[TriLUG] Broadband Speeds
cgbullock at cox.net
Wed Oct 8 10:49:28 EDT 2003
Ben Pitzer wrote:
>>You have to remember that cable is shared bandwidth, compared to
>>dedicated as with DSL.
>Actually, this is not strictly true. The key in any cable plant is the
>signal power. As the signal degrades across the wire, it is periodically
>boosted by a DC amplifier. There is a specific range of signal levels
>(dBmv) that the provider is trying to get at each house along the route.
>Now, if a new house is built in the middle of a line, a splitter must be put
>on the wire to serve that new house. This means that the signal at the amp
>prior to that splitter must be adjusted to allow for the approximately 3db
>difference in signal following the amplifier, or else the last person on
>that line before the next amplifier might have signal issues. What this
>means is that so long as the signal is good, the bandwidth is there.
>However, when there is alot of traffic on the line, the signal can get
>attenuated, because sometimes the transmit and receive power levels for
>modems can blur. So if two modems are transmitting at adjacent power
>levels, they might occasionally have moments where they are transmitting at
>the same power level when it hits the CMTS. That confuses the router, which
>asks one or both modems to adjust its power level. They do, and try again.
>Despite the commercials that we all laughed at from PacBell DSL a few years
>ago (Cable Hog graffiti on the garage door, snipping the cable line with
>hedge clippers, etc), not all users on a line will experience difficulty at
>the same time, unless there is a larger plant issue. Cable techs are
>looking for those issues ALL the time, and are typically quick to fix them
>when reported, in my experience.
>Now, I'll go ahead and potentially discredit myself by saying that
>everything you just read was learned a few months ago, and I may be a little
>fuzzy on some of those details. Folks familiar with RF technology feel free
>to correct me if I made some mistake, but I'm fairly certain I got the
This excerpt comes from DSL reports, which could possibly be wrong, but
I don't think so.
Sharing bandwidth - how many users per node? (#7135)
The number of modem users per node is important because due to the way
cable systems are built, nodes are the common aggregation point for all
the cable signal traffic in a particular neighborhood. This means all
users connected to one node share the bandwidth available to that node.
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