Sizing WAN Links, Part Two

In my last post, we considered one of the first tasks in traffic engineering: determining which calls will be traversing which links. We also explored a few ways you could collect the voice utilization data needed to complete the process of sizing a WAN link.

Continuing on with our example, let’s say we collected the following data for the Hokey Pokey Water Bottling Company:

  • Internal calls between Buffalo and Rochester – 2,000 minutes
  • Calls to Buffalo PSTN from Rochester users – 1,000 minutes
  • Calls to Rochester PSTN from Buffalo users – 500 minutes
  • Calls to Rochester LD trunk from Buffalo users – 250 minutes
  • Total voice traffic to be handled by WAN link – 3,750 minutes

A few notes before we proceed:

You must be careful to have a large enough sample or else your estimates may fall short. Ideally, you would have a year’s worth of data, select the busiest day of the year, and work from that data. Some traffic engineers will average the ten busiest days as well.

Consider that the busiest day for internal calls might not be the busiest day for long distance calls. It may be necessary to determine the busiest day for each call classification.

Our example doesn’t account for calls to and from the new centralized voice mail system at Rochester.

It’s tempting to assume that the call volume will be evenly distributed throughout the business day, but that’s seldom the case in practice. Instead, it’s common to assume that 34% the total daily traffic it will occur during a single hour of the day, which is referred to as the busy hour. In our example, 34% of 3,750 minutes equals 1,275 minutes of voice traffic the WAN will need to carry during the busy hour.

Long before the advent of VoIP, it was necessary to size central office (CO) trunks. Thankfully, statisticians have models which can determine the number of simultaneous call paths required to support a given volume of traffic with a certain probability of a call being blocked.

To use these models, you take the minutes of traffic and divide by 60 to get the hours of traffic (but instead of calling it “hours of traffic” the name for this unit of measurement is the Erlang.) In our example, 1,275 minutes divided by 60, equals 21.25 Erlangs of traffic to be carried by our network.

Now for the easy part. You use a calculator, such as the ones at, to determine the number of call paths required. In our example, our network will need to support 31 simultaneous calls to handle the busy hour with only 1 out of every 100 calls receiving a busy signal (blocked).

In the next post, we’ll examine how VoIP packets are built, and use that knowledge (along with the number of calls) to figure the amount of bandwidth required.

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1 comment

  1. Gene DeLarm Reply

    This is a very intersting article. Thats for the information.