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On Apr 15, 2012 11:07 PM, "Mark Evans" <mpe@xxxxxxxxxxxxxxxxxxxxxx> wrote:
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> On 15/04/12 11:54, Gordon Henderson wrote:
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> > 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.
Many of the Virgin media set top boxes have an ethernet connection and can be used as modems but have an appalling connection speed, so separate modems and routers are provided.
>
> > 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" :)
I don't know if they're still doing it, but BT used to call ISDN 'midband'. *shudder*
Grant.
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