This section provides a specification for a STREAMS interface which abstracts the services offered by a Signalling Data Link (SDL) as described in ITU-T Recommendation Q.701,^[2] Q.702,^[3] and parts of Q.703^[4]. Where possible, this specification remains consistent with Communications Device Interface (CDI).^[6] 

The Signalling Data Link (SDL) abstraction for STREAMS provides a Signalling Data Link Interface (SDLI) which is largely consistent with the Communications Device Interface (CDI)^[6] and enables a Signalling Data Terminal (SDT) service user (see next section) to access and use a varietry of conforming signalling data link service providers without special knowledge of the link's physical interface or internal device arrangement and driver implementations. 

The SDLI 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 SDLI providers. In some instances, it may be possible to use a CDI Style 1 or 2 WAN driver (e.g. for V.35 or DS0) as an SDLI driver without much modification. 

This interface specifies access to SS7 Signalling Data Link service providers, and does not define a specific protocol implementation. Thus, issues of network management, protocol performance, and performance analysis tools are beyond the scope of this document and should be addressed by specific implementation of a SS7 Siganlling Data LInk service provider. However, accompanying each provider implementation should be information that describes the protocol-specific behavior of that provider. Currently, there are plans to come up with a set of implementor's agreements/guidelines for common SS7 Signalling Data Link providers. These agreements will address issues such as SS7 Signalling Data Link PPA address space, management techniques, supported services, etc. 

This specification assumes that the reader is familiar with ITU-T Signalling System Number 7 network structure and reference models, SS7 Signalling Data Link services and service primitives, and STREAMS.