necka@cell.mot.COM (William J. Necka) (11/22/89)
In article <5385@uhccux.uhcc.hawaii.edu> bob@kahala.hig.hawaii.edu (Bob Cunningham) writes: >Would someone kindly tell me what the purpose is of the V.25 answer >tones and whether or not one wants an answering modem to offer >those first (in V.32 mode or not...)? >Bob Cunningham >Hawaii Institute of Geophysics, University of Hawaii >bob@kahala.hig.hawaii.edu The V.25 tones serve 2 purposes. The original goal was to disable echo cancelers on foreign phone systems, especially in Europe. The V.25 tone is a 2100hz tone that is sent by the answering modem for about 1.7 seconds followed by 75ms of silence and then the actual carrier frequency. In V.32 modems this tone was enhanced to include phase reversals and is actually part of the V.32 answering sequence. You will notice while listening to a V.32 answering sequence that the 2100hz has a "ding" "ding" sound to it. That sound is caused by the phase4 reversals. The V.32 standard has an alternate method of training which is not preceeded by the 2100 sequence; however no modem manufacturer has implemented V.32 that way to the best of my knowledge. Should anyone know of a modem that has this alternate method of traing up in V.32, please tell me and I can tell you of at least half a dozen V.32 modems that will have some type of problem connecting to it.