Call Blocking in Telephone Systems
In an ErlangB telephone system, N channels are available. New calls are assigned a channel until all channels are full.
Whenever all channels are occupied, a new call is blocked. That is, it is denied a channel. the assumption is that that the calls is lost and that the calling subscriber will not retry again.
This in contrast to an ErlangC system, in which new calls are queued, until they can be served.
Model
The number of active calls is a Markov process, i.e., at any instant t
the number of active calls statistically only depends on the number of active calls at t  Dt, with very small D t.
New calls arrive according to Poisson process with rate l calls per unit of time. Calls have a (memoryless) exponential duration with mean 1/m. Hence a call terminates with "rate" m and for i calls the rate of termination
of one call is i timesm.
 
Figure: Markov model.
The state (0, 1, ...or N) represents the number of occupied channels in a network with a total of N channels.


The probability of being in state i, thus with i channels occupied,
can be found from solving the "balance equations" (at equilibrium, the rate of moving into a state should equal the rate of moving out of a state).
One finds
A^{i} 1
P_{i} =  
i! N A^{i}
S 
i=0 i!
where A is the offered traffic expressed in erlang (l/m)
It can take a fairly long time before the measured blocking rate resembles the expected value. Click here to stop this animation and remove the animation display.
Since the continuoustime process is simulated in discrete time steps, some discrepancies are likely to appear betwen the simulation and the theoretical values, particularly for short average call durations. Thanks to wim Vallenduuk for pointing out that there is a problem here.
PASTA
The "Poisson arrivals see timeaverages" (PASTA)rule says that the probability that a newly arriving call sees a full system equals the timeaverage probability that the system is in state N. Thus the blocking probability equals P_{N}.
Computer Software
Example and Exercise
A GSM base station uses one carrier of 8 TDMA users.
Find the traffic load (in erlang)
for which the blocking rate is 1%. Answer