Are they really modems? Skipper says yes... but...
This is the USRX topic!
He says, describing the current Ascend/Rockwell product in detail:
1) A serial bit stream is output on your COM port to your modem.
2) The modem converts this data to audio frequencies.
3) The audio frequencies are carried on an analog line to the telco.
4) The telco converts the analog signal to a digital PCM signal.
5) The PCM signal is sent on a PRI to a MAX 4000.
6) The Rockwell chip in the MAX 4000 converts the PCM signal into audio frequencies (internally), and then into a serial bit stream which is processed by the MAX. The chip is doing a D/A/D conversion that used to be two separate steps.
If we had an X2 modem here (and presumably the new K56 stuff will work this way, too, when it's finally out...), it doesn't ever reach analog again... it's digital right off the PRI (which stands for Primary Rate ISDN) line right into the "modem". So it's not really a modem, in that it doesn't work with analog, but rather with a digital data stream. But we still call it a modem, because any alternative terminology decends rapidly into acronyms and/or Dilbertese.
7) On the return trip the MAX sends serial data to the Rockwell chip.
8) The Rockwell chip converts the serial data to PCM encoded audio frequencies.
Again, the X2 "modem" outputs a digital PCM data stream, not analog. No analog in the path, so far.
9) The PCM data travels to the telco, where it is converted to an analog signal and sent to your modem (USR no doubt).
10) The modem converts the audio frequencies into a serial bit stream and sends it to your COM port.
The modem does one D/A conversion while a codec (coder/decoder) does the other (PCM). The new 56K chips now combine these two functions.
I don't agree with Skipper, in that the original question was asked about "modern" hardware like Total Control, not some kind of old stuff which was retrofitted for digital telco. In the modern system, the only digital/analog conversions are in your modem, and in the codec in the telco near your house. The rest of the path is all digital. But we do still call the box or card that the ISP buys a "modem".
Proposals for a better name probably belong in the dog house. Or maybe over on the ZOOM topic, where it sounds like they need some new products to make! |
