Rogers high-speed internet

[Tim Writer]

James Knott <james.knott-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org> writes:

> Tim Writer wrote:
> > James Knott <james.knott-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org> writes:
> >
> >
> 
> >> Bell will now supply a modem/router/firewall combo, but that's useless for
> >> my work, which requires only a modem.
> >>
> 
> >
> > I don't get this at all.
> >
> > First of all, the Rogers "modem" is not a modem at all. It's a bridge.
> 
> 
> It's a modem in that it converts ethernet signals to something usable on
> the cable network.

I think we're both right.

A cable modem includes both layer 1 (the physical layer) and layer 2 (the
data link layer) of the OSI model. The physical layer of DOCSIS cable
modems uses QAM (quadrature amplitude modulation) to convert
(modulate/demodulate, hence "modem") between digital data and analog
signals. The data link layer or MAC uses TDMA or S-CDMA to mediate access
to the physical layer and convert between raw digital data to Ethernet
frames.

In contrast, a traditional analog modem provides only layer 1 and PPP is
used for layer 2. Without PPP, there are no frames (packets), just a stream
of bytes.

To users, a neighbourhood of cable modems is much like a LAN, as if the
cable modems make up one big hub into which everyone is plugged. I think
it's useful to think of it as a bridge because (to the end user) it acts
like a bridge. I believe cable companies chose to call it a modem because
everyone "knows" what a modem is.

> > Second, unless your work involves snooping on your neighbours, I don't see
> > how the network topology would affect you. At then end of the day, whether
> > you're using PPPoE (Bell DSL) or plain Ethernet (Rogers), you have a
> > publicly accessible IP, a suitable netmask (for the Internet side of your
> > connection), and a default route (gateway) to the rest of the Internet. IP
> > is IP is IP.
> >
> >
> 
> The modem/firewall/router combo isn't suitable, as the device I'm
> connecting has it's own firewall and address translation.  It is also used
> for VoIP and must have a "real" IP address, not the RFC 1918 address
> provided by the modem/firewall/router box.

We have exactly that setup in our office: Bell DSL service with a combined
DSL modem/router. As a result, we have two levels of NAT, one on the Bell
DSL modem/router and one on our firewall but it makes no difference. Nobody
ever sees the addresses on the (two node) network connecting the inside of
the Bell DSL router to the outside of our firewall.

This setup is very similar to a traditional T1 where the ISP provides a
router. The main difference is their router doesn't usually do NAT and they
configure it to route a (small) public subet so you can use public IPs on
your firewall.

I preferred the old DSL setup, with a separate DSL modem and PPPoE on the
firewall because there was no need for two levels of NAT but I guess Bell
figured most people want a router.

> Also, with ADSL, you don't see any traffic from your neighbours.

Sure, which is why I said you must be snooping on your neighbours. :-)

-- 
tim writer <tim-s/rLXaiAEBtBDgjK7y7TUQ at public.gmane.org>                                  starnix inc.
647.722.5301                                      toronto, ontario, canada
http://www.starnix.com              professional linux services & products
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