Size a SIP trunk from busy-hour call attempts and hold time: Erlangs, concurrent channels and the bandwidth they consume.
Offered traffic in Erlangs is simply the call-seconds carried in the busy hour divided by 3600. The channel count uses the normal approximation to Erlang B — the offered load plus a safety factor scaled to the square root of the load — then multiplies by the on-the-wire rate of the codec you selected. Under-provisioned SIP trunks return 503 responses at the worst possible moment, and over-provisioned ones are billed per channel every month.
SIP Trunk Capacity
Erlangs A = BHCA × AHT ÷ 3600; channels N ≈ A + z√A; trunk bandwidth = N × codec rate × (1 + signalling allowance).
Per-call bandwidth
G.711 on Ethernet = (160 B payload + 40 B RTP/UDP/IP + 18 B Ethernet) × 8 × 50 pps = 87.2 kbps.
Erlangs A = BHCA × AHT ÷ 3600; channels N ≈ A + z√A; trunk bandwidth = N × codec rate × (1 + signalling allowance). Offered traffic in Erlangs is simply the call-seconds carried in the busy hour divided by 3600. The channel count uses the normal approximation to Erlang B — the offered load plus a safety factor scaled to the square root of the load — then multiplies by the on-the-wire rate of the codec you selected.
Under-provisioned SIP trunks return 503 responses at the worst possible moment, and over-provisioned ones are billed per channel every month.
This calculator takes 5 inputs: Busy-hour call attempts, Average hold time, Codec (bandwidth per call on Ethernet), Grade of service (blocking target), Signalling and RTCP allowance. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.