TEST [7:41824] posted 04/18/2002
Is the group working?
From: bergenpeak [mailto:bergenpeak@xxxxxxxxxxxxx]
Sent: Tuesday, March 19, 2002 8:54 AM
Subject: Re: Is cable network really a shared medium? [7:38705]
The shared vs non-shared issue DSL providers mention is somewhat
misleading. In any residential cable or DSL network, you will
have stat muxing. In a cable network, this happens on the HFC
network. In a DSL network, this happens at the Agg router (the
one that terminates all of those DSL connections). The Internet
is one big stat mux. In either the DSL or Cable approach, the
customer observed performance will be a result of many factors,
including access network design (how many subs share the cable
or agg router), the behaviors of these other users, the regional
network design, the size and types of peering connections, and
where the users are actually surfing too.
My house has a long driveway that only I use. Does that mean
I'll get to work faster than the neighbors down the street
which live in an apartment complex and share a driveway with
In both approaches, one can prioritize traffic or partition bandwidth
to certain groups of users.
The current standard for how IP/ethernet frames are transmitted over
an HFC network is defined via the DOCSIS 1.0 spec. This specification
is available at www.cablelabs.com. This spec defines how to
support best-effort IP transport.
Support for additional features, include QoS, is defined in the
DOCSIS 1.1 spec. This document is also available at the above
Some details about DOCSIS cable networks:
* On the HFC network, a single downstream channel can support
~25-35 Mb/s (depending on the modulation being used).
* The upstream connection typically can support between 5-10 Mb/s
(depending on modulation and the size of the channel).
* The cable operator can opt, based on RF combining, how many homes
(fiber nodes) share a downstream or upstream. When service is
launched in an area, an operator might combine several nodes together
and as the take rate increases, reduce the amount of combining
(which effectovely reduces the number of customers who share the
* When a cable modem is brought online, it gets an IP address via
DHCP and then is loaded with configuration information (IP, L2,
and L4 filters), network management, etc information. These
filters prevent issues which arise when DHCP servers are
running in a customer's home, prevents my NETBIOS traffic from being
seen by neighbors, etc.
There are other technologies still deployed by cable operators to
HSD (LanCity, Motorola CDLP, Com21, etc.) which may not operate the same
Hope this helps.
sam sneed wrote:
> I just changed services from DSL to cable modem. I have heard from people,
> including verizon, that cable is not as secure as DSL becuase it is over a
> shared medium. I connected to my cable modem and fired up my packet
> I did not see anyone elses traffic on the line so i am assuming the
> is shared( a known fact about cable access) but is somehow filtered at the
> cable modem(bridge). Does anyone know if this assumption is true and the
> inside details of the how data is transmitted over the cable network? A
> to a whitepaer would be great.
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