Network and infrastructure
G.711
G.711 is the reference codec of digital telephony: voice is sampled 8,000 times a second at 8 bits, giving 64 kbit/s, with no compression.
Standardised in 1972 by the CCITT, now the ITU-T, it is the coding used inside the digital telephone network, long before voice over IP. It comes in two variants, A-law in Europe and µ-law in North America and Japan. Its quality is that of a classic line, at the cost of a high bit rate compared with modern codecs.
Why it matters for fax
It is the only common voice codec that lets a fax signal through, because it removes nothing a modem sends. This is called G.711 fax or passthrough. It remains vulnerable to packet loss and jitter, which it cannot correct.