>Another way of asking might be this: Could it be true that network >line >cards are really 600 ohms resistive worldwide, and that the >complex >reference impedance specified was chosen to approximate >the impedance >presented by the combination of the network >equipment and the real-world >loop? (With the intent that equipment >designed to provide a complex >source impedance would peform >better than equipment designed for 600 ohms)
Line cards source the line at 900 ohms. 600 ohms is only encountered on extensions of a PABX or line simulators. In other words subscriber loops are rarely 600 ohms. As Vic stated the variations are network equipment, length of loop, wire size, capacitive reactance, etc. The variations from loop to loop are infinite, even when sourced by the same CO equipment, no two are alike. The complex impedance requirements of most PTT's has evolved over a long period of time. As a consequence qualifying or justifying them is probably difficult. Also don't forget the network is primarily intended for voice communications. Voice requires much better return loss than a modem. In my opinion modem chips produced today will function well with a return loss of 6 - 7 dB. With either 600 ohms or complex impedance, but meeting approval requirements without buggering modem performance is generally something else. Duane
