When planning a private branch exchange (PBX), an audiotext system like CapiCall, a fax server like CapiFax, or a dial-in modem pool for remote access to a corporate server, there is a useful formula found by the Danish A. K. Erlang to find out the required number of lines. It also allows to guess how many operators must be in a call center to handle all or at least most incoming calls.
First of all, "Erlang" is a traffic unit, describing the total traffic volume of one hour. For instance, if you get 30 calls in one hour and each has an average duration of 5 minutes, the traffic figure will be (30 * 5) / 60 = 2.5 Erlang. It is obvious that you will need at least three lines to handle this traffic. But even then, due to the random nature of calls, you will still have a significant rate of callers who do not get through and hear a busy signal instead.
Extended Erlang B is quite similar to Erlang B, but it assumes that some calls are immediately represented to the system if they encounter blocking (a busy signal). Erlang C assumes that all blocked calls wait in a queue infinitely until they can be handled. We do not care about these two variants here since they cover situations which are unwanted under normal circumstances.
© 09/2004 Herwig Feichtinger, Shamrock Software GmbH