Codecs
G.711Also known as PCMA / PCMU, A-law and µ-law, Pulse-code modulation
Definition
G.711 is the ITU-T voice codec, standardised in 1972, that encodes narrowband telephone audio as uncompressed pulse-code modulation at 64 kbit/s: 8,000 samples per second at 8 bits each. It comes in two companding variants, A-law (PCMA) used in Europe and most of the world, and µ-law (PCMU) used in North America and Japan.
A-law vs µ-law
Both variants compress the 13–14-bit dynamic range of speech into 8 bits using a logarithmic curve, so quiet sounds keep more precision. They differ only in the curve, and conversion between them is lossless enough to be routine on international calls.
| A-law (PCMA) | µ-law (PCMU) | |
|---|---|---|
| Regions | Europe, Middle East, Africa, most of Asia and Latin America | North America, Japan |
| RTP payload type | 8 | 0 |
| SDP name | PCMA/8000 | PCMU/8000 |
| Bitrate | 64 kbit/s | 64 kbit/s |
Read more in G.711 A-law vs µ-law.
Bandwidth and quality
G.711 covers roughly 300–3400 Hz, the traditional telephone band. With 20 ms packets, each RTP packet carries 160 bytes of audio plus 40 bytes of RTP/UDP/IPv4 header, giving about 80 kbit/s at the IP layer and about 87 kbit/s on Ethernet, in each direction. Because it does not compress, it adds negligible delay and processing, survives transcoding well, and carries fax tones and in-band DTMF reliably.
G.711 is mandatory to implement in WebRTC and is the most universally supported codec, which makes it the safest default for SIP trunks and interconnects.
G.711 on gateway trunks
G.711 also has Appendix I packet-loss concealment and Appendix II voice activity detection with comfort noise, which many implementations use to hide lost packets and save bandwidth during silence.
GSMCalls offers G.711 and G.729 on the trunk side. Choose G.711 when bandwidth allows and your carrier or PBX expects PCMA/PCMU; see the features page.
Frequently asked questions
Should I use A-law or µ-law?
Match your region and carrier: A-law in most of the world, µ-law in North America and Japan. Offering both lets the far end pick.
How much bandwidth does G.711 need?
About 87 kbit/s per direction on Ethernet with 20 ms packets, or about 80 kbit/s at the IP layer.
Is G.711 HD voice?
No. G.711 is narrowband (up to 3.4 kHz). HD voice needs a wideband codec such as G.722, AMR-WB or Opus.