sidney: ...I've read that some DSL modems have a "half-bridge" mode just for this type of situation in which the modem handles the PPoE or PPoA login, then hands everything over to the next device. If your modem could do that you could plug it in to the WAN (Internet) port of the SPA-2102 and run the LAN (ethernet) port out to the rest of your local network.
I think this was where my plan came unstuck. My router doesn't support Half-bridge, only Full-bridge with or without NAT.
sidney: There are two points to this. One is to have all traffic go through the SPA2102, connected to the Internet via the Internet port and to the rest of your local network through the LAN (ethernet) port of the SPA2102. That gives the SPA2102 complete control so it can limit traffic on the LAN when it needs to keep the bandwidth clear for a phone call.
That's exactly how I connected it up, but I think the lack of Half-bridge support was what stuffed it up.
sidney: The other, more minor point, is to avoid having two NATs in series in your system by having the WAN port on the SPA2102 be assigned a public ip address using whatever method you can. If you can't do that, you can try a double-NAT setup anyway and see if anything breaks. You won't be able to forward any ports to the public from any of your computers, but if you don't want to run a server that might make no practical difference.
Unfortunately I can't live with double-NAT because I have VNC servers and an H323 VoIP box which need to be accessible from the internet. I figured that double-NAT was best avoided if possible.
sidney: Oh, I just looked up information about the LinkSys Wag54gp2 that maverick suggested and I see that it includes a PPPoA capable DSL modem as well as two line voip ATA and four port router and wireless access point all in one box. So it would do everything for you including QOS. You could sell your existing boxes on Trademe.
Yeah, I realised at the outset I could have gone for the WAG54GP2 but it was quite a bit more expensive than the SPA2102. For other reasons I need a Full-speed/Full-speed ADSL connection anyway, and the lack of QoS isn't causing us any problems at all. Compared to the approx. 35kbps down/up that VFX uses, we have around 5.5Mbps down and 500kbps up i.e. bandwidth to burn, so I think I'll stick with the existing setup unless any problems arise.
Thanks for posting such a detailed explanation which is better than I have seen anywhere else. Please let me know whether your proposed setup with the SPA2102 actually works in practice, I will be interested to know. And I'll also look forward to your comments on the effects of satellite latency on the VFX call quality.