ISP Information:
An electronic device that converts analog signals into digital signals. For example, the input port of a sound card can accept analog input from a microphone, and it uses an ADC to convert the analog signal to a digital signal that can be sent along to other parts of your computer.
ISP Glossary:
Analog to Digital Convertor - Reed wrote:Manish Bhasin wrote: Hi All, I have a system where a modem is connected to a residential voip gateway using a two wire connection. This residential gateway is talking to an H323 - ISDN gateway over a satellite connection. In normal voice calls I use G.729. But whenever there is a modem call, both the gateways upspeed to G.711 and I m using G711 A-Law right now. Disabling of Echo Cancellation in both the gateways is configurable (by default echo cancellation is disabled whenever there is a modem call). I am using modems from a spanish manufacturer called SATELSA and need to make just the low data rate modems work (V.21, V.22, V.22bis, V.23) The problems is that when I try to establish a call between two modems, calls go through using some models and at some specific data rates/modulation schemes but not other combinations of models and data rates. I tried to experiment with disabling echo cancellation just at one end and keeping it enabled at the other. Sometimes it helps to an extent that I see the circuit 109 (line signal detected) on at one end (e.g. at the PSTN gateway end when it is the called end and the echo cancellation is kept enabled at this end) but not at the other end. The call still fails. Now searching for ideas on how to improve this performance. Any help in this regard will be greatly appreciated. Warm Regards ManishWhy ??The average analog 2-wire dial modem has enough trouble workingover the channels for which they were designed to work. 3khzband-limited and all that. To try to operate them over a "lossy"medium like IP is going to be marginal and unreliable at best. Afew dropped or late packets and your done.Even with analog voice compression schemes such as ADPCM, mostmodems cannot maintain connections. Now you want to try are-digitized, lossy, variable delay path ??I agree with your over all point. However, ADPCM, and anynumber of other methods that are used to provide quasi-analogvoice channels using 32 Kbps for the digital transport do infact work with modems. I once had the opportunity to test AydenADPCM, Hughes DAMA, Scientific Atlanta DAMA, an AT&T packetsystem, and a couple of others, plus some older analog satelliteequipment. (Basically everything available in Alaska in 2000.)The common characteristics of all these were that they did nothave variable path delay (although some of them have significantphase jitter compared to standard 64Kbps circuits) and allprovided the equivalent of a 32Kbps bandwidth on the digitalside (except for the really old SA and California Microwaveanalog SCPC satellite systems).Basically they all function at 14.4 Kbps with a quality v.32bisor v.34 modem. Most would handle 16.8 Kbps, and some could hit19.2 Kbps (for example, the DAMA systems). In fact the oldanalog systems could be, in some cases, configured for wideenough bandwidth to even get 21.6 Kbps now and then, on anotherwise good channel.I found that a USR Courier and a SupraFAX modem were the bestthat I had. I tried several PCMCIA cards on my laptop, andnever did find one that would work well over these limitedbandwidth systems.But one thing came through loud and clear: you *cannot* tandemtwo of *any* of that type of reduced bandwidth channel and get adecent modem connection! I think 7200 bps was the highestconnect that I ever got, and it wouldn't stay up for long. 1200bps wasn't even stable on many connections. And on many therewas simply nothing that the modems could actually connect at.(The effect of this was that from villages I could make modemcalls to an ISP near the major switching centers like Anchorageand Fairbanks, but could not get a connect calling my owncomputer here in Barrow, because it always involved a tandemroute using at least 2 of those systems.)VoIP was just starting to appear when I retired, so I've neverworked with it. I cannot imagine that a modem connection wouldbe possible over a real world VoIP channel. Every droppedpacket causes a phase hit larger than a modem can tolerate.The phase j
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