Initial Attachment

In LPWA (Low Power Wide Area Network) type modems, but especially with NB-IoT modems, the network search functionality does not work as vigorously as regular LTE modems like the ones we have in our mobile phones. This is understandably due to the main concern topic of such modems:- power saving to enable long battery life. So, delays may occur in LPWA modems that are using LTE-M or NB-IoT radio access types. This occurs especially during the very first connection. After a connection is made, the last used network is stored in the SIM card and during the next boot up, the modem reads this information from the SIM card and attempts to attach to the same network again. That is why successive attachments are faster than the initial attachment.

For this reason, we normally ask the reset timer implemented in the device firmware to be at least around 15 minutes (or even more ideally). This makes sure the device firmware gives enough time to the cellular modem to search for networks and attach to one.

There are some other practices to speed up network searching in LPWA modems. These practices are implemented by AT commands sent from the device firmware to the cellular modem:

1. Cancelling Home PLMN (HPLMN) search
Generally, the modems are configured to search for the Home PLMN first as default. This is to make sure that the modem attaches to the home network of the SIM card. MVNOs, like emnify, do not have a home PLMN. So, this initial search is a waste of time with an emnify SIM. Therefore, we suggest switching off this functionality whenever it is possible to do so with the cellular modem used.

2. Reducing the number of bands to search.
Instead of making the modem search for all frequency bands that the module supports, it is a good practice to limit this configuration to the bands used at the location of the IoT device.

3. Enabling required RATs only.
It is a good practice to turn off the Radio Access Types (RATs) that are not going to be used. For instance, if you know that you will use NB-IoT only and your cellular modem supports LTE-M and GSM in addition to NB-IoT, it may be a good idea to enable NB-IoT only and restrict LTE-M and GSM. Or alternatively, if you need to use LTE-M and GSM only, it will be good to leave NB-IoT out.

CS/PS Attachment Mode

With cellular networks evolving into all-IP based networks, the need for registering to PSTN network via Circuit Switch (CS) attachment is diminishing. Hence, we see the network operators not using VLR component anymore. We see this trend in 4G LTE networks and especially in Low Power Wide Area Network (LPWAN) type connections like LTE-M and NB-IoT. The important type of network attachment is the Packet Switch (PS) attachment to the MME for such networks.

The cellular modems configured to do a CS/PS Combined Attach attempting to attach such networks without a VLR may receive a rejection for the CS attachment attempt. In such cases, the IoT device firmware should not consider this rejection of the CS Attachment attempt as a failure, and it should not take recovery actions like resetting the modem. This rejection to the CS Attachment should be omitted as long as a successful PS Attachment with the network is established.

The below screenshot is from an actual test where Quectel BG96 is attached to an NB-IoT network in Germany where VLR is not available in the network. Here, we see CS attachment requests are being rejected (code 3 - Registration denied) by the network and the PS attachment to MME is successful (code 5 - Registered, roaming). 

However, with some other networks, we see successful response to CS attachment in Combined Attach mode in NB-IoT connections. So, the device firmware should be able to handle these various conditions as mentioned above.