Regarding virtual channels...
Eric,
Your question is a valid one. Can one ISP prevent another ISP
from infringing on their performance when both share the same cable.
For residential Internet delivered over the cable plant, the norm is broadband
LAN technology. This technology allows transmission of digital data over one
or more 6 MHz channels of a CATV cable.
The broadband LAN is built on top of two 6 MHz channels, one transmitting data
from the headend to subscribers downstream, and the other transmitting data
from subscribers to the headend upstream. At the headend, a frequency translator
connects the two channels into a channel pair, so that data can be sent from one
customer to another.
The problem with the "virtual channel" is that currently, the upstream channels
share the same spectrum between all of the subscribers.
A weak attempt to illustrate my point is shown below.
While there may be 15 downstream channels, they will all use the same
upstream spectrum.
MHz 0 20 40 60 80
+------------------+------------------+------------------+------------------+
| upstream | | ch 2-4 |
spectrum
MHz 540 560 580 600
----------------+----------------+------------------+------------------+
| ch 69-... | | | | | | | | | | | |
6 MHz downstream channels
use same upstream spectrum
Downstream transmission from the head end is "broadcast", as the same
signal is sent on all the wires. Upstream transmission is inherently
"personalcast", where each subscriber is trying to place a different signal
onto the network. When going upstream, these different signals must
eventually share the same piece of transmission spectrum. However to
accommodate another ISP, or virtual channel, a separate upstream and
downstream channel would be required.
In any case, downstream channels will need to be given up by the MSO in order
to accommodate other ISP's. Highly unlikely, IMO. |
