SIM900 modem usage
===================

To enable SIM900 module support you need to put the following
udev rule into appropriate file in /{etc,lib}/udev/rules.d:

KERNEL=="gsmtty3", ENV{OFONO_DRIVER}="sim900"

On the i-Tetra tracking device, the SIM900 is accessed
via N_GSM mux device. We use ofono as SMS message
service and incoming voice calls service, so we
use /dev/gsmtty1 provided by N_GSM mux.