David Drori said: >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.
>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? Such very short dropouts are a common occurrence, for a variety of reasons: 1) T-1 spans resynchronizing 2) Metering pulses drowning out the encoded signal The result of such a dropout would be a lost V.42 data frame, triggering a retransmission, which appears to the user as added latency. If enough frames are lost, the modem may decide to retrain, causing a significantly longer delay. >From our own experience with facsimile modems, the modems are normally able to >resynchronize and ... Half-duplex fax protocols are significantly more robust than full-duplex protocols, due to the lack of (need for) adaptive echo canceling. >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. The result of picking up a handset on the line goes far beyond a short signal dropout. For testing this sort of behaviour, you need a telephone line simulator with programmable line impairments. / Lars Poulsen [email protected] +1-805-562-3158 RNS / Meret Communications 7402 Hollister Avenue Goleta, CA 93117
