s jority terface/timing These of devices are necessary in and in purchased more sophisticated devices This capability is part identifying of in- separately. connection usually breakout consists device is connected in series and between one for the the connection two elements connectors to with the DCE. the (one status The for 78 TELECOMMUNICATIONS of the various leads visible through indicators (usually light emitting diodes, LEDs). Some of the units use tri-state LEDs (red, green, off) to show if the interface lead is high, low, or off. In addition, the leads are generally brought out to pinjacks to allow connection to external test equipment. Most of these are non-interferring with data and control signal flow between the DTE and DCE. 5.7.3 Error Rate Testers The two most commonly used devices of this type are bit error rate testers (BERT) and block error rate testers (BLERT). They usually generate a known bit or block pattern. With most that are currently available, user selected test data can be generated as well as standard Fig. 5-6. Hewlett-Packard 1640B serial data analyzer. NETWORK MANAGEMENT 79 "fox" messages and pseudorandom patterns. The device monitors what is returned to it and provides a numerical readout of the BER over a specified time frame. This is a good overall test of the trans- mission facility and hardware. It gives a reasonable indication that the problem is random, solid, or occurs periodically in bursts. 5.7.4 Data Monitors The function of these units is to bridge the connection between a DTE and DCE on a non-interferring basis and to display data and events (Fig. 5-6) as the connected devices interact. The display is usually a CRT and an optional printer port is sometimes provided (not the printer). These units always have internal storage and many are equipped with an integral tape unit, or the means to connect to an external unit, in addition to semiconductor memory. 5.8 NETWORK SECURITY Security vulnerabilities increase as the communication link gets longer. The communications links 