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CSC358 Tutorial 1 Notes

Below are some notes and sample solutions to the tutorial questions. These
notes are only meant to provide the necessary amount of information for you to
verify your own work and to help you recall the discussions in the tutorial.
Simply reading these solutions does NOT convey the same learning experience as
attending a tutorial (not even close). The tutorial materials are a mandatory
and important component of this course, so don't miss any of them!
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Question 1:

Review lecture slides for answers.

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Question 2:

(a) 3Mbps / 150 kps = 20 

(b) p = 0.1

(c) Pr(n) = (120 choose n) * p^n * (1-p)^(120-n)  

(d) Pr(21 <= n <= 120) = sum n from 21 to 120 Pr(n) = 0.0079

(e) Discussed in the tutorial.

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Question 3:

(a) d_prop = m/s seconds

(b) d_trans = L/R seconds.

(c) d_end-to-end =(m/s + L/R) seconds.

(d) The bit is just leaving Host A.

(e) The first bit is in the link and has not reached Host B.

(f) The first bit has reached Host B.

(g) Want m/s = L/R so m = sL/R = 536 kilometers 

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Question 4:

(a) Discussed in tutorial.

(b) There are F/S packets. Each packet is S+80 bits. Time at which the last
packet is received at the first switch is (S + 80)/R * (F/S) sec. At this time,
the first F/S-2 packets are at the destination, and the (F/S-1)-th packet is at
the second router. The last packet must then be transmitted by the first router
and the second router, with each transmission taking (S+80)/R seconds. Thus,
the delay in sending the whole file is

Delay = (S+80)/R * (F/S + 2)

Take the derivative and set to zero, i.e., 

(d/d S)Delay  = (2/R) - 80F/R * S^(-2) = 0  implies  S = sqrt(40F)

Note: it is interesting that the optimal S only depends on F and does not
depend on R at all.