It is preferred to use LTE Cat. NB1 over GSM, because the power consumption is lower. If NB1 is unavailable then we are forced to use GSM but we want to return to NB1 as soon as it is available again. Is there a recommended way to implement this fallback mechanism?
I use the following commands to force the BG96 to search for LTE Cat. NB1 only:
at+qcfg=“nwscanseq”,03 // Set scan sequence to NB1->M1->GSM
at+qcfg=“nwscanmode”,3 // Scan LTE only (not GSM)
at+qcfg=“iotopmode”,1 // Search LTE Cat NB1 only (not LTE Cat M1)
at+qcfg=“band”,0,0,80 // Specify LTE Cat NB1 frequency band 0x80 (LTE B8)
The BG96 takes a very long time to search and register on the NB1 network but then it saves the registration details in non-volatile memory and subsequent registration attempts are quick.
If the NB1 network becomes unavailable then I want the module to fall back to GSM. Is there a good recommended procedure to do this? From observation it looks like the BG96 saves the fact that it registered on GSM in non-volatile memory and subsequent registration attempts will immediately go for GSM and not LTE Cat. NB1.
Is there a way to check (say every 60 minutes) that the NB1 network is available again and return?
If I use the commands above to force the BG96 back to NB1 then it start the whole search procedure from scratch and it takes a very long time. How can I tell the BG96 to use the previous NB1 network operator details for a fast registration?
If I understand correctly, you want to use both GSM and NB network, use NB first, if NB doesn’t work, use GSM, when NB works, then use NB, right?
If so,pls try the following configuration.
1-you need to first set the network search mode to auto. AT+QCFG="scanmode",0,1 //Automatic,
2-then set the network search sequence to NB-GSM-CatM AT+QCFG=“nwscanseq",030102,1 // NB-GSM-CatM
3,LTE network search mode configuration AT+QCFG="iotopmode",1,1 //LTE Cat.NB1 only
4,Band Configuration
According to the operators you are using to configure the actual situation.
5.at+cfun=0 and at +cfun=1 to make the configuration take effect.
Note.
The three network formats(NB,CATM,GSM) supported by BG96 are independent of each other, and there will be no redirection / fallback between network formats.
The module will not switch actively. It needs the information and permission from the network side before the module will perform the action. It is suggested to consult the operator.
If you select at+ QCFG = “nwscanmode”,0,1
1.It may be determined by the priority of the SIM card, such as 0: GSM:, 1: LTE
2.It may also be related to the signal strength of NB network.If the cell signal is below the intervention threshold, it will result in unsuccessful registration.
In this case we need log to analyze,please send email to support@quectel.com to get the support from local FAE, you will get the log tool and they will help to analyze the reason. Thanks!
Q1-Is there an AT command to query the SIM card and find out what the priority is?
Sorry,there is no such AT command yet, but it can be found in the log, and you will need to use a special catch log tool.
Q2-If I use AT+QCSQ what must the signal quality be for successful registration?
Please refer to the following information.
2G refer to rssi
4G refer to rsrp
<gsm_rssi>, <lte_rssi>
0 to 99, corresponding to -120 to 0 dBm
0 -113 dBm or less
1 -111 dBm
2…30 -109…-53 dBm
31 -51 dBm or greater
99 Not known or not detectable
<lte_rsrp> (Reference Signal Received Power)
Chapter 9.1.4: The reporting range of RSRP is defined from -140 dBm to – 44 dBm with 1 dB resolution
For Cat M, we can Refer to the RSRP and set the signal bar as below:
rsrp: -90 dBm good
rsrp:-100 dBm Normal
rsrp:-110 dBm little bad
rsrp:-120 dBm very bad
<lte_sinr> (Signal Input to Noise Ratio)
Limits: 0~250(corresponds to -20 to 30 dB)
SINR is a measure of signal quality as well but it is not defined in the 3GPP specs but defined by the UE vendor.
It is not reported to the network. SINR is used a lot by operators, and the LTE industry in general, as it better quantifies the relationship between RF conditions and Throughput. LTE UEs typically use SINR to calculate the CQI (Channel Quality Indicator) they report to the network.
Calculation of sinr: SNR=(*2-200)/10
SNR: 0 not very good, some interference
SNR:-5 bad, quite big interference
SNR:-8 very bad, big interference
<lte_rsrq> (Reference Signal Received Quality)
Limits : -20 dBm to -3 dBm
Apologies for bumping an old thread but I finally got a better understanding of this process.
Slides 7 and 8 provides information on RAT selection and re-selection. This helped clear some of my understandings of what the module is doing. Although not specified to which module I assumed BG96/95 to have been used.