In what language is Erlang written?

ANSI-C. 

Do you have an MS Excel add-in rather than a standalone program?

Yes. See DLLs.

Why does the Poisson model have a limit of 708 erlangs?

This limit is fixed in Versions 4.x.

What equations are behind the Erlang B (Erlang C, Etc.) models?

See the Equations page.

Does the program handle call center apps?

Lets work an example. Lets say......

Can I call the DLL functions from:

• Visual Basic (VB)?

  Yes. This is described by Microsoft on the following page. 
  http://msdn.microsoft.com/library/devprods/vs6/vbasic/vbcon98/vbcondeclaringdllprocedure.htm

  Also, download the free Erlang DLL Help file to see examples of how to call the Erlang DLL functions from Visual Basic.

• MS-Visual Studio V6.0

  Yes. Here are some notes from a customer:

    1. Determine exported functions: dumpbin /EXPORTS poisson.dll 

    2. Manually create a .def file to import by ordinal number:
       LIBRARY POISSON
       EXPORTS
       POIS_CalcNumServ@16 @1 ;

    3. Create .lib using .def file: 
       lib /def:poisson.def /out:poisson.lib 

    4. Declare function: 
       int FAR pascal _export POIS_CalcNumServ (double dErlangs, double dADA_AHT);
       (see list of all function prototypes in the "Header/Include Files" 
       section of the Erlang DLL Help File.

    ===================
    Notes
    ===================
    1. Using "lib /def:poisson.dll /out:poisson.lib" didn't work. I got the
       following link error: 
       "error LNK2001: unresolved external symbol _POIS_CalcNumServ@16".
       The only workaround was to use a .def file (lib /def:poisson.def /out:poisson.lib).

    2. I wasn't able to use 
       "__declspec(dllimport) int POIS_CalcNumServ (double dErlangs, double dADA_AHT);" 
       but had to use 
       "int FAR pascal _export POIS_CalcNumServ (double dErlangs, double dADA_AHT);" 
       to declare the function.

• VB.NET

  Yes. As a windows form app (vb.net) the DLLs can be called directly.

• ASP.NET Application?

  Yes. If the DLLs are UNLIMITED, no special precautions are needed. But if the DLLs are Personal DLLs, problems have been seen related to the DLLs being able to check the registry for the existence of a valid Erlang-G license key. One solution has been found to be: initiate a separate thread to handle the processing of the Erlang functions.

• C++?

  Yes. See the DLL Help file for information on how to turn off name-mangling in your function prototypes.

• Lotus 1-2-3?

  Yes. You can call the DLL functions using the LotusScript language, which is very similar to Microsoft VisualBasic. See this page on the Lotus web site. (Or visit http://www.lotus.com, and Search for "How do I call DLL functions").
  Yes. You can call the DLL functions using the LotusScript language, which is very similar to Microsoft VisualBasic. See this page on the Lotus web site. (Or visit http://www.lotus.com, and Search for "How do I call DLL functions").

• .NET?

  Yes. But, the because the size of a "Long" in .NET is 8 bytes, the function declarations must change all "LONG" declarations to "INT". 

Will any of these produce the number of trunks needed to support peak volumes?

    (Calls per Hour - in BUSY HOUR) * (Minutes Occupied by each caller)
     ----------------------------------------------------------------------------
                                      60

    REQUESTS = calls
    SERVERS = phone trunks
    QUEUES = phone queues

    REQUESTS = data packets
    SERVERS = data links (usually only one)
    QUEUES = buffers built into things like routers or TCP/IP stacks on PCs

Do the "Call Center" and "Erlang C" models support 30 minute intervals or only 60 minute intervals ?

Yes. These models ask for a CALL RATE expressed in units of 'Calls per hour', but that does not mean this call rate must be maintained for exactly one hour. 

For example;
If you drive a car for 10 km in 30 minutes, the speed is 20 km-per-hr.
If you receive 10 calls in 30 minutes, the call rate is 20 calls-per-hour

In what language is VoIP written?

ANSI-C. 

Should I use Erlang-B or Poisson models?

Question:
I am trying to determine the best traffic modeling formula to use for VoIP, such that we wish to know necessary bandwidth when converting to VoIP.

Answer:

1) Decide if your source population is

• (F) FINITE (e.g. an office with a known number of phones/employees) or
• (I) INFINITE (e.g. a call center receiving VoIP calls from a 1-800 number).

2) Decide what happens to calls that cannot get 'service' (e.g. talk) immediately. Do they

• (A) Just disappear and don't try again. [eg overflow from VoIP network to PSTN]
• (B) Keep trying or go onto a HOLD queue, but if the wait is too long...hang up
• (C) Go onto a queue and NEVER leave until they get service.

3) Choose model:

• (I) + (A) = Erlang B
• (I) + (B) = Poisson
• (I) + (C) = Erlang C
• (F) + (A) = Engset
• (F) + (B) = Binomial
• (F) + (C) = Delay

4) Use a tool (such as our Erlang-G program) to decide how many voice trunks you need. At this point - it is irrelevant if they are VoIP or non-VoIP trunks.

5) Take the result from step (4) and use the VoIP too to calculate the required bandwidth for your VoIP network.

Generally - Erlang-B is often used in VoIP calculations for infinite populations (or Engset for finite populations) BECAUSE there is often an overflow path (to PSTN?) so loosing a call is not critical; just a little more expensive.

But if there is no overflow path, then the Poisson model (infinite populations) or the Binomial model (finite populations) should be used.

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Last modified: November 06, 2003