Traffic engineering is an integral building block in voice capacity planning and in ensuring a certain level of service to users. In the traditional telecommunications world, certain terminologies and calculations are used, that now apply to the world of IP-based voice communication. Some of these terms and values are as follows:
Grade of Service: This is a probability that a call will be blocked by the voice gateway during the busiest hour. This value is expressed as the following fraction (or percentage):
[Number of Lost calls / Number of offered calls] * (100)
Erlang: An Erlang represents the average number of concurrent calls carried by a single resource (such as a trunk) in continuous use, where that average is calculated over some reasonable period of time. In practice, an hour of traffic it is used. The following example, taken from here shows how to calculate an Erlang:
If a group of user made 30 calls in one hour, and each call had an average call duration of 5 minutes, then the number of Erlangs this represents is worked out as follows:
Minutes of traffic in the hour = number of calls x duration
Minutes of traffic in the hour = 30 x 5
Minutes of traffic in the hour = 150
Hours of traffic in the hour = 150 / 60
Hours of traffic in the hour = 2.5
Traffic figure = 2.5 Erlangs
Centum Call Seconds (CCS): A CCS is a value that represents 100 call seconds, meaning a 100 second long call. The following formulas better describe a CCS:
100 call seconds = 1 CCS
3600 call-seconds = 36 CCS = 1 call-hour.
3600 call-seconds per hour = 36 CCS per hour = 1 call-hour per hour
1 call-hour per hour = 1 Erlang
Therefore, 1 CCS = 1/36 Erlangs
Busy hour: The 60-minute period in a given 24-hour period during which the maximum total traffic load occurs. This is sometimes called the peak hour.
Busy-Hour Traffic (BHT): The BHT value represents the number of hours of traffic that is transported across a trunk group in its busiest hour. This value is expressed in erlangs.
There are three most common Erlang traffic models that are used in the industry. These are:
Erlang B: The most commonly used of the three methods, Erlang B is a traffic model is a queuing mechanism used to calculate the number of lines that are required if the traffic figure is known (in erlangs) during the busiest hour. The model assumes that all blocked calls are immediately cleared. The Erlang B formula can determine the number of trunks, or lines, needed to handle a calling load during a one-hour period. However, the formula assumes that if callers get a busy signal, they will never retry, that is, all lost calls are completely cleared. This assumption means that Erlang B can underestimate the number of trunks needed.
Extended Erlang B: This model is similar to the Erlang method, but it considers the additional traffic load that is caused when blocked callers immediately try to call again. Unlike the Erlang B model, the retry percentage can be specified and therefore the problem of undersubscription or underestimation is avoided.
Erlang C: is a traffic modeling formula used in call center scheduling to calculate delays or predict waiting times for callers. Erlang C bases its formula on three factors: the number of reps providing service; the number of callers waiting; and the average amount of time it takes to serve each caller. Erlang C can also calculate the resources that will be needed to keep wait times within the call center’s target limits. This method assumes that there are no lost calls or busy signals, and therefore may overestimate the staff that is required.
Filed under: Design, Technical, erlang, erlang b, erlang c, voice design, voice engineering, voice traffic engineering
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