Numbering and Addressing: In telephone and data networks, the end equipments are more often single units than multiple devices units like PABX or LAN. Historically, a telephone, a computer, or a terminal has been the predominant end equipment. The numbering system for these networks has also evolved to identify single equipment end points. In ISDN, multiple devices at the end points are more of a norm than single units, in view of the multiple services environment. It then becomes necessary to identify a specific end equipment, e.g. facsimile or computer, to render the service. Identifying the specific equipments in a two-level process; first the end point is identified as in the case of telephone or data networks and then the equipment at the end point. ISDN addressing structure provides for this requirement. The component of the ISDN address which is used to identify the end points is known as the ISDN number, and the component for identifying the specific equipment at the end point is called the ISDN sub address.
The numbering plan for ISDN is evolved using the following guidelines:
1. It is based on, and is an enhancement of, the telephone numbering plan. It is
independent of the nature of the services (e.g. voice, facsimile or data) or the
performance characteristics of the connection (e.g. 32 kbps voice or 64 kbps
voice).
2. It is independent of routing, i.e. the numbering or addressing does not specify the
intermediate exchanges through which the services is to be put through.
3. It is a sequence of decimal digits. No alphabet or other characters are permitted
as part of the address.
4. Its design is such that interworking between ISDNs requires only the use of
ISDN number and no other additional digits or addressing signals.
Address Structure: The ISDN address structure is shown in Fig. ISDN number part has a maximum of 15 digits and the ISDN sub address part a maximum of 40 digits. National destination code is like an area code in telephony network and is of variable length. ISDN subscriber number is the one normally listed in the directories. It is the number to be dialed to reach a subscriber in the same numbering area. An ISDN number is a unique worldwide address and unambiguously identifies an end point connection. This end point may be:
1.single S or T reference point,
2.one to many T reference points at the same site, and
3.one of many S reference points using direct inward dialing feature.
A single S or T reference point may also be addressed by multiple ISDN numbers.
This feature is generally used in internetworking.
A sub address, although a part of the ISDN address, is not considered as an integral part of the numbering scheme. The sub address is carried in a separate field in the user-network interface message. It may or may not be present in a call setup message. The sub address is generally transparent to the network and it the equipment at the destination which analyses the sub address information for routing to the appropriate terminal. A typical address using both ISDN number and the sub address is shown in Fig. Here, an ISDN number identifies a T reference point and a sub address one of the many S reference points. Alternatively, as reference point may also be addressed by using direct inward dialing (DID) feature.