Information Technology Reference
In-Depth Information
aggregate trunk usage for a system and do not apply to any specific trunk or call. The
hour used for the Erlang calculation should be the busiest hour (peak hour) in the day.
If a group of users makes/receives 20 calls in the average busiest hour and each call lasts
and average of 10 minutes, the Erlangs are calculated as follows:
20 calls per hour * 10 minutes per call = 200 minutes per hour
Traffic volume = (200 minute s per hour) / (60 minute s per hour) = 3.33 Erlang s
The name Erlang came from the inventor, Agner Krarup Erlang, a Danish telecom engineer
and mathematician who defined many formulas still used today in the design of voice sys-
tems. The most commonly used formulas are known as Erlang B, Extended Erlang B, and
Erlang C.
There are three common Erlang models:
Erlang B
is a for mula that estimates the amount of trunking capacity required given an
Erlang value (busy-hour traffic) and a desired Grade of Service (also known as block-
ing percentage). It is the most common model used. Extending the previous example,
3.33 Erlangs (BHT) and a GoS of 1 percent results in an Erlang B value of nine trunks
required. An Erlang B calculator can be found at
www.erlang.com/calculator/erlb.
■
Extended Erlang B
adds a “retry” percentage to the Erlang B model. It assumes that
some blocked or failed calls will be reattempted, and therefore additional load is added.
■
Erlang C
queues excess calls instead of blocking them. This model is used to calcu-
late the number of agents required in a call center environment. It is based on meas-
urements of handling time, expected call volumes, and the amount of time a caller
spends with an agent. This model is used in call centers where calls are queued for
service.
■
Centum Call Second
A call second is equivalent to a single call lasting 1 second. A Centum call second (CCS)
represents one call occupying a channel for 100 seconds. It is the equivalent of 1/36th of
an Erlang. In other words, 36 CCS equals 1 Erlang (3600 seconds). The typical range is
around 6 to 12 CCS per port.
Busy Hour
The busy hour is the specific hour within a 24-hour period in which the highest traffic
load occurs. Most calls are placed and are of longer durations during this hour. It is also
called peak hour.
Busy-Hour Traffic
Busy-hour traffic (BHT) is the amount of voice traffic that occurs in the busy hour, ex-
pressed in Erlangs. It is calculated by multiplying the average call duration by the number
of calls in the hour and then dividing that by 3600.
