Automatic network selection


we are using EG91 in a lot (tousands) of our devices, and are experiencing some connectivity issues. In order to better understand these issues, I would like to know how automatic operator selection works in these modems. We are using MVNO SIMs by Wireless Logic, the devices are stationary.

  1. Does the modem check the signal strength, and choose the best network, or one network at random?
  2. Does the modem prefer 4G over 3G over 2G networks?
  3. What if a there is a very poor 4G network, and a stable 2G network? Is there a mechanism to decide what network will work better, or is one picked randomly?

Thank you,

In general, the terminal has a built-in preference for network standard selection. For example, 4G terminals have a preference for 4G network, followed by 3G, and then 2G.If you are using a 4G terminal, as long as the signal strength received by the terminal is greater than or equal to Qrxlev_min(configured by the base station side), the terminal will search for 4G cells but not 3G and 2G cells;If the terminal receives signal strength Qrxlev less than Qrxlev_min, the terminal may search 3G or 2G networks.
Terminal selection of corresponding network mode is not random, and is not completely determined by signal intensity.

Hi Herbert,

thanks for the reply.

  1. Does this mean that it should never be necessary to limit the modem to a certain network type?
  2. Could you please share some details about the network selection algorithm?
  3. Is it possible that a SIM “locks on to” (remembers) the network it is connected to, and once this network disappears, the modem refuses to automatically select a different one (until AT+COPS=0 is sent to it)?



  1. In principle, network types should not be restricted unless there are special requirements, such as locking a specific network type by locking frequency band, frequency point, or network mode.
  2. For details, please refer to 3GPP:
  3. The SIM card provides PLMN, authentication and encryption for terminals to select networks.The terminal selects the corresponding network operator according to the PLMN, which can be interpreted as locking a certain network,If the current network mode with the best priority is not suitable, such as 4G, the terminal will successively select the low-priority network mode in the same PLMN until no suitable network can be supported in PLMN.and then the terminal will enter the OOS(out of search) mode.

Hi Herbert,

thank you, this helps quite a bit.

Regarding PLMN selection:
We are using multi-IMSI SIM cards, so there are a number of selectible PLMNs per country and thus, as far as I understnd it, the SIM will return a list of PLMNs to the modem.

  1. Will the modem then only try one of those PLMNs, and enter OOS without trying the other PLMNs?
  2. How can we detect OOS? Will AT+CREG? return x,0? What will the return value of AT+COPS? be?
  3. Does the modem restart search on its own, or do we need to trigger that, e.g. using AT+COPS=0 ?

Thanks again for your help, much appreciated!


1.The terminal can search the network based on the PLMNS list. PLMNS that are not listed in non-emergency calls will not be searched.
2.Terminals may enter OOS(out of service) in cases of extremely weak signal strength, such as shielded rooms.“AT+CXREG” indicates network registration and has nothing to do with PLMN search.
3.You can use the AT+COPS=0 to re-search.

Ok, but what AT commands can we use to detect OOS?

No corresponding AT Command was available to query for OOS