Solutions to Chapter 4 - Communication Networks

Communication Networks (2 nd Edition)
Chapter 4 Solutions
Solutions follow questions:
a. Explain how a multicast connection may be implemented in a crossbar switch.
In a crossbar switch, multicasting can be implemented by copying the multicast input to multiple
outputs.
b. How does the presence of multicast connections affect blocking performance of a crossbar
switch Are unicast calls adversely affected
Unlike unicast switches, flow in multicast switches is not conserved. Generally, if multicasting is
implemented in a switch, the total traffic flow out of the switch is more than the flow into the
switch. Thus the probability of blocking is increased because the traffic inside the switch is
higher.
Unicast traffic is also affected by this increased flow because fewer resources in the switch are
available.
c. Explain how multicast connections should be implemented in a multistage switch. Should the
multicast branching be done as soon as possible or as late as possible
In a multistage switch with a given multicast traffic profile, each branching procedure represents
an increase in traffic flow beyond the branch point. Thus, to keep traffic load in the switch to a
minimum, branching should be done as late as possible.
31. Do a web search for crosspoint switching chips. What specifications do you find for number of
ports, bit rate/port Are dualcasting and multicasting supported Do the crosspoints need to be
changed together or can they be changed independently What considerations determine how fast the
crosspoints can be changed
Solution:
Some specs for Crosspoint fabrics chips:
Switch size: 2x2, 4x4, 17x17, 34x34, 36x37, 68x69, 64x64, 72x72, 144x144
Port speed: 1.6 Gbps, 2.5 Gbps, 3.2 Gbps, 3.6 Gbps
Dualcasting and multicasting is usually supported by crosspoint chips, as well as masking
particular ports, and also programming a single port or a subset of ports independently.
The speed of changes depend on the clock recovery and signal synchronization at the
port interfaces (receivers and transmitters at input and output ports of the crosspoint
fabric), and also the speed of programming the I/O Table registers.
32. What is the delay incurred in traversing a TSI switch
Solution:
The arriving frame is written onto the switch register and after the frame is completely written in, it
needs to be read out in permuted order.
33. Explain how the TSI method can be used to build a time-division multiplexer that takes four T-1
lines and combines them into a single time-division multiplexed signal. Be specific in terms of the
number of registers required and the speeds of the lines involved.
Solution:
Leon-Garcia/Widjaja 16