[TriLUG] Broadband Speeds

Ben Pitzer uncleben at mindspring.com
Wed Oct 8 00:40:40 EDT 2003


> 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
basics correct.

  Potential speeds of cable are significantly
> higher than that of DSL.  I have heard that DSL is more reliable but
> can't make an honest call about that.  Cable is like the days of dial-up
> when everyone got off work and tried to dial up and got a busy signal.

Actually, that is a very, very poor analogy, mainly for the reasons I just
outlined above.

> Cable acts the same way, unless the infrastructure is made to withstand
> a lot of people surfing at the same time.  Depending on the cable
> provider depends on the speed decrease, hopefully the cable provider has
> planned for this and the consumer should notice little difference if any.

Yes, that's the goal.

> --chris
>
> Stephen Hoffman wrote:
>
> >Ok, so with respect to speed for TWC, what are the actual download speeds
> >for Cable?  I am at a consistant 160Kb/sec (advertised at 1.5M), how does
> >this compare to the other providers?  I have heard stories of cable
> >getting painfully slow around 5ish when everyone is home and surfing, is
> >that the case here?  How about Upload speed?  Bellsouth DSL gives me
> >roughly 35Kb/s upload.  I am considering the switch, but want to
> make sure
> >I am getting as much bang for my buck as possible.
> >
> >TIA.
> >Steve
> >
> >
>
>
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