diff --git a/doc/sim900-modem.txt b/doc/simcom-modem.txt
similarity index 64%
rename from doc/sim900-modem.txt
rename to doc/simcom-modem.txt
index e9f051c3..0a813075 100644
--- a/doc/sim900-modem.txt
+++ b/doc/simcom-modem.txt
@@ -10,3 +10,13 @@ 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.