we interrupt traffic by taking one DSO or E0 channel out of service, or the entire aggregate. We replace the channel with a PRBS (pseudo-random binary sequence) or other sequence specifically designed to stress the system. Commonly we use conventional BERT (bit error rate test) techniques looping back. This idea of loopback is shown in Figures 21.3a and 21.3b. ESF and channel 0 data channel testing is nonintrusive. It does not interfere with customer traffic. Trouble can also be isolated, whether in the "go" or "return" channel of the loopback. Both intrusive and nonintrusive testing could be automated in the network management suite. Many frame relay equipments also have forms of in-service monitoring as well as a system of fault alarms. In fact, most complex telecommunication equipment has built-in monitoring and test features. The problem often is that these features are proprietary, whereas our discussion of DS1/E1 systems is they have been standardized by ITU-T and Telcordia recommendations and publications. This is probably the stickiest problem facing network management systems. That is handling, centralizing, and controlling network management features in a multivendor environment. 21.5.2 Communication Channels for the Network Management System A network management facility is usually centrally located. It must monitor and control distant communications equipment. It must have some means of communicating with this DS1 channel DS1 channel (near end) (far end) 2.048 Mbps E1 channel E1 channel 2.048 Mbps (remote end) 4-kbps data channel derived from time slot 0 HP Telecom analyzer in BERT mode Figure 21.3 (a) Loopback of a DSO channel with BERT test in place, intrusive, or on spare DSO. (b) Loopback of 4 kbps data channel derived from ESF on DS1 or channel 0 on E1. NETWORK MANAGEMENT equipment, which may be widely dispersed geographically. We have seen where DS1 and E1 systems provide a data channel for (OAM) operations and maintenance. Higher levels of the DS1 and E1 hierarchies have special communication c