In a message dated 5/15/2002, Colin Botha writes:
> Are there any souls out there who can shed some light on possible test > suites, test equipment, etc. please? > Hi Colin: It is certainly true that V.32bis modem performance is affected by many other impairments besides attenuation. In fact, attenuation alone would be a very poor way to evaluate modem performance. While the two most widely recognized transmission impairments that can affect modems are attenuation and noise, experience has shown that V.32bis modems are susceptible to additional impairments such as phase jitter, distortion, envelope delay, ADPCM compression, satellite delay, and echo. To address these impairments in a comprehensive and logical fashion, the TIA has published TIA/EIA TSB37A, which describes a comprehensive, objective model of the phone network. All of the relevant impairments are included, but a variety of test combinations have been used to make each impairment appear in approximately the same percentage of test lines that it appears in the actual network. Using this network model, it is possible for a modem vendor to objectively predict how well their modem will perform over the full range of possible connections on the phone network. On any simulated connections where the modem performs poorly, the designer can study the problem in a laboratory environment and evaluate the effects of various changes in the design. Test equipment for performing tests per TSB 37A and TSB 38 is available from Spirent communications (http://tas.spirentcom.com), which recently acquired the telecom test manufacturer TAS. Unfortunately, a system that performs the full TSB 38 tests can get expensive. The TAS 100 (http://www.taskit.com/prod_100.htm) is probably a good candidate for basic modem testing, although it can not fully simulate all of the test conditions in TSB 37A. It is also possible to have modem testing performed at an outside lab such as Henderson Communications (http://www.henderson-labs.com). This may seem attractive since it involves no capital expenditures for test equipment, but the testing fees can quickly add up to more than the cost of some basic test equipment. Also, when using an outside lab there is little opportunity for the type of test-modify-retest cycle that designers can use in the comfort of their own lab. In view of this, you might consider purchasing some basic test equipment for internal pre-testing, to be followed by more comprehensive tests at an outside lab. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
