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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.