Rogers high-speed internet

[James Knott]

Tim Writer wrote:
> 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.
>   

A lot of people seem to think everyone in the neighbourhood is on one
big lan.  This hasn't been the case for years, if at all.  The up and
down paths are on separate frequencies back to the head end.  This means
that you can only see the neighbours the same way as you would any one
else on the internet, that is via IP.  Take a look at the DOCSIS specs
to see why this is.  If everyone on cable is one one big lan, then so is
everyone on ADSL, for the same reason, that is everyone is a "neighbour"
back at the head end or CO, but not before.
>   
>>> 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 would mean providing translation for one TCP and up to 24  UDP
ports per device.  The firewalls don't provide that sort of filtering
very easily.

> 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. :-)
>
>   

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