text: what is an erlang?   the dimensionless unit named the erlang is a statistical measure of telecommunications traffic used in telephony. it is named after the danish telephone engineer a. k. erlang, the originator of traffic engineering and queueing theory.  in the traffic calculation, one erlang implies a single resource in continuous use (or two channels at fifty percent use, and so on, pro rata). for example, if a bank has two tellers and they are both busy the whole time, that would represent two erlang of traffic.  the traffic measured in erlang is used to determine if a system is over- or under-provisioned (has too many or too few resources allocated).  the traffic measured over many busy hours might be used for a t1 or e1 circuit group to determine how many voice lines are in use at the busiest hours. for example, if no more than 12 out of 24 channels are ever in use at any given time, the other 12 might be made available as data channels.  traffic measured in erlang is used to calculate grade of service (gos) or quality of service (qos).  there are a range of different erlang formulae, including erlang b, extended erlang b, erlang c and a related engset formula to calculate gos