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Re: [LUG] Routers and Modems

 

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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" :)
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