Airlines Link Control - a primitive data
communications protocol, devised by
SITA, and therefore peculiar to the airline industry. It works by having a central hub
poll outlying "interchanges" (cluster controllers). ALC is a 5
bit code, thus limiting the character set to the basic
alphanumerics (no lower case here!), and a handful of
punctuation marks. There is no
error correction, merely a rudimentary error detection. If a transmission error is detected, REENTER (or in more minimalist implementations RENT) is displayed on the terminal, and the user is expected to re-input the failed command. If the link fails altogether UNAVBL is displayed - the user then waits until some long-suffering techie restores service (AVBL).
In spite of its age and deficiencies, ALC is still quite widely used, as it is simple and it works. There is also a large amount of cheap, pre-owned terminal equipment available. Nowadays, one can expect to find ALC encapsulated in a marginally less archaic protocol (X.25, for example).