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Blocking
Probability
Based
on our experiences either calling or receiving calls from a
cellular phone, most of us know that not every call works. It
may not reach the person we are calling, it may drop the connection,
and so on.
Because designing coverage that is flawless 100% of the time is not
a realistic goal, RF Designers design their systems according
to a specified blocking probability and grade
of service.
Blocking Probability Defined Blocking
probability is the fraction of time a trunk request is denied because all
channels are busy. This probability is usually specified for
a given system. It is typically desired to be 2%.
A trunk group with a blocking probability of 0.05 means that all
trunks are simultaneously busy 5% of the time or a total of
3 minutes an hour.
Cell site blocking probability is the fraction of time a call
setup attempt at a particular cell site is rejected because all
channels are busy.
Grade of Service (GOS) Most
telephone networks, are designed to provide a specified grade
of service (GOS) during the busy hour.
The GOS is expressed as the percentage of call attempts that
are blocked during peak traffic periods because of a lack of
network resources (such as radio channels in cellular systems).
Typically, landline telephone networks are engineered to a 2% grade of service.
Most cellular systems are designed to a blocking rate of 1% to 5% during the busy hour, with 2% being the most common.
Outside of peak hours, the blocking rate is much lower.
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