ITU T.50

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ITU-T recommendation T.50 specifies the International Reference Alphabet (IRA, formerly International Alphabet No. 5, IA5), a character set encoding. ASCII is the U.S. variant of that character set.

The original version from November 1988 corresponds to ISO 646. The current version is from September 1992.


At the beginning was the International Telegraph Alphabet No. 2#ITA2, a five bits code.

IA5 is an improvement based on seven bits bytes.

International Telegraph Alphabet No. 2#ITA2

  • 5 T.50 (09/1992) In force
  • 4 T.50 (11/1988) Superseded
  • 3 T.50 (10/1984) Superseded
  • 2 V.3 (11/1980) Superseded
  • 1 V.3 (10/1976) Superseded


This standard is referenced by other standards such as RFC3966.[1]

It is also used by some Analog Modem such as Cisco ones.[2]

This standard is referenced by other standards such as RFC 3939 - Calling Line Identification for Voice Mail Messages.


  • Identical standard: ISO 646:1991 (Twinned)


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