City University of Hong Kong
______________________________________________________________________
Course code & title : EE6412 Signaling, Switching and Routing in
Telecommunication Networks
Session
: Semester B 2008/2009
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
______________________________________________________________________
Question 1
(25 marks)
(a) A simplified version of Session Initiation Protocol (SIP) only contains five signaling
messages: ACK, BYE, INVITE, OK, and RINGING. Complete the following diagram to
show how these signaling messages can be used to establish and disconnect a call
between two users.
[5 marks]
User A
User B
INVITE
Cal establishment between two users
Voice communication channel
Disconnect call by user A
Fig. 1: Signaling diagram between two users in establishing and disconnecting a call.
(b) How does the receiver in a VoIP system determine whether a packet is first in a talkspurt? Consider two cases: i) loss channel, and ii) lossless channel.
[6 marks]
(c) Caller with email address <jim@umass.edu> and IP address 217.123.56.89 places a
call to <keith@upenn.edu> using SIP through procedures (1)  (9) shown below. Briefly
describe these procedures.
[6 marks]
SIP registrar
upenn.edu
SIP
registrar
eurecom.fr
2
SIP proxy
umass.edu
1
3
4
5
7
8
6
9
SIP client
217.123.56.89
SIP client
197.87.54.21
1
(d) Suppose you want to implement an IP telephony system. Look up the RTP payload
format for G.711. Assuming 15 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) Describe the advantage of a three-stage switching network over a two-stage switching
network used as an expander?
[4 marks]
(b) 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]
(c) Calculate the grade of service (blocking probability) provided by the network in part
(b) when it operates as a route switch and is offered 120 E of traffic.
[4 marks]
(d) Sketch an STS network to connect m incoming lines to m outgoing lines, each
carrying n PCM channels. i) Explain how to make a connection from the second
incoming PCM highway in the first time slot to the first outgoing PCM highway in
the third time slot? Assume that all channels are free in the sixth time switch and all
channels are busy in all other time switches. ii) Explain how the S-T-S switching
network makes the corresponding switching for the connection mentioned in i)?
[10 marks]
Question 3
(25 marks)
(a) Draw a diagram of the simplified U.S. telephone network, including the long-haul
core network, central offices and end-systems. Using the diagram, give three examples to
illustrate three different routing situations required in such a telephone network.
[8 marks]
(b) 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 two
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]
(c) What is the result if Sequential Routing is used instead?
[4 marks]
2
(d) 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 be
used. Draw a sequence of diagrams 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]
Question 4
(25 marks)
a) Consider the MPLS network shown in Figure Q.4a.
[15 marks]
i. Describe briefly how to distribute the labels when the Label Switched Path is setup in
an MPLS domain.
ii. Suppose that a packet with destination address 128.89.25.4 enters the MPLS network
from the left. Show how the packet goes through the network using a diagram.
iii. Suppose that we want to perform flow aggregation so that packets with destination
network address 128.89.0.0 will join the same class of the packets with destination
network address 171.69.0.0. Write down the three modified tables which would make
this possible.
iv. Suppose that another packet with destination address 128.89.25.4 enters the MPLS
network from the left after flow aggregation mentioned in (iii). Show how the packet
goes through the network using a diagram.
v. In addition to the flow aggregation of (iii), suppose that we want to perform further
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 171.69.0.0. Write down
the three modified tables which would make this possible.
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
0
3
3
171.69
0
-
1
117.59
1
3
3
117.59
0
2
2
.
117.59
1
.
..
.
0
128.89
0
1
1
117.59
3
(b) Assume that the network shown below uses label switching technique for
communication between computer users and that only two label numbers (1 and 2)
can be used in each link. Considering a situation where Computer A wants to
communicate with Computer D, illustrate the disadvantages of using the same label
number along the path over using the different label numbers. Note that the number
associated with each link represents the label number which has been used.
[5 marks]
1
A
1
2
1
B
D
2
C
(c) Why does the label need to be removed one hop prior to de-aggregation point for the
case of label sharing in MPLS? In what situation is this prior label removing
unnecessary?
[5 marks]
-END-
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