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ISO 8348 : 1987/Add.2 : 1988
Network Service Definition - Network Layer Addressing

says that the maximum DSP length for two of the IDI formats (DCC and ICD) in
both decimal and binary syntax are:

ISO DCC decimal syntax          35 digits
ISO DCC binary syntax           14 octets
ISO 6523-ICD decimal syntax     34 digits
ISO 6523-ICD binary syntax      13 octets

Using the preferred encodings, this results in the following lengths for
the combined IDP and DSP (ie the complete NSAP):

ISO DCC decimal syntax          20 octets
ISO DCC binary syntax           17 octets
ISO 6523-ICD decimal syntax     20 octets
ISO 6523-ICD binary syntax      16 octets

The US GOSIP NSAP address structure has an AFI of 47 indicating that it is in a
binary syntax ISO 6523-ICD format, but has a total length of 20 octets which
seems to conflict with the above.

On the other hand I do not understand the rationale for differing maximums on
the lengths of NSAP addresses encoded using the preferred encoding. Is it
because they have approximately the same number of possible values (and hence a
more compact encoding of the decimal syntax would shorten the length of an
encoded decimal syntax NSAP to approximately the length of an encoded binary
syntax NSAP)?

Any clarification would be greatly appreciated.

Gary Nebbett, R.1004.7.20, CIBA-GEIGY, 4002 Basel, Switzerland
   Internet/uucp-Mail: /PN=GARY.NEBBETT/OU=CHCGBS21/@CIBA-GEIGY.CH
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   phone: +41/61/6974193