even possible to get A/B switching with an adjustable audible alarm on RS-232C leads such as Carrier Detect (CD). Thus, if CD goes Four-wire VF test or Modem monitoring equipment Patch Four-wire telco line Monitor in VF patch jack Fig. 5-1. Monitoring through a VF patch jack. 70 TELECOMMUNICATIONS Modem Modem Telco line 1 Telco line 2 VF patch cord Fig. 5-2. Patch cord connects modem 1 to transmission line 2. On-line front Monitor LED for carrier detect signal processor Back-up front end processor Fig. 5-3. Typical A/B (or fallback) switch. down (indicating that either the Telco line is lost or the modem has a problem), the operator would be immediately notified. 5.3 REDUNDANCY AND COMPATABILITY Most telcom planners, including this author, feel it is wise to give preference to equipment which has built-in redundant parts for crit- ical functions, such as power supplies and control logic, SO that if one goes down the other takes over immediately. For major elec- tronic switching systems such as a matrix you should get front panel patching for backup so that if the matrix goes down you can patch across it. It makes little sense to spend a quarter of a million dollars for a swtiching system and then not spend a few thousand more for this kind of redundancy-no matter how reliable you think the equipment is. There is a large tendency in the business towards standardization. Nonetheless, the compatability of your equipment with equipment NETWORK MANAGEMENT 71 you already have of plan to obtain is always an issue. It may be worth your while to call in a consultant who specializes in system design and knows pretty much what fits what. On the other side of this matter, be careful not to make compatibility with existing equip- ment too much of an issue because in some cases it may be both cheaper and more efficient in the long run to discard your present equipment and configure whole new network control facility. 5.4 NETWORK TESTING The integration of voice and data communications require test sys- tems that can perform fu