This section provides a specification for a STREAMS interface which abstracts the services offered by a Signalling Data Terminal (SDT) and a Signalling Data Link (SDL) as described in ITU-T Recommendation Q.701, Q.702, and parts of Q.703. Where possible, this specification remains consistent with the Communications Device interface (CDI).⁶ 

The Signalling Data Terminal (SDT) abstraction for STREAMS provides a Signalling Data Terminal Interface (SDTI) which is compatible with the Communications Device Interface (CDI) and enables a Signalling Link (SL) service user (see next section) to access and use a variety of conforming signalling data terminal service providers without special knowledge of the terminal's physical interface or internal device arrangements and driver implementations. 

The SDTI is based on a subset of the CDI with a number of protocol-specific extensions. In many cases, WAN interfaces which have been written as streams drivers to the CDI may be converted easily for use as SDTI providers. In some instances, it may be possible to use a CDI Style 1 WAN driver (e.g. for V.35) as an SDTI driver without modification. 

The Signalling Data Link (SDL) abstraction for STREAMS provides a mechanism for identifying, pooling, allocating and addressing signalling data links which are accessible to the host implementing this STREAMS specification. In addition to providing identification and addressing of signalling data links, it also provides for querying and qualifying signalling data link attributes (such as bit rate).