 or conclusions are drawn. As we say, "the proof of the pudding is in the eating." The best proof that we were right on our conclusion is to fix the purported fault. Does it disappear? If so, our job is done, and the network is returned to it "normal" (baseline) operation. What "conclusions" can we draw from this exercise? There are two basic ingredients to elemental network troubleshooting: expertise built on experience of the troubleshooter and the availability of essential test equipment. Troubleshooting time can be reduced (degraded operation or out-of-service time reduction) by having on-line network man- agement equipment. With ideal network management systems, this time can be cut to nearly zero. 21.5 SYSTEM DEPTH - A NETWORK MANAGEMENT PROBLEM An isolated LAN is a fairly simple network management example. There is only a singular transmission medium and, under normal operating conditions, only one user is transmit- ting information to one or several recipients. It is limited to only two OSI layers. For troubleshooting, often a protocol analyzer will suffice, although much more elaborate network management schemes and equipment are available. Now connect that LAN to the outside world by means of a bridge or router, and network management becomes an interesting challenge. One example from experience was a VAX running DECNET that was a station on a CSMA/CD LAN. The LAN was bridged to a frame relay box that fed a 384-kbps channel with an E1 hierarchy (i.e., 6 E0 channels) via tandemed microwave links to a large facility at the distant end (550 km) with similar characteristics. This connectivity is shown diagrammatically in Figure 21.1. Such is typical of a fairly complex network requiring an overall network management system. To make it even more difficult, portions of the network were leased from the local common carrier, but soon to be cutover to own ownership. 21.5.1 Aids in Network Management Provisioning Modern E1 and T1 digital systems3 are provided a means for operational monitoring of performance. The