International Alphabet No. 5

Lore & Background

IA5 was developed as a seven-bit code to replace the five-bit International Telegraph Alphabet No. 2 (ITA2), offering a larger character set. The standard went through several revisions under ITU-T Recommendations V.3 and later T.50, with versions in 1968, 1972, 1976, 1980, 1984, and 1988, before being renamed the International Reference Alphabet (IRA) in the 1992 version. The character set is identical to ISO/IEC 646:1991, and ASCII is the U.S. variant of IA5. The encoding is used in ASN.1 as the IA5STRING type, restricted to IA5 characters, and is referenced by standards such as RFC 3939 for calling line identification in voice mail messages. Some analog modems, including those from Cisco, also employ IA5.

Reader's Guide

IA5's significance lies in its role as a foundational seven-bit character encoding that bridged older telegraph codes and modern digital communication. By expanding from five bits to seven, it enabled representation of a wider range of characters, including both uppercase and lowercase letters, digits, and punctuation, which was essential for international text exchange. Its adoption as the International Reference Alphabet (IRA) in 1992 standardized the encoding globally, and its identity with ISO/IEC 646 ensured cross-platform compatibility. The encoding's use in ASN.1's IA5STRING type and in telecommunications standards like RFC 3939 demonstrates its lasting utility in structured data and voice messaging. The fact that ASCII, the dominant character set in computing, is a U.S. variant of IA5 underscores its legacy as the direct ancestor of modern text encoding. IA5 remains in force as of the 1992 revision, continuing to underpin systems that require a reliable, internationally agreed-upon character set.

Did You Know?

• IA5 is based on seven-bit bytes, an improvement over the five-bit ITA2.
• The current version of the standard is from September 1992 and is called the International Reference Alphabet (IRA).
• ASCII is the U.S. variant of the IA5 character set.
• IA5 is used in the ASN.1 IA5STRING type and is referenced by RFC 3939.