Short note: Grade of service

Grade of Service (GoS) is a measure of the quality of service of a telephone network in the busy hour. In a loss system it is the proportion of calls that are lost because all servers are busy:

$$GoS = \frac{\text{number of calls lost}}{\text{number of calls offered}} = \frac{A_{lost}}{A_{offered}}$$

In a delay system it is the probability that a call waits longer than a stated time.

Key points

• GoS lies between 0 and 1; a smaller value means better service. Typical design values: 0.002 for local exchange paths, 0.01 for trunks, 0.02–0.05 for cellular channels.
• It is fixed for the busy hour, so service is better at other times.
• For Poisson traffic offered to N trunks, GoS equals the Erlang B blocking probability $B = E_{1,N}(A)$.
• For a connection passing through several stages or links in series, overall GoS ≈ sum of the individual GoS values (when each is small): $B \approx B_1 + B_2 + \dots$

Example

In the busy hour 1000 calls are offered and 10 are lost: GoS = 10/1000 = 0.01 (1%).

Use

GoS is the design target in traffic engineering: given the offered traffic and GoS, the number of trunks is found from Erlang B (or Engset/Erlang C) tables. A tighter GoS needs more circuits and so costs more.