ork is primarily for record-keeping and subsequent analysis, failures are not SO vital and costly since lost or delayed records can be reentered and retransmit- ted. On the other hand, if your network is an interactive transaction system for bank tellers, airline reservations, travel agencies, or per- haps a time share operation, then downtime might cost thousands of dollars per minute. For these operations a modest investment in capital equipment begins to look very economical. NETWORK MANAGEMENT 69 5.2 ELEMENTARY MONITORING With only a few lines you could probably get by with VF patching and some degree of monitoring. A VF patch jack is a device which is placed on the telephone line between the modem and the telco line. The "normal-through" connection simply allows the signals to pass through the patch. The double-pointed arrow (Fig. 5-1) represents the normal-through connection. Barrels on the front allow the in- sertion of plugs for performing tech control functions. Note that the monitor barrel's connection is tied to the normal-through: thus in- sertion of a plug "picks up" the signal and connects it to a moni- toring device while still allowing the signal to flow uninterrupted. Figure 5-2 shows two such patch jacks connected with a patch cord. On the left, the plug is inserted into the modem barrel of the jack. Note that it breaks the normal-through connection, thereby routing the modem 1 signal onto the patch cord and connecting it to the jack. There, the plug is inserted into the telco barrel thus breaking the normal-through between modem 2 and telco line 2 and routing the modem 1 signal onto telco line 2. Whether patching or switching, analog or digital, the basic principle is the same and it is one of the most fundamental concepts in network control. You might want to combine your monitoring access with a res- toration capability as shown in Fig. 5-3. Here you have a simple application of standard A/B switching or fallback switching which enables you to "spare" the data terminal equipment (DTE). It is 