Q. What do you mean by sub address?

A sub address, though a part of the ISDN address, isn't considered as an integral part of the numbering scheme. Sub address is carried in a separate field in user-network interface message. It may or may not be present in a call setup message. Sub address is normally transparent to the network and it the equipment at the destination that analyses sub address information for routing to the appropriate terminal. A typical address using both ISDN number and sub address is displayed in figure. Here an ISDN number identifies a T reference point and a sub address one of the many S reference points. On the other hand, as reference point can also be addressed by using direct inward dialing (DID) feature.