I have a cellular data card that connects through ttyS0. I can talk to it via Putty using AT commands, or from Python using /dev/ttyS0. When I enable a ppp0 interface over ttyS0 to communicate with a cellular service provider server, I can talk to the web but I can no longer talk to the device itself. It seems that I have to disable the ppp0 interface in order to resume AT communication with the cellular data card. I'm wondering if I'm running up against some kind of default route or something. There must be a way for me to leave ppp0 up (for a web server running in the background) and still be able to retrieve information (eg. gps data) from the cellular card. I've searched everywhere for help in mapping virtual interfaces to physical interfaces, routing, etc, but I'm no closer to a solution. Has anybody got a suggestion?