Number of lines required
The number of lines required is under a predefined grade of service e.g. 0.5% of GOS or 0.1% of GOS at peak busy hour ( normally will be the typical type of services, different services will have different peak busy hour)
e.g. local telephone lines
at 0.5% of GOS at peak busy hour,
erlang B is using with full availability, 10 lines only offers to carry 3.96 erlang.