Table of Contents:

• 1. Issue Output
• 2. What is Signal Strenght?
• 3. What to check?
  • 3.1 Check whether module detect the SIM card?
  • 3.2 Whether you use a NB-IoT SIM card that the module supports
  • 3.3 Whether the antenna connection is good
  • 3.4 Check Software Settings
    • 3.4.1 Extend attach time
    • 3.4.2 Lock Frequency Band

1. Issue Output

NB-IoT module can't connect to NB-IoT Network and Keep Showing Signal Strenght:99. Below is an example output:

2. What is Signal Strenght?

Signal Strength is same as AT+CSQ command output. When you run AT+CSQ and NB-IoT module return AT+CSQ=99,99. It means NB-IoT Sensor doesn't attached to the NB-IoT network. If attached to network, the first 99 should be 2~31.

CSQ Definition

3. What to check?

3.1 Check whether module detect the SIM card?

You can see if the output shows the IMSI number. If NB-IoT module detects SIM card successful, There will be a IMSI number shown, this is from the SIM card. If SIM card not detect, please check:

• If SIM card is placed in correct direction.

• Make sure there is power off / power on after insert SIM card.

3.2 Whether you use a NB-IoT SIM card that the module supports

• Make sure the SIM card you use is a NB-IoT SIM card. It is different from normal SIM card we use for mobile.

• If you use a NB-IoT SIM card, check whether the Frequency Band of this operator are in the support list of the NB-IoT Sensor.

3.3 Whether the antenna connection is good

See  the connection of Antenna Path is loose.

3.4 Check Software Settings

In the NB-IoT Sensor which power by battery. The sensor will try to connect to NB-IoT in a specify time (For example 5 minutes), if sensor is not able to attach network in 5 minutes. Sensor will stop retry and put the NB-IoT module in sleep mode to save battery life. So in the case that attaching network needs more time than 5 minutes, it will cause module hard to attach NB-IoT network. There are two solution to solve this.

3.4.1 Extend attach time

User can use AT Command to extend network acquisition time.For example AT+CSQTIME=10 will extend the network acquisition time to 10 minutes.

3.4.2 Lock Frequency Band

NB-IoT module by default support multi-bands, and it will try to use these bands to attached network one by one. User can lock the frequency band to the SIM card band to speed up the process.

See how to lock frequency band.