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required wan bandwidth, Erlang or number of simultaneous call.

Hi everybody.

1)  Based on my book, there are two ways we calculate required bandwidth using cisco online tool.

   We can  calculate required wan bandwidth using Erlangs,grade of service, codec type etc.  The bandwidth thus calculated is based on maximum hour usage of the day or Erlangs.

  We can also calculate the required bandwidth using number of simultaneous calls that need to be supported.

My question in real life which  method is widely used  to calculate required wan bandwidth for voip?

2) Let say we have two brand new  sites ;each site has only 100 phones. For simplicity , we are only concerned with voip traffic to calculate the required bandwidth.

Site1---                                                        Site2

Now one method my book mentions is to calculate

Busy_Hour_Call_Minutes = [50,000 / 22] * .15

Erlangs= Busy-hour-call -min/60

Once we get Erlangs, we could use cisco online tool to calculate required bandwidth.

In order to calculate Erlangs , we must have some statistics of the calls usage like  telephone bill which will reflect the total minutes of usage for a site.

What if we are designing a network from a scratch?  Just like in example above. These two sites are new and therefore no data  for phone usage  is available . In such case how can one calculate required bandwidth based on busiest hour usage(Erlang)? Don't we use  "simultaneous number of calls that need to be supported in such case rather than Erlangs to calculate required bandwidth ?

thanks and have a great weekend.

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