[Gllug] ADSL modem -- WTF?

[Russell Howe]

On Thu, Jul 15, 2004 at 01:35:51PM +0100, Tethys wrote:
> 
> I went up to TCR the other lunchtime to try and get a simple ADSL
> bridge/router for my parents' imminent ADSL line. When I went and
> asked, the salesdroids all asked if I already had a modem[1]. WTF?
> Given that there is no modulation/demodulation going on, it's got
> to be a marketing ploy, as that's a term people are used to. But
> what is it? You can buy a standalone ADSL modem, an ADSL router,
> or a single box with the two combined. If the "modem" is the bit
> that handles the PPPoA, then what's the router (i.e., what makes
> it specifically an ADSL router as opposed to any other IP router)?

Cisco apparently used to do SOHO DSL kit which I want to get my mitts
on. It consisted of a DSL modem with a phone wire on one side and an ATM
port on the other (apparently DSL uses ATM somewhere along the line,
hence PPPoA being more efficient than PPPoE, which would have to be
PPPoE over ATM). Cisco then did the router, which had an ATM port and
one or more Ethernet ports.

What I want (and what apparently murb can supply for me :)) is a modem
which has an ATM interface, an ATM card for my firewall and an
appropriate connection cable. I can then bridge the PPPoA onto the ATM,
and terminate the DSL on my Linux firewall. The modem just being there
to 'convert' PPPoA coming in over the phone wires via DSL to PPPoA on an
ATM wire. A DSL-ATM bridge, essentially.

I currently have the equivalent using PPPoE and Ethernet, however this
gives you a reduced MTU (1492 vs 1500 for PPPoA) and since my ISP (or
BT, or both) is broken and assumes that your MTU is 1500, doesn't send
out ICMP fragmentation needed and will not fix their systems ("We don't
support PPPoE"), I want to switch to the ATM-based setup ASAP.

The other option is to get a PCI card which can do the ATM stuff onboard
- this would likely perform better than the above solution anyway. Yes
there are the Conexant-based cards people have mentioned, but
apparently the drivers are more than a little flaky. I've heard very
good things about the Bewan-based PCI cards, and someone has obviously
been so impressed with them and their Linux support they set up
http://www.linuxadsl.co.uk/ to extol their virtues. My boss uses one in
place of his old USB Speedtouch and swears by it (and at the Speedtouch).

> All I want is a box with two leads -- one into my phone socket and
> off to my ISP, and one into my firewall (and on to the rest of my
> network). It needs to be able to route my IP range frmo the outside
> world and on into my network. It looks like the D-Link DSL-502T
> should do that. Does that sound reasonable?

Needs to route your IP range, or needs to NAT your LAN? big big
difference.

See the end of my email for a verbatim cut-n-paste from an email I sent
to support at nildram.net

> Incidentally, has anyone else noticed how hard it is to buy a
> non-wireless ADSL router these days?

A DSL router with an IP address is a dangerous, dangerous thing. I
wouldn't trust these devices one little bit.

For the diagrams:

My router does support PPPoA, but I don't want to terminate the PPP on
the router. If I use PPPoA, I have to do this: (view using a
fixed-width font)

    [ Internet ]
         |
      DSL line
       PPPoA
         |
  /---------------\
  | 82.133.120.56 |
  |               |
  |  DSL router   |
  | PPPoA client  |
  |               |
  |  82.133.8.9   |
  \---------------/
          |
   ethernet (x-over)
        cable
          |
/---------------------\
|    82.133.8.10      |
|                     |
| Firewall PC (Linux) |
|                     |
|    82.133.8.11      |
\---------------------/
          |
    ethernet cable
          |
         LAN


As you can see, this has wasted 2 IP addresses - the router takes two,
but doesn't need one - I only have 3 usable addresses remaining!
82.133.8.12-14. A bit wasteful considering that I have 7 addresses at my
disposal.

What running PPPoE allows me to do is this:

    [ Internet ]
         |
      DSL line
       PPPoE
         |
  /---------------\
  |               |
  |  DSL router   |
  |  bridging DSL |
  |  to ethernet  |
  |               |
  \---------------/
          |
        PPPoE
     data stream
          |
   ethernet (x-over)
        cable
          |
/---------------------\
|   82.133.120.56     |
|                     |
| Firewall PC (Linux) |
|    PPPoE client     |
|                     |
|    82.133.8.9       |
\---------------------/
          |
    ethernet cable
          |
         LAN

This leaves me with 82.133.8.10 through .14 usable - a whole 5
addresses, surely more than anyone could ever need!

However, because of MTU issues, it is unreliable.

The ideal (full 1500-byte MTU, plus fewer encapsulation layers) would be
to run ATM between the DSL router and the firewall. That would allow an
incoming PPPoA data stream to be bridged onto the ATM (can't bridge
PPPoA onto ethernet) and have a PPPoA client on the firewall, listening
on an ATM interface on the firewall.

-- 
Russell Howe       | Why be just another cog in the machine,
rhowe at siksai.co.uk | when you can be the spanner in the works?
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