logical flow of messages and information according to the rules and a 16kbit/s D channel and is often referred to as 2B+D and is a of the protocols in use on the interface. These tests can operate world-wide standard. The Primary Rate Interface has two versions in a functional and non-functional manner. one made up of 30 64kbit/s B channels and a 64kbit/s D channel (30B+D) that is found in Europe, Africa and South America and one 61.5.3 Different levels of test equipment made up of 23 64kbit/s B channels and a 16kbit/s D channel (23B+D) that is found in the USA, Japan and Canada. A wide variety test equipment is currently available for ISDN, but the design of ISDN test equipment is still evolving. Most test 61.5.2 ISDN testing compared equipment available for ISDN to date can be categorised by price and functionality. The test products for ISDN have traditionally The Basic Rate Access is a new access method defined by CCITT been split by testing capability such as, protocol testers, physical for ISDN and is based on the twisted copper pair used for the line testers and transmission testers. existing analogue telephone circuits. The Primary Rate Access is The fragmentation of ISDN testing in this manner suits the R&D already used for transmission and was initially deployed in inter- laboratories where testing can be easily split into these disciplines. local exchange networking for PCM systems. However for those who have to install and commission ISDN this Testing procedures for Primary Rate circuits already exist, al- methodology is both cumbersome and expensive. A new generation though the testing methods used are normally restricted to physical of ISDN tester is now becoming available that combines elements and transmission and framing tests. Because ISDN makes use of the of all three areas into a simple to use functional tester that can prove existing G704 framing structure of the Primary Rate circuits the to be highly cost effective. Dedicated testers however still have their only extra requirement for ISDN t