erlang
A dimensionless statistical measure of the volume of telecommunications traffic relative to the capacity of a single channel.
erlang: traffic intensity unit for telecom channels
An erlang is a dimensionless number that expresses how fully a telecommunications channel or circuit is being used. One erlang means the channel is occupied continuously; 0.5 erlang means it is occupied half the time on average. The unit allows engineers to compare traffic load against available capacity in a common language, whether they are managing a switchboard with ten lines or a digital trunk with hundreds of channels.
The erlang is calculated by measuring the total duration of all calls in a given time period, then dividing by the length of that period. If a phone line handles 30 minutes of calls in an hour, that is 0.5 erlang. If it handles 60 minutes of calls in 60 minutes (continuous use), that is 1 erlang. In practice, traffic engineers sample busy-hour usage: the 60-minute period each day when call volume peaks. This busy-hour traffic figure, expressed in erlangs, becomes the basis for provisioning.
The unit is named after Agner Krarup Erlang, a Danish telephone engineer who developed the mathematical models for telephone traffic in the early 1900s. His work, particularly the Erlang B and Erlang C formulas, lets planners calculate how many circuits are needed to handle a predicted traffic load while tolerating a specified percentage of blocked calls. Erlang B is used for systems that reject overflowing calls; Erlang C is used for systems with queuing.
Practical use and limits
In modern telephone exchanges and VoIP networks, traffic is still measured and reported in erlangs. A PBX handling 500 erlangs at busy hour needs sufficient trunk capacity to the public network, or calls will be blocked or queued. Internet service providers use erlang-derived metrics to size links and equipment. However, the erlang assumes calls arrive randomly and have random durations, which is a simplification; real traffic patterns cluster around events and times of day.
The erlang has also become a shorthand in billing and service-level agreements. When an operator sells n erlangs of capacity to a customer, that customer is entitled to occupy the circuit continuously for n hours per hour, averaged over the billing period. The unit bridges technical measurement and commercial transaction.