SIP channel calculator
Work out how many SIP channels a trunk needs from the calls in your busiest hour, using the Erlang B formula that telephone networks have been sized with for a century, and the bandwidth those calls take.
How the calculation works
A SIP channel carries one call at a time, so a trunk needs enough channels for the calls happening at once in its busiest hour. Calls do not arrive evenly, so a trunk sized for the average is busy far more often than it looks.
- Traffic in erlangs. One erlang is one channel busy for a whole hour. Calls per hour multiplied by the average call length in hours gives the traffic: 60 calls of 3 minutes is 3 erlangs.
- Erlang B. For a given traffic and number of channels, the Erlang B formula gives the share of calls that find every channel busy and are blocked. The calculator finds the fewest channels that keep blocking at or below your target.
- Bandwidth. Each call needs bandwidth in both directions at the same time. The figures include IP, UDP and RTP overhead at 20 ms packets; Ethernet framing and VPN or tunnel headers add a little more.
Measure your busiest hour from your phone system's call records rather than estimating it, and remember that a held, transferred or conferenced call can use more than one channel. Erlang B assumes blocked calls go away; queues and call-backs need Erlang C.
SIP trunks and hosted voice
We provide SIP trunks and hosted voice to partners, with channels sized to your traffic and burst capacity when you need it.