Editing Grade of service (section)

==Erlang's lost call assumptions==
To calculate the Grade of Service of a specified group of circuits or routes, [[Agner Krarup Erlang]] used a set of assumptions that relied on the network losing calls when all circuits in a group were busy. These assumptions are:<ref name="flood" />

*All traffic through the network is pure-chance traffic, i.e. all call arrivals and terminations are independent random events
*There is statistical equilibrium, i.e., the average number of calls does not change
*Full availability of the network, i.e., every outlet from a switch is accessible from every inlet
*Any call that encounters congestion is immediately lost.

From these assumptions Erlang developed the Erlang-B formula which describes the probability of congestion in a circuit group. The probability of congestion gives the Grade of Service experienced.<ref name="flood" />