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POAutomatic 3G modem discovery
 

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  1. POAutomatic 3G modem discovery
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    <p><strong>Background</strong></p> <p>I am currently building a piece of software using Linux &amp; C to establish and monitor a connection with multiple <code>3G</code> modems automatically and simultaneously. This is currently working fairly well.</p> <p>I am reading the <code>/proc/tty/driver/usbserial</code> file to discover all GSM modems. </p> <p>I then have a list of modem device ID's which tells me the interface (ttyUSBX) to use for <code>pppd</code>, and which to use for the AT commands (network name, RSSI etc).</p> <p>A list entry contains;</p> <p><em>Device ID (12d1:1001) ,serial tty (3) ,modem tty (1)</em></p> <p>This means that for device <code>12d1:1001</code> the first <code>ttyUSB</code> associated should be used for the modem, and the third is for the <code>AT</code> commands, leaving the second unused.</p> <p><strong>Question:</strong> What I want to try and do is remove the need for this list. Is there any way to easily discover which interface should be used for AT commands and which for the modems ppp link? </p>
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    1. COI think the usual way to handle this is by having udev rules to discover the hardware and create appropriately-named entries. Keep in mind that some GSM modems are messier than others - they require a mode switch to go into modem mode from "virtual CD" mode. I would just borrow these rules from a popular distro. Alternatively, you can probe the ports with some AT commands.
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      1. USGreg Inozemtsev
    2. COThanks Greg, I am currently looking into udev rules to handle this. In terms of probing ports, I have USB modeswitch set up to handle all the switching, usually after the modeswitch a modem will register with three ttyUSB interfaces. One of which won't work at all with pppd or when sending AT+ commands, which I can detect fairly easily. The remaining two, seem to "work", in that AT commands are fine but pppd gets stuck at the authentication stage. Are there any AT commands that would help me determine if pppd can be used on that interface, that you are familiar with?
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    3. COThe three ports are (most likely) for AT commands, data connection, and GPS receiver. The first two will respond to AT commands, but only one can establish a data link. The reason there are two is so that the application can keep using AT commands (for example, to query signal strength) even when the data link is up. The GPS data is probably in NMEA format, but needs to be started with an AT command. But I can't think of how to distinguish between the data and command ports by probing, sorry.
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      1. USGreg Inozemtsev
 

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