Bandwidth Requirements for SIP Trunking

You already know that SIP trunking allows businesses to save money on the cost of their phone service.

It’s likely to be a primary reason that your customers are interested in SIP trunking.

But before you go helping your customers make the switch, ensuring your customer has the right amount of bandwidth can save precious resources and time: too much bandwidth means spending money that could be used elsewhere while too little bandwidth means dropping calls and losing potential clients or frustrating potential business partners.

There are plenty of complicated formulas to determine exactly how much bandwidth your customer needs, and while those formulas are helpful, we also know you don’t have the time to put such formulas to use.

That’s why we’ve come up with a simple way for SIP trunking dealers to determine how much bandwidth they need for their SIP trunking deployments.

Step One: Determine the codec and how much bandwidth it consumes per call.

Most SIP trunking services use either G.711 codec, which consumes 64 Kbps per call, or G.729, which consumes 8 Kpbs per call. There are other codecs that be used, but these two are the most popular.

You can provide your customers with good SIP service using either codec, but if your customer is bandwidth constrained, stick with G.729, as sometimes upgrading your customers bandwidth is not possible due to cost.

Step Two: Determine call volume for a typical hour.

If you’re dealing organization that doesn’t rely on the phone calling, you can probably figure one call per every three or four employees per hour.

Call intensive businesses, such as companies with call centers or sales organizations, would require you to use the Erlang B formula (yes, this is one of those formulas we are trying to keep you away from—but we promise, with websites like this Erlang B calculator, determining volume is simple).

Erlang B allows you to determine one of three factors as long as you have the other two:

  1. BHT or busy hour traffic. This is the number of hours of your heaviest call volume times.  For instance, if 10am to 3pm are your heaviest call times, you would enter 5 in the Erlang B calculator listed above.
  2. Blocking. This is the percentage of how many calls you want blocked (or given a busy signal). Most businesses enter .01 in this field or 1% of calls blocked with 99% answered.
  3. Lines. This will probably be your unknown field if you are trying to determine bandwidth. The number of lines (one call per line) you will need available during your busy hours with your 1% blocking rate. For instance, if you enter 5 hours in the BHT field with a .01 blocking rate, you will need 11 lines available.

Step Three: Time for a little math.

The final step in the process is to multiply the number of current calls (channels) your customer needs by the amount of bandwidth each call will take (based on the codec selected).

For example, if your customer has six employees they will likely have two current calls at most times and if the provider offers G.711 codec, the bandwidth necessary for quality calling will be at least 128 Kbps.

For 11 concurrent calls with the same type of codec, the required bandwidth would be 704 Kbps.


Taking a little time to determine your necessary bandwidth can save you time, money, and customers.