to each device, and only relates to how that device has interacted with the network. To gain a complete picture of the behaviour of the network requires access to the functionality provided by a network monitor. A network monitor will continuously record information about all 4 wire 2B+D 4 wire 4 wire 144kbit/s 2.048Mbit/s CEPT/G703 or T1 of the traffic that it sees on the network, including erroneous (30B+D) packets. Furthermore, the monitor can independently collect data 1.544Mbit/s Standard (23B+D) about how the various network devices interact, forming the net- interfaces e.g. RS 232 work's perspective of the communication patterns. Remote monitoring devices, connected to all of the network segments, will, provide information to the management station to Figure 61.14 ISDN Primary Rate Access complement that provided by the other network devices. In this way, the management station will be able to form a complete picture of how the network is operating. 61.5.1.1 The interfaces ISDN test equipment The structure of ISDN is such that the two ISDN interfaces, the BRA and the PRA are used for customer access, it is therefore This section gives an overview to the testing requirements for apparent that the two interfaces are the most commonly tested ISDN, and the test equipment associated with carrying out such component in ISDN (see Figure 61.15). The two ISDN interfaces BRA and PRA, are well defined by the testing. CCITT a branch of the United Nations, International Telecommuni- The structure of ISDN, including that of the interfaces of ISDN cations Union. ISDN as defined by the CCITT however is not just are examined, determining the types of tests that can be performed. restricted to the interfaces but is defined as being a collection of The testing of ISDN interfaces is compared to that of traditional components and functions: access types, devices, protocols and telephony testing and the differences and similarities are sum- interfaces. Testing of ISDN is therefore not restricted to the inter- marised. faces of 