ITU G.711

G.711 is a high bit rate (64 Kbps) ITU standard codec. It is the native language of the modern digital telephone network.

Although formally standardised in 1988, the G.711 PCM codec is the granddaddy of digital telephony. Invented by Bell Systems and introduced in the early 70’s, the T1 digital trunk employed an 8-bit pulse code modulation (PCM) encoding scheme with a sample rate of 8000 samples per second. This allowed for a (theoretical) maximum voice bandwidth of 4000 Hz. A T1 trunk carries 24 digital PCM channels multiplexed together. The improved European E1 standard carries 30 channels.

There are two versions: A-law and U-law. U-law is indigenous to the T1 standard used in North America and Japan. The A-law is indigenous to the E1 standard used in the rest of the world. The difference is in the method the analog signal being sampled. In both schemes, the signal is not sampled linearly, but in a logarithmic fashion. A-law provides more dynamic range as opposed to U-law. The result is a less ‘fuzzy’ sound as sampling artifacts are better suppressed.

Using G.711 for VoIP will give the best voice quality; since it is the same codec used by the PSTN network and ISDN lines, it sounds just like using a regular or ISDN phone. It also has the lowest latency (lag) because there is little to no need for buffering, which costs processing power. The downside is that it takes more bandwidth than other codecs, up to 84 Kbps including all the UDP and IP overhead. However, with increasing broadband bandwidth, this should not be a problem.

G.711 is supported by most VoIP providers.

ITU G series recommendations