by johnny nemo » Fri May 19, 2006 1:06 pm
Thanks to my education, you now know that EFAXING DOES NOT REQUIRE A FAX MODEM.<br><br>You also learned that you can use your telephone with your computer via VoIP, as opposed to using a fax modem.<br><br>Now you will learn that Digital modems are, in fact, still MOdulator/DEModulators.<br>Obviously, there isn't an analog to digital conversion when you're going pure digital, however..... a cable modem sends and receives data in two slightly different fashions.<br>In the downstream direction, the digital data is <!--EZCODE BOLD START--><strong>modulated</strong><!--EZCODE BOLD END--> and then placed on a typical 6 MHz television channel, somewhere between 50 MHz and 750 MHz. Currently, 64 QAM is the preferred downstream modulation technique, offering up to 27 Mbps per 6 MHz channel. This signal can be placed in a 6 MHz channel adjacent to TV signals on either side without disturbing the cable television video signals. <br><br>The upstream channel is more tricky.<br>Typically, in a two-way activated cable network, the upstream (also known as the reverse path) is transmitted between 5 and 42 MHz. <br>This tends to be a noisy environment, with RF interference and impulse noise. Additionally, interference is easily introduced in the home, due to loose connectors or poor cabling. Since cable networks are tree and branch networks, all this noise gets added together as the signals travel upstream, combining and increasing.<br>Due to this problem, most manufacturers use QPSK or a similar <!--EZCODE BOLD START--><strong>modulation</strong><!--EZCODE BOLD END--> scheme in the upstream direction, because QPSK is more robust scheme than higher order modulation techniques in a noisy environment. The drawback is that QPSK is "slower" than QAM. <br><br>So, you may be all pissy-pants about being instructed, but the fact of the matter is that I've taught you more than you knew before, else you never would have said some of the stupid things you said.<br><br>So, I'm gonna let you pass with a D - and school is out.<br><br>n/t <br> <p></p><i></i>