ALC

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).

A couple corrections to Mit Wahsmircs' writeup:

• ALC is sometimes known as "Airline Line Control" and if you work for Galileo, "Apollo Line Control". I don't know which of the first two is more correct.
• I believe that ALC was actually invented by IBM and SITA later designated this spec as P1024B
• I've never seen a 5 bit implementation of ALC, always 6 bits. When you think about it, 32 characters doesn't even allow enough space for 0-9 along with A-Z, and numbers are definitely supported.

More ALC trivia:

• ALC uses the IPARS character set
• ALC is transmitted "reverse inverted". For example, capital A is 0x31, but it's transmitted as 0x73. This makes it a major PITA to read off the wire.
• ALC uses a CRC for error detection.

One other reason ALC has survived is that it requires minimal technology to implement. Airlines and GRS companies operate in every country in the world, including those with little or no telco infrastructure, where implementing even a protocol like X.25 would require a huge capital outlay.