G.711 (mu-law and A-law)
G.711 is the lowest-common-denominator narrowband audio codec on the PSTN. It is uncompressed PCM at 8 kHz sample rate, 8-bit logarithmic samples, yielding 64 kbps bitrate. Two regional flavors: mu-law (US/Japan/Canada, payload-type 0) and A-law (Europe/rest of world, payload-type 8).
Why G.711 matters
Every PSTN gateway, every SIP phone, every PBX supports G.711. It is the universal fallback when codec negotiation has nothing else in common. Quality is good for voice (MOS ~4.2), at the cost of bandwidth.
Tradeoffs
| Codec | Bitrate | MOS | Frame | PSTN-ready |
|---|---|---|---|---|
| G.711 | 64 kbps | 4.2 | 20ms (160 bytes) | Yes (universal) |
| G.729 | 8 kbps | 3.9 | 10ms | Yes (most carriers) |
| G.722 | 64 kbps | 4.5 (wideband) | 20ms | Wideband only on some routes |
| Opus | 6-510 kbps | 4.5+ | 2.5-60ms | VoIP/WebRTC, transcoded for PSTN |
Regional flavors
mu-law (PT 0) compands negative half slightly differently than A-law (PT 8). Endpoints transcode between them; quality loss is negligible. Always offer both: m=audio <port> RTP/AVP 0 8 ....
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