"One thing Larry - I've found this x2 to be pretty picky connecting to my x2-capable ISP at times when it will connect reliably to my other 28.8K-only ISP. I can't really detect any audible line noise so it must be something very subtle, we have been going through some wide temperature swings lately. "
I've heard that same problem from others. Here is my theory.
Regular V.34 modems use a modulation method which I liken to FM radio, since it uses phase information extensively. However, 56K modulation is amplitude modulation, for the most part, so I liken it to AM. You know from experience that FM radio is more "stable" even when the atmospherics are not, than AM is. An FM radio can get excellent receoption in electrical storms, while AM radio can be impossible to listen to under the same circumstances, and can also be affected by temperature, humidity, etc.
Since 56K modems use "amplitude modulation" (PAM = Pulse Amplitude Modulation), they will be sensitive to anything which affects the dynamic range and distortion of the phone lines. It doesn't take much of a problem to mess them up; for example, the ear can't detect changes in amplitude much less than 3 dB, but a modem is sensitive to much smaller variations. I's suspect that the phone lines in your locale ARE sensitive to temperature, and that your 56K modem is having problems because of amplitude distortions of some sort. V.34 uses both amplitude and phase, so it is less sensitive to amplitude distortion.
Well, I pulled that one out of my hat, just like Bullwinkle used to do on TV, remember?
"Normally one could blame the line at the other end if it
works fine with a different ISP, but if I'm not mistaken I'm going digital from the local
CO straight into the x2 ISP's facilities. "
You know, I have a "digital" connection of this sort with my ISP too. When I call that number, I am connected through US West to a high capacity digital trunk (a bunch of T-1 or T-2 lines), which go directly to the ISP, into a Livingston digital box. It digitally "emulates" a modem, so that when I communicate with it, my modem thinks it is talking to another V.34 modem. This eliminates one A/D conversion and one D/A conversion, which reduces the effect these steps would normally have on the connection, and allows me to connect at 33600 most of the time (8 out of 10 tries).
Even though this is theoretically a "superior" connection, I have a lot of trouble with it doing precisely what you describe, ie, dropping the connection right in the middle of something, the way analog modems drop off when an extension phone is accidentally picked up. There is nothing that can happen to cause this (the ISP isn't timing out or anything like that; they've turned the timer off for us), other than a flaw in the digital system somewhere.
When I call into one of the straight analog lines, I only get 33600 about 20% of the time, BUT IT NEVER DROPS THE CONNECTION after it is connected, the way the "digital" connection drops at random. I wonder if you are having the same problem, and I wonder if it is something to do with digital problems that mimick analog problems? Just because the system is "digital" doesn't mean it will work right, it only means that data is transmitted using wideband binary signalling rather than narrowband modulation. Both methods are a form of modulation, and the rest of the system can have problems, too.
I've been disappointed in this, as I'd expected MORE reliabel connections through the digital box, not less reliable connections. If I discover anything further about this problem, I will let you know. Frankly, the ISP tech support people just think I'm nuts, so I don't know if I'll get much chance to solve it.
Larry |
