Gary, The reason I asked the question:
"Could you share with us what you meant by the statement: "a V.90 modem with marginal return loss may not work very well. " was not from the point of return loss or balance that's a simple problem to solve. The question was asked to understand how it effects V.90 performance. As I stated in a previous email most analog modem chips will function at maximum performance with a return loss of 6 - 7 dB. V.90 will not. I'm humble enough to know that I will be learning all my life. As a consequence I am great believer in asking questions. That's how I learn many things. In my opinion simply asking for a second opinion does not exhibit ignorance. It exhibits a mind open to different fields of view and interpretations. Even though I bill myself as a telecom consultant there is no claim to know all. I doubt anyone does. The transmission theory classes I struggled through did not cover designing equipment for subscriber loops used as a digital transmission medium. The problems, to name a few, are standing waves (return loss), holding circuits, line current, a mix of wire gauges, loading coils, capacitive reactance, variable loop lengths from 200 to 25K meters and in some cases barb wire. No matter how well it's modeled subscriber loops are very hostile medium for V.90 and xDSL. In the last 30 years most of us have solved the subscriber loop problems effecting analog modems. It's fairly easy to get return loss figures across the band in the 25 to 40 dB range at 600 ohms and 18 to 25 dB with a complex impedance. But, in my opinion V.90 is a special case and xDSL even more so. Anyone that has designed transmission equipment that pushes bandwidths to limit and beyond understands it is fairly straight foward to design for perform at maximum throughput from point A to point B. But, when the equipment has to perform at maximum throughput on millions of subscriber loops that's a much tougher problem. I hope that wasn't too defensive or pedantic. Duane >> Hi Ted Duane, >> >> Could you share with us what you meant by the statement: "a V.90 modem >> with marginal return loss may not work very well. " > >I'm quite surprised that a telecoms consultant is not certain of the >problems caused by a mismatch of the terminating impedance to the >source impedance..... > > >> >> I assume you are not referring to return loss during DTMF transmissions. > >Its possible that poor return loss could affect performance in >conjunction with some systems (please see EG 201 121) but I think Ted >is referring to transmission of data. > > > > >Best Regards > >Gary Gorton >Senior Telecommunications Engineer > >--------------------------------------------------- >TUV Product Service >12, Marshall Road >Hillmead, Swindon >Wiltshire UK >SN5 9FZ > >T. +44-(0)1793-871990 >F. +44-(0)1793-872990 >W. http://www.tuvps.co.uk >---------------------------------------------------
