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Traffic Engineering and Sizing


Answer Seizure Ratio (ASR). 1

Average Hold time (AHT). 1

Network Effectiveness Ratio (NER). 2

Erlang. 2

·       Erlang B. 3

·       Extended Erlang B. 3

·       Erlang C. 3

·       Erlang Loading Factor: 3

BHCA.. 3

Formula. 3


Answer Seizure Ratio (ASR)

ASR gives the relationship between the number of seizures that result in an answer signal and the

total number of seizures. This is a direct measure of the effectiveness of the service being offered and is usually expressed as a percentage as follows:


ASR Formula

Measurement of ASR may be made on a route or on a destination code basis.


Average Hold time (AHT)

AHT is a metric for the average duration of one transaction, typically measured from the customer's initiation of the call and including any hold time, talk time and related tasks that follow the transaction.


Network Effectiveness Ratio (NER)

NER is designed to express the ability of networks to deliver calls to the far-end terminal. NER expresses the relationship between the number of seizures and the sum of the number of seizures resulting in either an answer signal, or a user busy, or a ring no answer, or in the case of ISDN a terminal rejection/unavailability. Unlike ASR, NER excludes the effects of customer behavior and terminal behavior.


NER Formula



NER Formula




An Erlang is a unit of telecommunications traffic measurement.  Strictly speaking, an Erlang represents the continuous use of one voice path.  In practice, it is used to describe the total traffic volume of one hour.

For example, 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



Hours of traffic in the hour


150 / 60

Hours of traffic in the hour



Traffic figure


2.5 Erlangs

Erlang traffic measurements are made in order to help telecommunications network designers understand traffic patterns within their voice networks.  This is essential if they are to successfully design their network topology and establish the necessary trunk group sizes.

The main Erlang traffic model are listed below, with links to the free online calculators on this Web site:

  • Erlang B
    This is the most commonly used traffic model, and is used to work out how many lines are required if the traffic figure (in Erlangs) during the busiest hour is known.   The model assumes that all blocked calls are immediately cleared.

Erlang calculator

Blocking is the failure of calls due to an insufficient number of lines being available.  E.g. 0.02 mean 2 calls blocked per 100 calls attempted.

  • Extended Erlang B
    This model is similar to Erlang B, but takes into account that a percentage of calls are immediately represented to the system if they encounter blocking (a busy signal).  The retry percentage can be specified.
  • Erlang C
    This model assumes that all blocked calls stay in the system until they can be handled.  This model can be applied to the design of call center staffing arrangements where, if calls cannot be immediately answered, they enter a queue.


·         Erlang Loading Factor: Usually we use a multiplier instead of Erlang B or C methods. This multiplier is called Erlang Loading Factor.


Use the following formula to calculate the Erlang value:

Traffic in Erlangs = (Number of calls in the busy hour * AHT in sec) / 3600 sec


The number of call attempts in the busy hour is called Busy Hour Call Attempts (BHCA).
In the standard business environment, the busy hour of any given day accounts for approximately 15 to 20 percent of the traffic for that day.



·         BHCA=CAPS*60*60

·         CPS = (Total Ports  x Erlang loading) / (ASR x AHT)

Erlang = BHCA * AHT/3600

AHT is in sec

 Erlang per Sub = BHCA per Sub * AHT in Sec/ 3600


Online Erlang Calculators




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