Finding the right number of channels from busy-hour traffic, an Erlang B table and 1% blocking, including bandwidth.
The most common question when signing a SIP trunk contract is: "How many channels should I buy?" Too few channels lead to a "lines busy" signal at peak times and lost customers. Too many is wasted money. Telecom engineering's century-old answer is the Erlang calculation.
What is an Erlang?
One Erlang means one channel being continuously occupied for an hour. You find traffic intensity by dividing total talk minutes in the busy hour by 60. For example, if the busy hour has 600 minutes of talk, the traffic is 10 Erlangs.
The simple formula
Busy-hour traffic = (calls in the busy hour × average call length in minutes) ÷ 60.
Example: 150 calls in the busy hour, average length 4 minutes → 150 × 4 ÷ 60 = 10 Erlangs.
How many channels are needed?
An Erlang B table gives the number of channels for an acceptable blocking probability. One percent blocking (only 1 call in 100 gets a busy signal) is the practical standard:
| Traffic (Erlangs) | Channels needed (1% blocking) |
|---|---|
| 5 | 11 |
| 10 | 18 |
| 20 | 30 |
As you can see, channel count does not grow linearly with traffic; efficiency per channel improves as traffic grows. For 10 Erlangs, 10 channels are not enough; you need 18.
Inbound and outbound on the same trunk?
Yes, a SIP trunk is two-way. Inbound and outbound traffic add up. If you use a separate contact centre trunk, calculate it separately because contact centre load is far more volatile.
Growth allowance
If you expect 20 percent growth over the next 12 months, multiply the calculated traffic by 1.2 and read the channel table. On Ahize SIP trunk plans you can raise channels month by month, so you do not need to set the allowance too high.
Bandwidth
Each channel uses about 100 kbps with the G.711 codec and about 30 kbps with G.729. For 30 channels you need 3 Mbps of dedicated upstream bandwidth with G.711 and 1 Mbps with G.729.
Summary
- Work out the number and length of calls in the busy hour.
- Calculate the Erlang traffic.
- Find the channel count for 1% blocking.
- Add a growth allowance and check bandwidth.
If you like, send your call records; Ahize engineers will run a free channel analysis.
A note for contact centres
In outbound campaigns (power dialer) the system dials several numbers at once, so channel needs can be 1.5 - 2 times the number of agents. Take this into account when planning the dialer ratio.