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.

SIM800 modem usage
==================

When using sim800 modem, the udev rule is the same as the
sim900 rule :

KERNEL=="ttyS3", ENV{OFONO_DRIVER}="sim900"

Because the SIM800 and SIM900 code are the merged into one driver.