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Number of circuits

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  • #33259

    I have a PBH (peak busy hour)between 15:30-18:00, with 40 contemporary calls average. Maximum number of contemporary calls,in the PBH, are 90-100. How many circuits do I need?


    Alex Yuen


    Peak busy hour is refering to a particular busy hour in which is the peak of the whole day.
    Assuming PBH call is 100, you still need to know the call holding time in this specified peak busy hour.
    Assuming average call holding = h in seconds,
    traffic in erlang = h * 100 / 3600
    Then for a given GOS, you can find the circuit requirement.

    Hoping this information can be useful to you.

    Alex Yuen

    PS: you can contact me for traffic related problem.


    Surely if you have 40 contemporary calls, then you have 40 Erlangs, and you need 53 circuits if blocking is 0.01 (1%) and 60 circuits if it is 0.001 (0.1%). Using the Erlang-B Calculator on line on this site. Mind you if the peak number of contemporary calls is 90 to 100 then you need 90 to 100 circuits to carry them, as calls are not unifor but very peaky. Would be interesting to know what is behind the circuit that cause this call demand. Also Peak Busy Hour is actually 2 and a half hours, so engineering to the average will under size, though engineering to the peak of 100 could oversize unless the peak is measured over a half to one hour duration.
    Assuming 100 Erlangs and 0.1 (10%) blocking then 97 circuits are needed, increases to 117 if blocking is 0.01 (1%). Interestingly, at 0.15 (15%) blocking, 90 circuits are needed, while at 0.413 (41%) only 60 circuits are needed. Thus telecom costs for this link could be increased by 25% to 50% depending on how the problem is approached and what service level is demanded! In answer to the original question, at least 53, probably 60, maybe 100+ depending on what the traffic is going to or coming from. If it is provided over a carrier circuit, you probably need to have 3-5xT1 (24 channels per T1) or 2-4xE1 (30 channels per E1) and it may depend on what the customer is prepared to pay for. If the end user application is a call centre, then the customer may not want to lose a call so 120 circuits are needed. On the other hand it may be a 200 extension office PABX where it seems every extension makes an outgoing call around 5PM and is otherwise fairly constant with less than 1 in 4 extensions on calls over this time. In this case even 60 circuits may be too much.
    One needs to understand the reason for the peak calls and the inconvenience to the caller/called party if the calls are blocked.


    For a gateway switch, how we sall calculate total traffic(erlang). Will it be only outgoing , only incoming or adding both

    Alexis Reyes

    Depends on TG definitions (TwoWay or OneWay TG), How Many circuits do you have in these TG. If this Switch is 100% gateway then the incoming traffic must be equal to the outgoing traffic then the name of the TG is different but the circuits are the same. Finally the total traffic is the number of erlang of one of the TG defined.
    I think that you have the same capacity on both TG (in & out).

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