Dear TREGers, Many, many thanks to all those who replied to my recent queries about trans-hybrid loss. Your answers helped us to "stick to our guns" with a customer, who now also fully agrees with us, and has apparently solved some software difficulties that were causing poor near-end echo cancelation. It seems that we had already done just about all we could to ensure that the THL for our design would be no worse than others.
I have some questions relating to the ability of (half-duplex) group 3 facsimile modems (V.17/V.29/V.27ter) and (full-duplex) data modems (V.34/V.32bis, etc.) to withstand short drop-outs. I suppose that any of you who develop modem DSP software, or who work with those who do, may be able to offer advice on this subject. Can any of you provide a definitive answer as to how these modems will behave in the event of the speech connection between two such modems being occasionally lost for 1 millisecond? By "occasionally", I mean once per call, or possibly once every few minutes, at a random time after the speech connection has been established between the two subscribers' equipment. I am only referring to a speech drop-out, with the DC connection between each modem and its telephone exchange being unaffected. During the drop-out period, there is silence. If you feel that a 1 millisecond break could cause serious problems, how short could the interruption be without causing the modems to drop the connection, or to cause more than a few seconds' delay before resynchronization and data channel recovery occur? What is/are the mechanism(s) that causes the connection to fail in the case of such drop-outs? Clearly, data are lost during such an interruption, but the question is whether the connection will recover after such an interruption, and if so, how long it would take for the modems to resynchonize, and how far-reaching the repercussions might be. Our interest is in how standard quality modems on the market in the described circumstances, rather than how some unusually well-designed modem happens to work. >From our own experience with facsimile modems, the modems are normally able to resynchronize and in most cases the impact will be retransmission of V.21 handshaking or a block of image data at V.17/V.29/V.27ter. However, I have not done an exhaustive study on the abilities of the various fax modems (and their controlling software) to cope with an impairment of the type described. We have no direct experience with V.34 and other data modems, except that we have found that if someone picks up a telephone on the same extension to which one of our V.34 modems is connected, it is virtually guaranteed that the connection will rapidly be dropped altogether. Best regards, David Drori ----------------------------------------------------------- Novarex Enterprises Ltd., POB 2833, 306-6, Mevo Hapartisanim Street, Jerusalem 91028, Israel. Tel: +972 2 5810995 Fax: +972 2 5813750 -----------------------------------------------------------
