OSI/AS Configuration and Management Manual

NSAP Addressing
OSI/AS Configuration and Management Manual—424119-001
3-13
Converting NSAP Addresses
Converting NSAP Addresses
Theoretically, an NSAP address constructed according to the rules provided by the
network addressing authority must be converted before it can be included in an NPDU
(network protocol data unit) for transmission over a physical link, as follows:
1. Convert it into computer-readable format—from the basic NSAP address (the
concept) to a format recognized by the Network Layer protocols.
2. Encode it and include it in NPDUs (done by the Network Layer protocols).
An encoded NSAP address is unique in the Network Layer protocols all nonprotocol
contexts (national language, system, or any other locally devised semantics) have been
removed. Because it is unique, this encoded address should be used in all written and
verbal communication. The HRPF (hexadecimal reference publication format) has been
defined for this purpose, and is discussed in Notational Format for Encoded NSAP
Addresses on page 3-14.
Step 1: Converting to Network Layer Format
NSAP addresses first must be converted into a computer-readable format, as follows:
•
AFI, IDI, and decimal-syntax DSP values
Since these values are expressed in decimal digits, they can be easily used as input,
as is.
•
DSP values: binary syntax
Binary-syntax DSP values are represented in hexadecimal (two digits per octet).
•
DSP values: character-based syntax
ISO 646 syntax or national-character syntax DSP values are useful only in a system
having the desired character set in its native mode. Therefore, character-based
syntax is not easily communicated in networks having dissimilar systems or in
networks connecting people of different nationalities. If, however, there is a method
of representing the non-native characters in computer-readable form (using some
conversion process), such DSP values can be used.
Step 2: Encoding Network Layer Protocols
Once NSAP addresses are entered into the system, they are encoded by the Network
Layer protocols and included in NPDUs for transmission. (The encoded NSAP
addresses have a maximum of 20 binary octets.) These protocols use a binary encoding
method called the preferred binary encoding method, defined in ISO 8348, Addendum 2.
The binary encoding is straightforward for the AFI values, the IDI values, and the
decimal-syntax and binary-syntax DSP values. However, DSP values in character-based
syntax must be represented by numeric codes (in decimal or hexadecimal).
The preferred binary encoding method is designed to neutralize any advantages one
character code may have over another. Therefore, a DSP value in ISO 646 syntax, for
example, does not resemble its binary encoded value. For network addressing domains