Start with channels
A SIP channel is one concurrent call. Your carrier limits channels and calls per second (CPS) separately, and running out of either causes failed calls that look like a bad route. There are two ways to size channels.
- Dialer agents. For outbound dialing, channels are agents times dial ratio. Fifty agents on a predictive dialer at 3 lines each need 150 channels at peak. Preview and progressive dialing run close to 1 line per agent.
- Busy-hour traffic. For inbound or mixed traffic, multiply calls in the busiest hour by average call length in minutes and divide by 60. That is traffic in Erlangs. Erlang B then tells you how many channels keep blocking at or under your target, such as 1%.
Add headroom on top for retries, short peaks and campaign changes. 20% is a sensible start. The channels and CPS sizing guide covers CPS, which this calculator does not.
How bandwidth per call is calculated
Each call sends a steady stream of RTP packets in both directions. The codec sets the voice payload and the packetisation time (ptime) sets how often a packet goes out. Every packet also carries 40 bytes of IP, UDP and RTP headers, plus 18 bytes of Ethernet framing on a LAN. SRTP adds a 10-byte authentication tag.
payload = codec kbps × ptime ÷ 8
packets = 1000 ÷ ptime (per second)
kbps = (payload + headers) × 8 × packets ÷ 1000
G.711, 20 ms: (160 + 40) × 8 × 50 = 80 kbps (87.2 with Ethernet)
G.729, 20 ms: ( 20 + 40) × 8 × 50 = 24 kbps (31.2 with Ethernet)The headers are fixed per packet, which is why G.729 at 8 kbps still needs 24 kbps on the wire. Longer ptime means fewer packets and less overhead, at the cost of slightly more delay and bigger gaps when a packet is lost.
Choosing a codec
| Codec | Wire rate at 20 ms | When to use it |
|---|---|---|
| G.711 | 80 kbps | Default for call centers. Best quality, no transcoding, works with fax and DTMF in-band. |
| G.729 | 24 kbps | Low-bandwidth sites. Quality is lower and some carriers transcode it. |
| G.722 | 80 kbps | Wideband between your own endpoints. PSTN legs still end up narrowband. |
Our trunks accept G.711 A-law and µ-law and G.729. Offer one codec first in your SDP to avoid transcoding. Answering-machine detection is more accurate on G.711, which matters for predictive campaigns.
Beyond raw bandwidth
Packet rate often limits capacity before bandwidth does. 150 calls at 20 ms is 15,000 packets per second in total, and small firewalls and consumer routers can drop packets well below their rated throughput. Check the packet figure against your firewall spec.
Last reviewed by the CLIRoutes routing team. Spotted a numbering change? Tell us.