Questions[modem](server)

I have an internet connected Ubuntu server, connected by Ethernet to a residential ADSL line. This is adequate, most of the time, but I can't rely upon the ADSL link to be as reliable as I'd like. Murphy's law has already dictated that down-time comes when it is least convenient.

I'd like to use a 'pay-as-you-go' 3G USB dongle to provide a fail-over. I'm lacking information both about sample configurations that work for other people - and about the most suitable (inexpensive) hardware for my purposes. The objective is that, when my ADSL line is disconnected, the server detects this and dials up the 3G service - monitoring the ADSL line and switching back seamlessly when its connection is restored.

The principle service I care about is an OpenVPN tunnel - over UDP to a remote server. The other useful service would be Squid - so I'd still have web-access from my LAN when ADSL is disconnected.

Questions:

1. How difficult would this be to set up?
2. Would I be forced to 'roll my own' - or are there packages/howtos I've overlooked?
3. What (inexpensive) hardware is recommended (concerned with unattended reliability - mainly)?
4. Any anecdotes of success or failure from others who've tackled this already?

there's a computer ("server") that has no Internet connection, and it is located in a place with no GSM signal. The only way to get connection to it is an analog modem (US Robotics 56Kbit/s). This modem is connected to a national PSTN line with a geographical number, and the server (running a modern Linux distribution) is configured to offer a dial-in serial terminal. To clarify, when you phone to this PSTN line, the modem handshakes (I think it's the V90 protocol), then you're presented with the login and then you are in on ttyS0. There's no IP/PPP protocol stack, just the serial terminal.

I would like to connect to this server in any way that does not require me to carry around a physical analog modem and (what's worse) finding around a physical PSTN line which is really a scarce resource at least in business context here in Italy. To clarify: the PSTN line on the server side is the only way through, but I would like to be able to connect to it without a physical PSTN line on the client side.

Some ideas that came through my mind but I do not know if they can be really implemented or not:

• Having a mobile phone connected through bluetooth/USB. Old phones exported a ttyUSB that handled AT commands, but then there was no way (AFAICT) to get the audio stream. I don't know if it's technically possible to write an Android app that makes a phone call, then encodes the audio into a semi-raw UDP stream.

• Routing the call through VOIP. Googling around, some people seems to have had mixed success with doing analog modem calls through a VOIP termination with g711 (uLaw/aLaw). It looks like it is possible to get at least 9600bps, which is enough for me.

• Even if one of the above is possible, is there any software available that emulates the full V90 protocol/handshake/whatever and en/decodes audio into bytes?

• Is there any online service that offers a battery of analog modems to subscribers? Doesn't sound a business I would personally invest on, but you never know :)

• Is there any physical object that is the modern UMTS equivalent of an analog modem, like a box connected through USB where you put the SIM inside, it does all the magic, and gives you a ttyUSB on your PC?

• Assuming that I can physically access the server just once, is there anything I can install (either hardware or software) on the server and that would make my life easier? The only requisite is that must go through a standard national PSTN line (dual twisted copper), there's no other way to get any other signal. The modem looked like the best option, at first, but you never know.

Thanks!

I've got an interesting one for you. I've been going over my server monitoring and notification systems (Nagios based), and realized that if our internet connection goes down, there's no way for it to notify me. I already have a modem listening (Via CentOS 5) on a spare POTS line so that I can dial-in in case our internet goes down. I was wondering if I could come up with a script (Shell, Python, etc) that can dial out and play a recorded message (wave file I'm guessing) when it's picked up. I know Windows supports voice calls over a voice modem, I was wondering if a solution existed for Linux...

I know asterisk can probably do it, but isn't that overkill (A full blown VOIP system just for a notification mechanism that will hopefully never be used)? And wouldn't it interfere with the modem's primary function as a backup network interface (PPP spawned via mgetty)?

I've done some searching, and haven't really come up with much. I know how to dial out from the command line, but only as a modem (not as voice). Worst case, I could set it up to dial out as a modem, and then just realize that if I get a call with modem sounds from that number that it's the notification... Any insight would be appreciated...

I'm not sure specifically the differences between a normal Dial-up modem and an ADSL one.

Here is my problem:

I recently found an old box in the basement, which I installed Ubuntu (linux) on. I have both a NIC, and an old dial-up modem on it.

My modem that I connect to the internet with was provided by my ISP when I first joined their service, but lately it has been overheating and I've been having my connection dropped.

Seeing as how I have this linux box with a modem, I was wondering if it was possible that I could connect to my ISP using this dial-up modem.

Here's an example:

Currently: Wall (phone cord) -> ADSL Modem -> Router -> PC, PC, PC

Theoretically: Wall (phone cord) -> Linux box (as modem AND router) -> PC, PC, PC

What's the difference between a switch, a router, and a modem?