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EE489 In-Class Test # 1 – Winter Term 2007

  1. 50 minute exam, ending at 9:50 am. There are five main questions offering a raw total of 50 possible marks: Get marks wherever you can.

Question 1 - 10 marks

Question 2 - 10 marks

Question 3 - 10 marks

Question 4 –10 marks

Question 5 – 10 marks

  1. Closed-book. Only a formula sheet (8.5”x11” page provided by you), a calculator, and writing instruments are allowed. Use the backs of the exam pages for rough calculations, if necessary. No scrap paper allowed. If extra paper is needed, ask the instructor.
  2. Write answers on the test paper in the spaces provided (write on the back of pages where necessary). Show your work. Check that you have SIX pages including this cover sheet. Do not unstaple your exam.
  1. Sign the class list before you leave.
  2. Do the questions you find easiest first. If stuck on one question goon to get the marks you can from the next one.

Marking

Question 1 / /10
Question 2 / /10
Question 3 / /10
Question 4 / /10
Question 5 / /10

Total

/ /50

Question 1(2 marks per part. 10 marks total)

A network has 20 nodes. Using simple sketches, equations, or other reasoning, work out

(a) How many (bidirectional) links are needed to create a full mesh network for these 20 nodes?

(b) How many two-link routes exist between any one pair of nodes in this network?

(c) How many links are needed if a pure tree of three hierarchical levels is used to interconnect the 20 nodes? (If needed to work out your answer, assume one top node, 4 second-level nodes, the rest third level).

(d) Does it actually matter in (c) how many hierarchical levels there are if the network is a pure tree? How many different routes are there in a pure tree between each node pair?

(e) State two ways or aspects in which practical networks are actually hybrids with structures in between a full mesh and a pure tree.

Question 2(10 marks total)

With the aid of a few sentences and simple sketches where helpful…

(a) Explain what is meant by “traffic concentration” and by “multiplexing” and how they differ.(3 marks)

(b) What is an echo suppressor? What is an echo canceller? When is one or the other used and what is the main difference between them? Explain in your own words the operational concept of an echo canceller. (4 marks)

(c) State three advantages, and three disadvantages of converting an analog network into an all-digital network. (3 marks)

Question 3(5 marks for each part, 10 marks total),

(a) For each of the following traffic theories, state:

(i) if the numbers of sources (M) is infinite or finite,

(ii) the relevant blocked call handling model, i.e., BCC, BCH, BCW,

(ii) suitability or accuracy for use at high blocking (Yes/No)

Erlang B:
Poisson:
Binomial:
Erlang C:
Engset:

(b) Which of these theories requires statistical equilibrium and in your own words, what is the concept of statistical equilibrium as it applies to traffic theory?

(c) What guideline or criteria suggests when an Engset calculation would be more appropriate than an Erlang B calculation?

(d) Which of the five theories above generally predicts the highest carried traffic, and why?

(e) One group of ten trunks and another group of 100 trunks both operate at 1% blocking under nominal conditions. (i) In which group are the servers individually busier onaverage and why? (ii) When traffic is raised unexpectedly above the design level,one group has higher blocking than the other. Which one is it? (N.B. No calculation is needed)

Question 4(5 marks each part, 10 marks total)

(a) Using Tables attached, specify the number of trunks needed to satisfy the following requirement. Show all steps.

Nominal busy hour conditions: Expect 150 calls / hour, 5 min / call on average. Provide < 1% blocking

Allowance for overload: Blocking should not get worse than 5% upon a 30% overload

(b) Also using Tables, if trunks must carry 0.3 Erlangs each to be economically justified, then at what level of offered traffic is it deemed economic to add one more trunk to an existing group of 5 trunks?

Question 5 (10 marks possible in total)

Explain the role of the “hybrid” circuit in the telecommunications network. Touch on: (i) the function it performs, (ii) the issue of balance return loss – what does it arise from?, (iii) how transhybrid loss affects the subjective quality of connections from an echo standpoint, and (iv) how it affects the potential stability of connections.

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