diff --git a/How-to-upgrade-4G-extension-firmware.md b/How-to-upgrade-4G-extension-firmware.md
index c3446b8..5174a9d 100644
--- a/How-to-upgrade-4G-extension-firmware.md
+++ b/How-to-upgrade-4G-extension-firmware.md
@@ -187,4 +187,19 @@ even if you switch id from 9011 to 9001 ,mmcli may still not able to see the 4G
 
 Therefore, there are risks involved in flashing the 4g ext, and it is necessary for you to have a certain understanding of these background information. During the testing process, it is also not possible for me to test whether all SIM cards worldwide can easily connect to the internet through **usb0**.
 
+# Useful AT commands
+
+* check sim card status
+```
+echo -en  "AT+cpin?\r\n" |sudo socat - /dev/ttyUSB2,crnl
++CPIN: READY
+OK
+```
+* check signal quality
+```
+echo -en  "AT+CSQ\r\n" |sudo socat - /dev/ttyUSB2,crnl
++CSQ: 20,99
+
+OK
+```