Solutions to Chapter 4 - Communication Networks

Communication Networks (2 nd Edition)
Chapter 4 Solutions
P b Load [Erlangs]
1% 43.342
10% 56.059
b. Explain why requests for channels from handoffs should receive priority over requests for
channels from new calls. How does this change the Erlang load calculations
In telephone conversations, service interruption is less acceptable than denial of service due to a
busy network.
The calculations in part (a) assumed equal priority for all calls. If priority is given to current calls
that are being handed off, the Erlang load calculations are more complicated. Newly attempted
calls in this scenario have a lower priority and, thus, a higher P b .
64. Suppose that an analog cellular telephone system is converted to digital format by taking each
channel and converting it into n digital telephone channels.
Solutions follow questions:
a. Find an expression for the number of channels that can be provided in the digital system using
the parameters introduced in problem 62.
Assuming that there will be n times more control channels in this new scheme, the number of
channels in each cell is:
B
( 1−
a)
b n
R
b. Consider the AMPS system and assume that it is converted to digital format using n = 3.
How many Erlangs of traffic and the new system support at 1% blocking probability 5%
This calculation is exactly the same as that in problem 4.62, except that now the number of trunks
is tripled, so c = 160.
To support 1% blocking or 5% blocking, the traffic can be:
P b Load [Erlangs]
1% 141.167
5% 157.046
65. Suppose that a CDMA system has the same number of channels as the digital system in problem
64, but with a reuse factor of 1.
Solutions follow questions:
a. How many Erlangs of traffic can be supported in each cell by this system at 1% blocking
probability 5%
This CDMA system has 416 channels. Assuming that 21 channels are still used for call setup
and the reuse factor is 1, each cell has 395 channels, so we have:
c = 395
a = Erlangs of traffic
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