🚀 go-pugleaf

> Someone Wrote:
> > Cable does not use CSMA/CD.  It has always, including the proprietary Motorola system,
> > used polling for the upstream channel.

It depends on the type of cable system.  The only system I've worked
with (albeit a *long* time ago) was
an Ungermann/Bass Broadband Ethernet...this was 10Mbps CMSA/CD over
broadband coax.

I thought that even the very first (pre DOCSIS) cable modem system used
CSMA/CD.  I also though that even DOCSIS reverts to a CSMA/CD-type
access method if you diable the slotting processes.

"Randy S." wrote:

> You're not saying that not being CSMA/CD would avoid the upstream throttling issue are
> you?  Because as John correctly says, this is a TCP/IP limitation, not an Ethernet one.
> 

No, what I'm saying is that with CSMA/CD you have ONE shared upstream
channel, thus saturation would
be easier to occur, so I'd be even more concerned.

What I think John is saying is that DOCSIS cable allocates upstream
slots to each user.  Slotting
should provide some minimal amount of bandwidth to each user, however if
the total
slot allocation is so over-subscribed that a few (dozen?) users are
using their total allocation
can cause upstream saturating.

The key comes down to just how oversubscribed is the upstream?

If you've got 500 users with 256,000bps contending for 2,000,000 that's
> 50:1...which is
2x-3x what ADSL typically is.  These are just numbers I'm pulling from
the air, I defer to someone
that knows cable infra to post real numbers.

lb.

-- 
-------------------------
Lawrence Baldwin
Protocol Scientist
678-624-0924
http://www.mynetwatchman.com - I'm under attack, now what?