Tips for planning SIP trunk bandwidth

The advantages of using Session Initiation Protocol (SIP) trunks to transport voice calls are well known. You merge your voice traffic right onto your IP WAN service—no separate telephony network to manage. Virtual trunks can grow as your traffic loads demand, thanks to the IP nature of the trunks. 

Most people know that an SIP trunk is different than traditional trunk lines. You measure capacity by bandwidth, not lines or connections. When deciding how much SIP bandwidth you need, think about:

• How many SIP trunks and concurrent calls paths (CCPs) do you need at peak?
• Do you need to add extra bandwidth to support additional traffic?

SIP trunks and concurrent calls paths

Let’s start with number of SIP trunks. How many CCPs do you need on your SIP trunk at the peak? You might know this information from setting up TDM primary rate interface (PRI) physical circuits in the past. Like PRI trunks, the more calls your location makes, the more CCPs you need.

Bandwidth

Convert number of CCPs into bandwidth. The formula will vary depending on what codec you use. A codec encodes and decodes digital data streams (like voice over IP).

Two of the most common codecs are G.711 and G.729a. Here are sample bandwidth results using the same simple formula for each: 

G.711: SIP Trunk Peak Bandwidth = Peak CCP x 80Kb. If you need to be able to make 200 simultaneous calls, 200 x 80Kb = 16Mb. So 16Mb is the minimum bandwidth you’d need at your IP port to handle VoIP traffic.

G.729a: SIP Trunk Peak Bandwidth = Peak CCP x 32Kb. This is the same basic formula, but with a more efficient codec that requires less bandwidth. So 200 x 32Kb = 6.4Mb.

Or you can do the math using an Erlang calculator.

The bottom line is, it should show that a single SIP trunk can replace multiple PRIs.

This is a simple calculation that doesn’t factor in a number of variables, but one of those, Class of Service (CoS) should be explored. Basically, for this example, obtain at least this much bandwidth—16Mb for G.711 and 6.4Mb for G.729a—for your dedicated IP connection, and you are on your way.

Each PRI has 23 voice channels for carrying 23 simultaneous phone calls. In this example, using physical PRI circuits to support 200 peak calls would require nine PRI lines. For every additional simultaneous call up to 23, you need one more PRI line installed—which can grow pricey quickly.

With SIP, you could only need one circuit with one SIP trunk provisioned over your existing WAN access link. There would be no need to install any new physical circuits, but there would be a lower cost and faster turn up.  

What about my PBX?

Did you know you can hand off your SIP as a PRI interface so you don’t have to change equipment? That means you get the bandwidth and the savings of SIP without overhauling your entire PBX system.