The basic channel associated signalling (CAS) package provides a baseline package. Basic CAS defines abstract events and signals that are common to all CAS protocols. For some protocols, basic CAS provides all of the functionality required to satisfy the line interface, such as North American single-stage dialling applications or R1 signalling. For other protocols, basic CAS provides a baseline package for which additional protocol specific extension packages can be defined. Further line signals and events required for international signalling protocols can be provided through extension packages.

The basic CAS addressing package extends the basic CAS package to add the signals and events required for basic addressing. This would satisfy addressing functionality for many protocols such as North American single-stage dialling applications or R1 signalling.

The robbed bit signalling (RBS) package, operator services and emergency services package and the operator services extension package can be used to provide functionality required in addition to the basic CAS package.

The support of these packages is optional.