srg@quick.COM (Spencer Garrett) (09/15/88)
In article <39163@pyramid.pyramid.com>, csg@pyramid.pyramid.com (Carl S. Gutekunst) writes:
- The TrailBlazer uses a proprietary modulation scheme called Dynamic Adaptive
- Multicarrier Quadrature Amplitude Modulation (DAMQAM). Rather than the single
- high-frequency carrier used by other modulation schemes, the TrailBlazer uses
- 511 different low-frequency carriers. Each is QAM modulated at a rate of 12
- baud (that is, 12 state changes per second); so each carrier has a data rate
- of 48 bits per second. Each carrier can be slowed down to 8 or 4 baud (32 or
- 16 bps), or dropped entirely. On real phone lines, quite a few carriers are
- dropped, and a number are slowed down to 8 baud; so the theoretical speed of
- 24528 bits per second is reduced to a real maximum of 18031. (Why an odd num-
- ber? I don't know.) Note that since DAMQAM modulation broadcasts across the
- entire bandwidth of the telephone line, it is half duplex -- only one end can
- be transmitting at a time.
Not quite right. The Blazer runs at 7.58 baud (132 ms) when it has
data to send. Rev 4.0 and later use a shorter time when they're idle.
There are 511 discrete frequencies each of which can encode 0, 2, 4, or 6
bits, but the baud rate doesn't change. The modem can't or won't
use all of the channels at once (I don't know why). It seems to
max out at 2380 bits per packet, and 18031 is just 2380 * 7.58,
more-or-less.