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TrafficFinder! TM  -  Telecommunications Traffic Basics

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Help Topics: Calls are presented to a device such as a PBX system that has a limited number of resources (servers or trunks). The resources are usually limited due to their expense. There are normally less resources than people so some blocking occurs.  In the scenario below: 3 of 4 calls are serviced 1 of 4 calls are blocked so the blocking rate is 25% or P.25

Basic Traffic Calculator

Trunk Estimator

Erlang B Overflow

Agent Calculator

Peaked Traffic (Neal-Wilkinson)

Traffic Tables

Traffic Principles


Traffic Models:


Erlang B

Extended Erlang B

Erlang C

Erlang B Peaked(Neal-Wilkinson)




Traffic Tables:

Poisson Traffic Table

Erlang B Traffic Table

Erlang C Traffic Table

Erlang B Peaked(Neal-Wilkinson)

If the system is designed properly, people can share limited resources with an acceptable amount of blocking. Designing a system involves estimating the traffic usage and then engineering the facilities needed to provide an acceptable level of service to its users.
Traffic is measured in increments of time. One Erlang is equal to one hour of continual use. Another measurement of traffic is the CCS (Centum Call Seconds, or 100 seconds) One hour is equal to One Erlang or 36 CCS.

In theory, one trunk can handle 36 CCS or One Erlang of traffic per hour.  This is usually not the case though because calls are presented in a random order.  This would only be possible if users took turns and only requested a trunk when others were finished.  In practice, trunks are only capable of handling a small portion of their total capacity because they sit idle waiting for someone to request their use.
 There are time proven statistical formulas for determining the number of Servers or Trunks needed to provide a certain percentage of blocking.  There is a relationship between the amount of traffic, the number of trunks and the percent blocking.  The following table uses the Poisson statistical formula and depicts the relationship.
Erlangs of Traffic

Trunks Needed for 1% Blocking (P.01)

Trunks Needed for 10% Blocking (P.1) Trunks Needed for 50% Blocking (P.5)
10 19 15 11
20 32 27 21
30 44 38 31
40 56 49 41
50 68 60 51
60 80 71 61
70 91 82 71
80 103 93 81
90 114 103 91
100 125 114 101