Download 07/08 Semester B - City University of Hong Kong

City University of Hong Kong
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Course code & title : EE6412 Signaling, Switching and Routing in Telecomm Networks
Session
: Semester B 2007/2008
Time allowed
: Two hours
________________________________________________________________________
This paper consists of 4 questions.
1.
Answer ALL four questions.
________________________________________________________________________
Materials, aids & instruments permitted to be used during examination:
1.
Non-programmable calculator
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1
Question 1
(25 marks)
(a) Draw a picture of the simplified U.S. telephone network, including the long-haul core
network, central offices and end-systems. With the help of the picture, give three examples
to illustrate three different kinds of routing situations needed to handle in such a telephone
network.
[9 marks]
(b) Can RTP be used to provide reliable real-time communication? If yes, explain how. If
no, explain why not. Also, if no, should RTP be re-designed to provide reliable real-time
communication?
[8 marks]
(c) Suppose you want to implement an IP telephony system using voice codec G.711.
Assuming 10 msec sampling duration and 64 Kbps voice signal rate, discuss the
transmission efficiency of the resulting system for the following cases: (i) one sample per
packet (ii) two samples per packet and (iii) three samples per packet. Please comment on
the results.
[8 marks]
Question 2
(25 marks)
(a) Design a fully interconnected three-stage network with 1,250 incoming and 1,250
outgoing trunks with minimum number of crosspoints. How many crosspoints does the
network contain? Note that n = (N/2) 0.5.
[5 marks]
(b) If the occupancy of the trunks is 0.6 E and connections are to be made to particular
outgoing trunks, estimate the grade of service. State any assumptions you make.
[5 marks]
(c) Redesign the above network by increasing the number of the secondary switches to
provide a grade of service better than 1 in 500. How many crosspoints does this network
require? Draw the resulting design.
[9 marks]
(d) If connection is required to a particular outgoing route, but any free trunk on that route
may be used, what are the answers for parts (b) and (c), respectively?
[6 marks]
Question 3.
(25 marks)
(a) A TST network has twelve incoming highways and twelve outgoing highways, each
carrying 32 PCM channels. The average occupancy of the incoming channels is 0.3 E.
[9 marks]
(i)
Draw an equivalent space-division network.
(ii)
Estimate the blocking probability as an expander.
(iii) Estimate the grade of service when an incoming call must be connected to a
selected outgoing highway but may use any free channel on it.
(b) A TST and STS switch both have 32 incoming and outgoing highways, each
having 12 PCM channels. The STS network has 32 time-switch links, so both networks
give the same blocking probability. Compare the numbers of crosspoints and bytes of
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storage required for these two networks with the assumption that each memory location in
the connection/speech store requires one byte memory.
[8 marks]
(c) A two-stage digital switching network is to make connections between m incoming
PCM highways and m outgoing PCM highways, each having n channels. There are two
cases: 1) Each call from an incoming PCM highway is to be connected to a particular
channel in a selected outgoing PCM highway, and 2) Each call from an incoming PCM
highway is to be connected to a selected outgoing PCM highway but may use any free
channel on it. For T-S and S-T switching networks, determine the grade of service in
the above two cases when the network has 25 incoming and 25 outgoing 20-channel
PCM systems, each channel having an average occupancy of 0.3 E.
[8 marks]
Question 4.
(25 marks)
(a) (i) Suppose toll switch A wants to set up a connection to toll switch B using random
routing. Assume that there are a direct path and two three-link alternate paths between
switches A and B. If the probability that any link (including direct link) is full is 0.03, what
is the probability that A cannot setup a connection to B?
[4 marks]
(ii) What is the result of (i) if sequential routing is used instead?
[4 marks]
(b) Consider a dynamic routing method, called Most Loaded Routing (MLR), which
chooses the alternate route with the least available capacity if the direct path is blocked. If
all the alternate routes are blocked, the call is rejected. Comment MLR with Sequential
Routing (SR) in term of call blocking probability. Please give the rationale to justify your
comments.
[5 marks]
c) (i) Fill in the details of the following tables after routing information is exchanged
between MPLS routers.
[4 marks]
In
Address Out Out
Label Prefix I'face Label
..
.
In
Address Out Out
Label Prefix I'face Label
..
.
..
.
In
Address Out Out
Label Prefix I'face Label
...
...
0
128.89
0
1
You Can Reach 128.89 thru Me
You Can Reach 128.89
and 171.69 thru Me
1
You Can Reach 171.69 thru Me
3
(ii) Fill in the details of the following tables after labels are assigned and distributed.
[4 marks]
In
Label
Address
Prefix
..
.
..
.
Out
Out
I'face Label
..
.
..
.
In
Label
Address
Prefix
..
.
..
.
Out
Out
I'face Label
..
.
..
.
In
Label
Address
Prefix
..
.
..
.
Out
Out
I'face Label
..
.
..
.
0
0
1
Use Label 9 for 128.89
Use Label 4 for 128.89
and Use Label 5 for 171.69
1
Use Label 7 for 171.69
171.69
(iii) A packet with destination IP address 128.89.25.4 enters the left most MPLS router.
Show the changes of the header field of the MPLS packet when the packet is forwarded
along its label switched path on the above MPLS domain.
[4 marks]
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