Unwanted carrier switching

Hi, we are using the HL76xx with a SierraWireless roaming SIM and the modem seems to be switching between carriers. It appears this occurs when the signal is weak. This carrier switching causes a ppp session restart which causes issues with our application. Is there a way to reduce the propensity to change carrier and remain on the same carrier?

how about using AT+COPS to manual select the operator?

Thanks. That could be an option, but that means operator/carrier connection management is done by the application, I’d expect the HL modems to have some smarts built-in that will manage this for us and that is configurable.

If we were able to us AT+COPS, is there a way to detect the carrier or cell tower with the strongest signal?

have you tried +CPOL for choosing preferred PLMN list?

Didn’t you can scan the cell by +KCELL?


Its a pretty complicated question (even though it seems simple).

So I presume you are using a Sierra Smart SIM? If that is the case then this is going to be normal operation which is highly dependent on scenario. The SIM will essentially use the best network is can see balancing off a number of factors, pretty sure signal strength is not among them. Is it just low signal or does it actually lose the network even if briefly? If so then the SIM probably takes the opportunity to select what it views as a better network, which will obviously require the application restart the data session.

As I say its pretty complicated and highly scenario dependent.

Re the COPS question, COPS has to be set to 0 to allow any SIM/module firmware to manage the network selection so no you cannot set it to automatic with additional modifiers such as ‘I want the strongest signal’. If its auto then the cell selection is done by the module firmware under the instruction of the SIM network selection.



Hi Mark, thanks for your answer.

For the HL85xx we are using Smart SIM and for the HL76xx we are using Easy SIM. Would these behave differently in such scenarios?

In a nutshell we want to understand how the SierraWireless modems work and how to optimise them for our requirements. We have relatively low bandwidth, ~10MB per month, but sending data at least every minute. Our modems are on boats, so can travel. Is there guidance documentation or whitepapers (I can’t find any) on how to optimise for such scenarios? Or how the SierraWireless modem modules behave?

Its a pretty complicated question (even though it seems simple). :wink:

Our customers using these modems are now global and experiencing re-connection issues and periods of disconnection. One current example in the US is that the device with a HL76xx modem is oscillating between AT&T and T-Mobile in its current location. When on T-Mobile it doesn’t seem to be performing as reliably as on AT&T. Why the device is deciding to use T-Mobile over AT&T is currently unclear, and we would like to understand why. We are diagnosing these problems remotely and making some assumptions.

We are currently using AT+COPS=0 and I would like to avoid managing the network ourselves, the modem should do that.


In terms of cell selection and firmware behaviour the ‘white paper’ are the 3GPP specs, which are complicated and not easy to read. There isn’t a ‘noddy’s guide’ because there are so many external factors and scenario’s.

With regards the question about the easy and smart SIM, technically yes they might behave differently for a given scenario, as the Smart SIM applies some intelligence through its Java applet and the easy SIM relies entirely on the modem selection so is a bit random (although that is simplifying it a bit).

Bottom line is that the modem and SIM’s raison d’etre is to be attached to a network and with the test SIM the best possible network so as an application it is best to leave them to it as they are designed to do it as fast as possible.