> Cable networks are laid out in loop form
> primarily because
> the coaxial cable in most neighbourhoods is not bi-directional,
> the majority
> of the data can only flow in one primary direction. If anyone is familiar
> with the old IBM Token Ring networks, it is a similar concept. Basically
> one house is connected to the other, connected to the other, connected to
> the other... until both ends of the ring meet back at the cable company's
> equipment.
Kai, I'm sorry but I don't feel this is even a legitimate simplification.
Television cable is simply a wire, not all that different from the original
Ethernet wire, and the wire works either direction. There is no looping
from house to house as in Token Ring, nor will your connection go down
because my computer crashed as in Token Ring. There are various line
amplifiers between the cable office and your house, and these are typically
one-way devices. What the cable company has to do in order to offer cable
modem service is bypass each of these line amplifiers with a reverse
direction amplifier, that amplifies only the up-stream frequency band. So,
the wire really is bi-directional (just like original Ethernet), there is no
ring involved.
There is a fairly complicated protocol involved to try to avoid collisions
(and detect when they occur) on the upstream side, much more sophisticated
than Ethernet because of the long distances (and hence delays and
differences in signal level) involved. If you'd like to read more, try
http://www.cable-modems.org
They have a fairly nice tutorial.
BTW, I think most DSL providers have twigged to the fact that their upstream
connection costs are significant. At least around here, they now charge
extra for static IP addresses (as do the cable companies). They don't like
to talk about what their connection is, but a tech once told me there was
only a single T3 (roughly 44 megabits/second) between my central office and
their server location. Sounds like a lot, until you start figuring out how
many homes are within a 5 mile radius of the CO (and hence likely served by
it). And the current residential DSL agreement specifically prohibits
setting up any sort of server. There is a substantial extra charge (like
triple) for a business DSL that allows you to run a server.
Randall
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