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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 15/04/12 11:54, Gordon Henderson wrote: > Even with Cable - they are still modems. Cable Internet uses a system > called DOCSIS - and it's an analogue carrier signal modulated with the > digital signal over co-ax cable - unlike Co-Ax Ethernet which is > baseband (You put 5V on the cable to indicate a "1" and 0V to indicate a > "0"). You can use different carrier frequencies to carry different > information - so the way you get Internet and TV at the same time is to > use different carrier frequencies and filters to split the signals out. > DOCSIS uses a carrier of (IIRC) 10MHz - it can get away with much higher > carrier frequencies than ADSL as it's co-ax cable and thus not prone to > radiating the signal out, nor accepting interferance in. There's actually a cable modem built into many of the set top boxes VM use. Though rather than provide a customer connection (through the ethernet port on the back) this appears to be used to provide a data link to the box itself. e.g. for the video on demand feature. > If your home broadband is supplied on copper then the digital signals > are carried over it modulated on-top of an analogue carrier signal. In > ADSL, the carrier is between 32KHz and about 1.2MHz - the lower > frequencies are used for the upstream and the higher frequencies for the > downstream. The bands are divided into "buckets". Part of the > negotiation phase involves testing each frequency to see which work and > which don't - and various factors affect this - line length, local In some cases these can be variable. e.g. if the connections are not completly water tight. > conditions, etc. Some technologies extend the upstream frequeneices into > the downstream ones to give you more upstream at the expense of less > downstream speed (Annexe M in this country) > > So just because we can't hear it, doesn't mean its not via a modem. The > principles are the same, the carrier frequencies have been shifted from > the audio range to radio range. The most obvious example of "getting it wrong" would be calling ISDN "broadband" :) -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk+LRnMACgkQsoRLMhsZpFee4gCfW1l8greQ5RJIAxMXvlkOF98w FYIAn3hpF3VdkwQCI1ExEiXgBWTQeCNq =UQ/e -----END PGP SIGNATURE----- -- The Mailing List for the Devon & Cornwall LUG http://mailman.dclug.org.uk/listinfo/list FAQ: http://www.dcglug.org.uk/listfaq