This article examines PLMN selection process using Vodafone NB IoT to hi-light some challenges to getting service especially for first time power-on, to attach to network can take 10’s of minutes.
See the Quectel_BG95&BG77_Network_Searching_Scheme_Introduction_V1.0 for details of algorithm:
A lot of the time this is the simplified procedure for the 3GPP rules for 2-stage cell selection/PLMN selection procedure.
- The higher EMM layer protocol selects an PLMN to search for, then the lower RRC/radio layers finds a cell for that PLMN (cell selection).
- The cell selection is outlined in the document above, just searching each enabled band for an NB IoT cell.
The cell selection will search each enabled band to find any instance of that PLMN. This is the part that take a long time for NB IoT compared to CAT.M1 or LTE, and can take minutes depending on how many bands are enabled by the application.
Generally speaking the automatic mode EMM PLMN selection procedure can be summarised(ref. 3GPP 23.122 in particular 18.104.22.168.1.):
- Check if registered PLMN (RPLMN) information has been stored from previous attach. If so then search for that RPLMN using stored band/EARFCN information. This is what should happen most of the time to optimise power-on to attach time.
- Search for HPLMN-EHPLMN (home network or equivalent). This home PLMN can be seem from first 5-6 digits of the IMSI. In Vodafone NB IoT case this is “90128” which is some generic international country/network, so this cell selection will always fail to find a cell. This part takes the majority of the time in Vodafone case. For this reason the attached document recommends restricting the list of enabled bands as much as possible.
- Search for network listed in SIM list for UPLMN, OPLMN (user or operator preferred network list).
- Try to attach to any other detected network (roaming), at some random point the module will try network “50503” and get attach success!
This is why automatic network selection mode on Vodafone NB NB IoT takes many minutes for the first time attach.
Manual selection (AT+COPS=1,2,”50503″) bypasses all this and directly searches for “50503”. A work-around is if you are able to add “50503” into the SIM file that is OPLMN – operator preferred list.