Download 06/07 Semester B - City University of Hong Kong

City University of Hong Kong
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Course code & title
:
EE6412 Switching and Routing in Tele Networks
Session
:
Semester B 2006/2007
Time allowed
:
Two hours
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This paper consists of 4 questions.
1.
Answer ALL four questions.
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Materials, aids & instruments permitted to be used during examination:
1.
Non-programmable calculator
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1
Question 1.
(25 marks)
(a) Given that n = (N/2)1/2 and that the number of links between stages is equal to the number
of the incoming trunks, design a three-stage network for connecting 120 incoming trunks
to 120 outgoing trunks. If there are more than one solution, determine which one is better.
Draw the end result.
[7 marks]
(b) If the above switching network is used as a route switch in a telephone network, derive its
blocking probability. Assume that the occupancies of links between the primary switches
and the secondary switches be p, between the secondary switches and the tertiary
switches be q, and the outgoing trunks be r, respectively.
[6
marks]
(c) Assuming that 30 erlangs of traffic is offered to the above network, calculate the blocking
probability.
[6 marks]
(d) For the offered 30 erlangs of traffic, what is the blocking probability if the numbers of
links between stages and the number of secondary switches are increased by 20%.
[6 marks]
Question 2.
(25 marks)
(a) A three-stage fully interconnected switching network is to connect 600 incoming trunks
(M) to 1000 outgoing trunks (N). It is to use switches assembled from blocks of size 55.
Design a suitable network and determine the number of switch blocks required. Also,
draw the designed switching network. Note that m = M (N + M)0.5 where m is the
number of inlets in primary switch modules.
[7 marks]
(b) In what sense a multiple-stage switch is better than a single-stage switch? What are the
disadvantages of a multiple-stage switch when compared with a single-stage switch?
[4 marks]
(c) Sketch a STS network to connect m incoming lines to m outgoing lines, each carrying
n PCM channels. Explain how to make a connection from the third incoming PCM highway
in the second time slot to the fifth outgoing PCM highway in the first time slot? Assume that
all channels are free in the sixth time switch and all channels are busy in all other time
switches.
[6 marks]
(d) An S-T-S network has 16 incoming and 16 outgoing highways, each of which conveys 24
PCM channels. Between the incoming and outgoing space switches there are 20 links
containing time switches. During the busy hour, the network is offered 200 E of traffic
and it can be assumed that this is evenly distributed over the outgoing channels.
(i)
Derive 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.
[8 marks]
2
Question 3.
(25 marks)
(a) Suppose toll switch A wants to set up a connection to toll switch B using random routing.
Assume that there are one direct path, three two-link alternate paths and one three-link
alternate paths between switches A and B and any of those paths can be used. 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]
(b) What is the result if least loaded routing is used instead?
[4 marks]
(c) Consider a four-node fully connected telephone network. Assume each link has two
channels and only one-link direct paths and two-link alternative paths are allowed to use.
Draw a sequence of pictures to show how the system can go from an empty state to a fully
utilized state for the following two cases: 1) all calls are directly routed and 2) all calls are
alternatively routed. Compare and comment these two cases especially in terms of network
efficiency.
[9 marks]
(d) Consider a circuit-switched network with a three-link path (OABD) connected in
series with link capacity N1, N2 and N3, respectively. The offered traffic are OOABD,
OOA1 and OAB2 , OBD3 from node pairs (or link) of OD, OA, AB, and BD to the path
OABD, link 1, link 2 and link 3, respectively.
[8 marks]
(i.) Express the blocking probability for link 1 and link 2, denoted as B1 and B2,
respectively. Let E(O, N) is the Erlang B formula with offered load O and link
capacity N;
(ii.) Express the blocking probability for the path OABD;
(iii.) Express the offered traffic to link 2 and link 3 due to path traffic and link traffic;
(iv.) Express the carried traffic for link 1 and link 3 due to path traffic and link
traffic. Are they in general the same?
Question 4.
Consider the MPLS network show in Figure 3.
(25 marks)
(a) Describe how to forward an IP packet at the core label switching router in an MPLS
domain.
(b) Suppose that a packet with destination address 117.59.25.4 enters the MPLS network
from the left hand side. Draw down a diagram to show how the packet goes through the
network.
(c) Suppose that we want to perform flow aggregation so that packets with destination
network address 117.59.0.0 will join the same class of the packets with destination network
address 128.89.00. Write down the three modified tables which would make this possible.
(d) Suppose that another packet with destination address 117.59.25.4 enters the MPLS
network from the left after flow aggregation mentioned in (c). Draw down how the packet
goes through the network.
(e) Suppose that we want to perform traffic engineering in the MPLS network shown in
Figure 3 so that packets with destination address 171.69.13.9 will now go the upper path of
the network. Write down the three modified tables which would make this possible.
3
In
Label
Address Out
Out
Prefix I'face Label
In
Label
Address Out
Out
Prefix I'face Label
In
Label
Address Out
Out
Prefix I'face Label
7
128.89
1
4
4
128.89
0
7
7
128.89
0
-
2
171.69
1
5
5
171.69
1
7
2
117.59
1
-
2
117.59
1
3
3
117.59
0
2
..
.
..
.
..
.
..
.
0
128.89
0
1
1
117.59
Figure 3
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