What is a High-Loss System?
An Erlang is a unit of measurement of traffic density in a telecommunications system. It describes the total traffic volume of one hour, or 3600 seconds.
For example:
60 calls in one hour, each lasting 5 minutes (on average), results in the following number of erlangs: total minutes of traffic = number of calls x duration = 60 x 5 = 300 hours of traffic in the hour = total minutes of traffic / 60 minutes = 300/60 = 5 therefore, traffic figure = 5 erlangs
When Agner Krarup Erlang developed the Erlang-B and Erlang-C traffic equations, they were developed on a set of assumptions (see grade of service). These assumptions are accurate under most conditions; however in the event of extremely high traffic congestion, Erlang's equations fail to accurately predict the correct number of circuits required because of re-entrant traffic. Network designers use the erlang to understand traffic patterns within a voice network and use the figures to determine how many lines are required between a telephone system and a central office or between network locations. Erlang calculations can briefly be described as follows::
- Erlang B -- the most commonly used traffic model. Erlang B is used to work out how many lines are required if the traffic figure during the busiest hour is known. This model assumes that all blocked calls are cleared immediately.
- Extended Erlang B -- similar to Erlang B, this model can be used to factor in the number of calls that are blocked and immediately tried again.
- Erlang C -- this model assumes that all blocked calls are queued in the system until they can be handled. Call centres can use this calculation to determine how many call agents to staff, based on the number of calls per hour, the average duration of class and the amount of time calls are left in the queue.
A high-loss system is a system where Erlang's equations fail, and congestion breeds further congestion at peak times. In such systems, it is first necessary for many additional circuits to be made available so that the high loss can be alleviated. Once this action has been taken, congestion will return to reasonable levels and Erlang's equations can then be used to determine exactly how many circuits are really required [1]. An example of an instance which would cause such a High Loss System to develop would be if a TV-based competition were to announce a particular telephone number to call at a specific time. In this case a large number of people would simultaneously phone the number provided. If the service provider had not catered for this sudden unanticipated peak demand, extreme traffic congestion will develop and Erlang's equations cannot be used [1].
References:
- Kennedy I., School of Electrical and Information Engineering, University of the Witwatersrand, Personal Communication.
- Erlang: http://www.webopedia.com/TERM/E/erlang.html
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Last edited by Mookho Tsilo on Feb 17, 2006 6:36 am US/Central.