At times like these portable data communication testers are very useful. A user can check out the vendor's equipment handshaking capability by using a data communication tester to simulate the pro- tocol that the network will use. Similarly, vendors can assure users that the equipment is operational, even though there is no commu- nication link at the site. In more complex networks that use bit-oriented protocols at higher levels, the portable testers can be modified to troubleshoot links at those levels. The same testers can be modified to handle simple char- acter-oriented, asynchronous protocols at lower network levels. This same flexibility makes network testers useful. 76 TELECOMMUNICATIONS Mainframe Computer Codex Racal Front Concentrator Modem Modem Concentrator Satellite Hughes Landline Western Electric Microwave Raytheon Graphics entry Exxon Tektronix Mohawk Patch Burroughs Honeywell Inforex Fig. 5-4. Typical multivendor communication network. 5.7.1 Line Analyzers Line impairment tests are performed using analog measuring equip- ment and are conducted mostly on telephone lines. Tests of this na- ture measure all types of line characteristics that the reader may or may not be familiar with (e.g., frequency response, signal/noise ra- tio, envelope delay distortion, frequency translation, impulse noise, etc.). The hardware required for these tests is sophisticated (Fig. 5- 5) and the operator must be skilled at interpreting the results. This hardware should only be required at a user's site when extremely troublesome lines are encountered and proof is needed to convince the carrier that action need be taken. Usually, digital instrumenta- NETWORK MANAGEMENT 77 Fig. 5-5. Hewlett-Packard 4935A transmission impairment measuring set. capability lem, tion, is while sufficient is providing to identify data that accurate there enough to pinpoint the tone generator, required, and power a low-cost level meter line monitor will is a usually problem. with suffice. a If speaker, some audio prob- test 5.7.2 Breakout Boxe