Traditionally, network selection process is not something your cellular connectivity provider will allow you to interfere in: MNOs have strict agreements with other operators across the world and whenever your M2M/IoT device happens to reach the areas outside its “home” network, it will connect to the preferred partner of your provider.
However, in case of a global network coverage provided by EMnify, M2M/IoT devices can attach to multiple radio access networks per country. Unlike traditional SIMs, issued by a local mobile operator that only have access to a specific radio network on a national level (its “home” operator), the EMnify SIMs are equipped with Multi-IMSI capabilities to choose the best fitting identity on the best available network. This feature significantly increases the resilience and grants modems the possibility to choose a network with the strongest radio signal.
There are several factors influencing which network devices will attach to, like settings stored in the SIM, and in the modem firmware, configuration of the visited network, home network, device’s current location and its antenna quality and position.
Let’s take a closer look at the network selection process.
Network Selection Procedure
Modems can be configured to work in 2 different modes when selecting the radio network: automatic (unmanaged or steered) and manual. Most commonly, M2M/IoT devices will use the automatic mode, which is also a recommended configuration when using EMnify SIMs.
Automatic Network Selection
When the modem turns on (or following recovery from the lack of coverage) it will perform a network scan to check which radio networks are available at its current location and will try to attach to the network according to the rules defined in 3GPP TS 23.122 standard:
Managed Automatic Network Selection
EMnify’s Mobile Core Network has been designed to manage the network selection from the network side in conjunction with the settings on the SIM: it has a global view on the available radio networks, their supported features, and live network status, temporarily or permanently steering the SIMs away from ‘unhealthy’ networks to the best-performing ones.
Additionally, our Mobile Core Network lets you reconfigure the “Operator Controlled Preference List” over-the-air, maximizing the number of successful attachment attempts. This intervention does not limit the device to join any available networks, it just gives a hint on which network to try first.
Manual Network Selection
For some very specific use cases, there might be a need to let the device select the networks itself.
In this scenario, the device first determines which networks are available by using the AT+COPS=? command (Defined in 3GPP TS 27.007), then the modem sends a list of all available networks to the device. The networks on the list are then sorted according to the same criteria that apply to the automatic network selection.
In the next step, the device sends the AT+COPS command to attach to a specific network. Note that it is also possible that the selected network will reject the attach attempt, therefore some fallback logic should be implemented, allowing the device to attach to an alternative network and restore connectivity.
The Forbidden List
When the device is rejected from joining a network it will receive an error code indicating the reason. In some cases, the modem will add this network to the so-called forbidden list stored on the SIM. When a network is added to that list the device will not try to join it again (only if manual network selection is applied).
However, the network’s coverage and quality are continuously changing, so it is recommended to try to attach to this blocked network later. To overcome this issue, the EMnify SIMs wipe the forbidden list when a blocked network is available again or if the device has lost a network connectivity completely – this ensures that your devices will always stay connected.
Network selection decisions play an important role in the design process of any cellular-enabled M2M use case. Regardless of whether you require a completely automatic network access, easily managed or the manual one – EMnify is your right partner providing maximum flexibility for your business.
by Masha Kovaleva
originally published 29.05.17