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Erlang Calculation

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  • #62924
    Sineesh C

    How we can calculate Erlangs ?

    Suppose we have 35 channels.
    Blocking probability is 0.02
    As per Erlangs B Calculation
    Erlangs is 26.4.

    How it is possible?

    My doubt :
    We have 35 resources or Channels.If all resources are using continuously for one hour, it is 35 Erlangs. If we are considering blocking of .02,the available Capacity should more than 35 Erlangs. Is it?



    it doesn’t work this way, you are looking at it from the system point of view. The erlang law is all about consumer QoS 🙂
    if you have 35 channels, how much traffic can this sytem carry assuming that subscribers should not be rejected more than 2% of the attempts, and assuming (and that’s important) a random arrival of calls.

    The answer to this question is 26.4 erlang.
    If the system carries more than this, the rate of calls which are rejected will be more than 2%.


    Sineesh C

    Hi Pix,

    Thanks for the reply.
    A little more clarification required.

    Let we have 35 channels,
    Then only 26 user(since capacity is 26.4 Erl) is possible to confirm the blocking probability less than 2%. Is it ?


    hello sineesh,

    1 user doesn’t use 1 erlang, does he ?
    If that is the case, then yes, you should dimension 35 channels to handle 26 subscribers.

    But I doubt that is the case. Usually in GSM, 1 user = 25 mErlang (0.025erl)



    Hi Sineesh,

    i think u r a bit confused of the definition of erlang. of course, as u mentioned an erlang s the volume of traffic carried in one hour. Even though it very well means that if a subscriber uses the traffic for continuous one hour he gives us 1 erlang, it doesnt mean that all the subscriber uses call for one hour.

    u cannot mingle no. of channels with erlang. Coz there s no concept of giving a single channel to a subscriber. There are hopping principles etc etc. An ocean called channel concepts. Where as Erlang is jus a unit. Jus to denote that how many erlangs or how much money he has given us.

    Hope u r clear.

    Sineesh C

    Hi Prabeen and Pix,

    Normally one user doesn’t use one Erlangs.

    Average user of one User is .09 Erl in BSNL 0.042 Erl in MTS, 0.02Erl in Tata.
    All the above values are assumptions.

    For Example in BSNL Average Erlangs is 0.09Erl

    It means that each user is active for the period 9 percentage of an Hour

    Means 60(min) *9/100 = 5.4 min

    BSNL assuming MS active for the period of 5.4minutes in one Hour

    My doubt is ,

    If we have 35 channels, it will support 35 simultaneous calls.Assume each call holding
    for one Hour.
    One MS holding one hour means, this MS using 1 Erl.

    So 35 MS using 35 Erlangs.

    So the system of 35 channels providing 35 Erlangs Capacity with 0% Blocking probability (35 channels 35 Users)

    So With 2 % Blocking , We can say the system has more than 35 Erl Capacity.

    This is my concept. I know there is some mistake in this .Please help me to find out


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    Hey Sineesh,

    Beautiful calculations and predictions but i think its a bit wild… Wat say Pix??

    Comin to point: U r again trying to converge Channel with Erlang. SO, again am gonna say the same. 35 channel not gonna support 35 subscribers alone. U cannnot predict the number of subscribers possible with number of channels.

    Its supported by number of TRX. Via no. of TRX u can find how many erlangs can these many TRX support. Say: I have 2 TRX. This can support around 7 erlangs with 2% GOS. Now the 7 Erl shall be divided with 25merl (Average capacity of 1 subscriber) which gives around 280 subscribers. This is the way to calculate no. of subscribers possible with given no. of TRX.

    There is no concept of calculating subs with no. of channels, as no perticular channel will be dedicated to a sub unless ur system is a nonhopping one.

    Post = Understood?????

    Sineesh C

    Hi Mr.Praveen ,

    With 7 Erl , 280 simultaneous user is not possible. 280 is the maximum number of
    connection that the system can provide based on the assumption that the average utilization(not
    average capacity)of one user is 0.025 Erl.This value is not constant.It will vary for
    different operator

    I am not telling that with 35 Channels supports only 35 connection.But 35 channels
    can support only 35 simultaneous Calls.

    Please tell me how many simultaneous calls your system can support with two TRX ?


    yes, 35 channels can carry 35 calls simult.
    but that’s not the point of the erlang law. The erlang law is ALSO taking into account the statistical (random) arrival of calls, and what QoS the subscriber experiences.
    The number of simultaneous calls is useless, it is an instantaneous indication of your system load.

    Sineesh C


    In system with 35 channels,

    Can I hold 35 call for one Hour (for test purpose) ?


    Iam tired……

    Sineesh C

    Are u tired ?.Then i will hold, you just watch



    Yes, of course.
    35 channels = 35 simultaneous calls
    35 parking spaces = 35 parked cars

    problem is that if mobile n°36 tries to get a channel, it will be rejected.
    Here, your blocking rate is : 1/36 = 2.7%


    and who knows what happens to rejected MS. They start drinking, get high, and ends up shooting random iPhones in the streets. Not pretty. Not pretty at all.

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