NET 351 Computer Network Technologies

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NET 351 Computer Network Technologies

Written Assignment #2 (individual)

The objective of this assignment is to review the material we have covered in chapters 6 and 7.

Note: Your answers must be typed (if not, you will get ZERO), and the late assignments policy will be applied.

Due Date: See the date on Blackboard

IMPORTANT:  you must follow the instructions below. If not, you will lose marks

  • Have all your answers typed
  • Be clear in your answers
  • Don’t copy-past content from Internet, slides, or any other source (write in your own words)
  • Follow the instructions in answering the questions when given (e.g. each step in a new line)
  • Include references (e.g. web site links, book chapters, course chapters, …)


Exercise 1 (15 marks)

Consider the scenario shown in the Figure below, in which there are four wireless nodes, A, B, C, and D. The radio coverage of the four nodes is shown via the shaded ovals; all nodes share the same frequency. When A transmits, it can only be heard/received by B; when B transmits, both A and C can hear/receive from B; when C transmits, both B and D can hear/receive from C; when D transmits, only C can hear/receive from D.

  1. Use the figure below to present and discuss the hidden-terminal problem in wireless networks
  2. Suppose now that A sends a message to B, and D sends a message to C, at the same time. Will there be any collision between the two messages? Why (i.e. explain your answer)?

Exercise 2 (15 marks)

In GSM networks:

  1. Identify two reasons for handoff.
  2. Who initiates handoff in case of moving from one BS to another BS, and both BSs are controlled by the same MSC? You should be clear about which entity. E.g. just saying something like “the origin node” is not enough. What is the name of the node? And what does the “origin node” mean?


Exercise 3 (20 marks)

Suppose an analog audio signal is sampled 8000 times per second, and each sample is using 10 bits.

  1. What would be the resulting bit rate of the PCM digital audio signal (i.e. what is the minimum required bandwidth)?
  2. Is it a good idea to send this signal over a link whose bandwidth is 64 Kbps?
    1. If YES why?
    2. If not, what solution can you propose to make this feasible (i.e. make it possible to send the signal over a 64kbps link and having the signal to be played at the destination in a smooth way)?


Exercise 4 (30 marks)

Consider the figure below. Suppose that the video is encoded at a fixed bit rate, and thus each video block contains video frames that are to be played out over the same fixed amount of time. The server transmits the first video block at t0, the second block at t0+∆, the third block at t0+2∆, and so on. Once the client begins playout, each block should be played out ∆ time units after the previous block.

  1. Suppose that the client begins playout as soon as the first block arrives at t1. In the figure below, which blocks of video (including the first block) will have arrived at the client in time for their playout? i.e. they are not late. NOTE: you need to list the blocks, e.g. blocks 1, 4, 6
  2. Suppose that the client begins playout now at t1+∆. which blocks of video (including the first block) will have arrived at the client in time for their playout?
  3. What is the smallest playout delay at the client, such that every video block has arrived in time for its playout? i.e. no block is late.

Exercise 5 (20 marks)

Alice is at her PC and she wants to call Bob, who is also working at his PC. Alice’s and Bob’s PCs are both equipped with SIP-based software for making and receiving phone calls.

  • Assume that:
    •  Alice knows the IP address of Bob’s PC; i.e.
    • Alice PC IP address is:
    • Bob port number for media: 1200
    • Alice port number for media: 3500
  • Question: Draw a sequence diagram to illustrate the SIP call-establishment process. You should show the port numbers used for the exchanged messages (for both SIP and media), along with the main content of the different messages.



A Note on Collaboration

While students are encouraged to discuss ideas, copying another student’s answers is not allowed and a mark of zero will be assigned to all copied assignments.

Online Material

I would recommend that you save your time and not search for solutions on the Web for the case studies.

What to Submit?

Please type your answers in a Word document and submit a print-out of that document.

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