by A with a timeout interval of 3 frames. Frame timers are independent, a timer is started immediately after each frame is sent, triggering a timeout event whenever the timer expires on that frame. Assume A has unlimited frame buffers.

a) Using Go Back N, complete filling in the diagram of the sender and receiver frames and Acknowledgements assuming that the Ack 1 is lost. Indicate discarded frames on the receiver.

b) State the maximum number of frames buffered by A.

c) How many additional frames are transmitted due to the single transmit error?

Click for diagram only.

7) (10) The following diagrams the frames sent by A with a timeout interval of 3. Frame timers are independent, a timer is started immediately after each frame is sent, triggering a timeout event whenever the timer expires. Assume A and B have an unlimited number of buffers.

a) Using Selective Repeat, complete filling in the diagram of the sender and receiver frames and Acknowledgements assuming that the Ack 1 is lost. Indicate discarded frames on the receiver.

b) State the maximum number of frames buffered by A and B.

c) How many additional frames are transmitted due to the single transmit error?

Click for diagram only.

8) (5) A channel has a bit rate of 4 kbps and a propagation delay of 20 milliseconds.

a) What is the minimum frame size for which stop-and-wait such as Protocol 2 yields an efficiency of at least 50 percent?

b) What would be a nearly ideal timeout period for Protocol 3 running over the channel?

9*) (5) Compute the checksum on the message containing as data two of your initials in 8-bit ASCII code using the polynomial x⁴+x¹+1. Show work.