ISDN but encompasses the protocols and functions of the Testing of ISDN is also examined for the following key areas: devices. 1. Physical testing. 2. Transmission testing. 61.5.1.2 ISDN components 3. Protocol testing. The channel is the basic unit of service within ISDN, and is a 61.5.1 Integrated Services Digital Network fundamental component in its testing. The Basic and Primary rate interfaces are made up by a number of these channels. Three types The Integrated Services Digital Network is one evolving from a of basic channel are defined, Bearer channels (B), Delta channels telephony Integrated Digital Network (IDN) that provides end-to- (D) and High capacity channels (H). A B channel is a 64kbit/s unit end digital connectivity to support a range of services, including of clear digital bandwidth, based on the data rate required to carry voice and non-voice services, to which users have access by a one digitised voice conversation. B channels can carry any type of limited set of standard multi-purpose interfaces. Two interfaces are digital information (voice, data or video). A D channel is a signall- defined by CCITT for ISDN, the Basic Rate Access (BRA) (see ing channel that carries the information needed to set up and tear ISDN test equipment 61/17 Basic rate (2B+D) Basic rate (2B+D) Signalling SS#7 Switch Switch Q.921 /Q.931 Q.921 /Q.931 Signalling Signalling Primary rate (30B+D or 23B+D) Primary rate (30B+D or 23B+D) Figure 61.15 The Integrated Services Digital Network down switched connections for the relevant B channels. Connec- as the framing of the signal for the particular service. These tions set up by the D channel are normally B channels that are within tests normally check for errors on a transmitted signal, as well the same physical connection as the D channel, although in the case as checking for errors in framing. of Signalling System Number 7 this is not always the case. Protocol testing, the protocol tests being used to check the The Basic Rate Interface is made up of two 64kbit/s B channels 