Explain ISDN Addressing with a example.

A sub address, though a part of the ISDN address, is not seems as an integral part of the numbering scheme. Sub-address is carried in a separate field into the user-network interface message. This may or may not be present in a call setup message. The sub address is usually transparent to the network and this equipment at the destination that analyses the sub address information for routing to the appropriate terminal. A usual address using both ISDN number and the sub address is demonstrated in figure. Now, an ISDN number identifies a T reference point and a sub address one of several S reference points. Otherwise, as reference point may also be addressed through using direct inward dialing (DID) feature.

Fig - Example of ISDN Addressing