RE: Bandwidth [7:92568] posted 09/08/2004
WAN technologies are full duplex.
Can't speak for DSL or wireless, but can provide some insight
on Cable. The RF spectrum on cable plants are divided
into a forward RF path and a return RF path. The precise spectrum
allocation plan differs by country but in general, the forward path
is the spectrum from ~40-50 Mhz to 1 Ghz. The return RF spectrum
(home->head-end) is from ~15 Mhz to 40-50 Mhz. So, there's
not nearly as much return spectrum availble. Further, other
factors (frequency, other sources that impact errors rates within
the spectrum) impact the modulation techniques which can be applied.
One the downstream, higher modulation (64QAM or 256QAM) rates are
used which supports from 27 to 40 Mb/s per a 6 Mhz RF channel. On
the return path, typically only QPSK or 16QAM can be used
which results in only 5-10 Mb/s per a 3.2 Mhz channel. Thus, the
current physical properties of the cable infrastructure is one
where much more downstream capacity is available. Other things
can be done (eg how the cable company performs RF combining) so
that the return path bandwidth is used over a limited number of
Note that cable is also full duplex; but since the return path is
shared, there's a scheduling mechanism used to dynamically assign
time slots to cable modems on the return RF to eliminte
transimission collisions (the interested reader can read the
DOCSIS specs if interested in details on how this works)
Mel Chandler wrote:
> I was sitting here thinking about bandwidth and I didn't know
> off the top of
> my head if Frame Relay or other WAN technologies are Full
> duplex or half.
> My gut instinct says half. So if you were utilizing all of
> your 1.544Mbps
> bandwidth sending a file, then no one could receive any data.
> This brings to mind another question about DSL, Cable, and
> providers. Why do they have a limit on the upstream
> bandwidth? Are these
> also half duplex connections? Why limit the upstream bandwidth
> stringently than the downstream? Does it cost them more money
> for upstream
> If anyone has all the answers or could point me to a good
> reference, I would
> be most greatful. Thanks.
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