|****** Moderator's Note ***** | |Please explain what "G.711u" means, and why you chose that option. I'd |also appreciate you telling us how much it cost.
Others have covered G.711; I'll add only that my application is mostly internal and even 10Mb/s Ethernet has plenty of capacity. Until very recently my DSL was 768k/128k which is marginal for a single G.711 stream. Now that I have a whopping 1M/384k I may do more with external VOIP.
The short answer to the cost question is that I spent way too much money and an absurd amount of time getting things working to my liking. You may recall I posted about some replacement firmware I wrote for a multi-ring adapter. This was one of several significant software exercises that were part of the effort.
Asterisk is of course free and I already had a machine on which to run it. There were some bugs (one related to directed call pickup and the others to the Festival speech synthesizer interface) that took me a while to fix owing somewhat to my lack of familiarity with Asterisk's structure. This was a rather disconcerting process because I found references and voodoo workarounds for the symptoms of these bugs going back years. I hope I was just unlucky to hit some unresolved problems and this won't be an ongoing maintenance thing.
I decided to use my existing Cisco routers for POTS ports since I was already familiar with the management interface. Initially I tried the NM-HDA-4FXS module in my Cisco which (as the name implies) gives you 4 FXS ports. It also accepts add-in cards which can give you 8 FXO ports. I wanted 5 FXO ports so this seemed like a good deal with populated cards at $200-$300 on eBay.
Unfortunately, the NM-HDA FXO ports have two serious flaws. One flaw is that when the router reboots the ports busy out the line until the router comes back up. This is a bit of a safety concern and, of course, the router may never come back up. I was able to "fix" this problem by lifting the pins of the IC that was being used to busy the line. It was busying as if it was a ground-start line so my modification did not affect loop-start operation. The other flaw is that CID reception is very unreliable. There seem to be both hardware issues with signal level and firmware timing problems that are unlikely to be fixed since there are newer interfaces available.
Having invested a fair bit of time in learning Cisco VOIP configuration I decided to try a newer generation interface: VIC2-4FXO (~$150) cards in NM-HD-2VE (~$100) carriers. These seem to work fine. The related FXS interface (VIC-4FXS/DID) is ~$200. In retrospect I could probably have done better with non-Cisco ports, but now that everything is working...
Dan Lanciani ddl@danlan.*com