Steering

When a device is outside its home network, it is roaming. In most countries there is more than one network it could use. Steering is how the SIM’s home operator influences which one that is.

Why operators steer

An operator has roaming agreements with many networks, and they are not all equal. Some cost less. Some support the radio technology a device needs, such as LTE-M or NB-IoT, and some do not. Some are having problems on a given day. Steering lets the home operator send devices to the networks that suit them best.

How it is done

There are two main ways.

• Lists on the SIM. The SIM holds a list of preferred networks for each country. The device tries those first. The home operator can update the list over the air, without anyone touching the device.
• Turning registrations away. When the device tries to join a network the home operator does not want used, the home operator can refuse the registration. The device then tries another network.

No steering and light steering

With no steering, the device picks a network using the standard rules every phone and module follows, and the home operator does not try to change its choice.

With light steering, the SIM carries a preferred list, so the device tries those networks first. If none of them is available it can still use another.

Heavier steering, where registrations are refused, gives the operator more control. It can also mean a device takes longer to connect, because it may be turned away before it is accepted.

What steering means for your devices

Steering affects which networks your devices use, how quickly they connect after moving or restarting, and how they behave when a network fails. The table below shows how each DataSIM SIM profile handles it.

Where the decision is made

Network selection is a function of the device, not the SIM. In automatic mode the device’s non-access stratum follows 3GPP TS 23.122 (how a device chooses a network). It reads lists from the USIM, scans for networks and decides. Steering of roaming (SoR) is any mechanism the home operator uses to influence that decision. The GSMA’s guidelines for it are IR.73, which sets out what a home operator may do and the limits that protect the visited network and the subscriber.

SIM-based steering

The USIM holds the Operator controlled PLMN selector with Access Technology list (EF OPLMNwACT, TS 31.102). In automatic mode, after the home network and the user-controlled list, the device tries the networks on this list in priority order, each with the access technologies it is listed for.

The home operator updates the list over the air, by SMS or by a data bearer to the SIM. A new list does not usually move a device that is already registered straight away. Where the device supports REFRESH in “Steering of Roaming” mode, it can act at once; otherwise it takes effect at the next network selection, for example after a restart, loss of coverage or the periodic search for a higher-priority network.

That periodic search is controlled by a timer the operator sets on the SIM (EF HPPLMN). TS 23.122 allows 6 minutes to 8 hours, with a default of 60 minutes. For devices that only support LTE-M or NB-IoT the range is 2 to 240 hours, with a default of 72 hours, which saves power but means a list change can take days to act on such devices.

Network-based steering

The home operator can also act when the device registers on a visited network. The visited network asks the home network to authorise the registration, and the home network can answer with a rejection. The device then tries another network and the operator accepts it once it reaches one it prefers, or once the device has tried enough times.

The reject cause matters. Some causes, such as PLMN not allowed (cause 11 in TS 24.301), put the network on the SIM’s forbidden list, and the device will not try it again until the list is cleared or a manual registration succeeds. IR.73 v10.0 (8 June 2020) describes which answers have the least impact on the subscriber, and for one of its methods it caps the refusals: after four rejected attempts on a network, the next attempt must be accepted. That covers manual selection and places where the refused network is the only one available.

Each refusal costs time and, for a battery device, energy. A device that is refused several times can take noticeably longer to connect after a restart.

Steering in 5G

In 5G the home network’s UDM can send a steering list to the device during registration, inside the NAS signalling and protected so the visited network cannot change it. TS 23.122 Annex C describes this control-plane SoR. The device updates its lists and can move to a preferred network without being refused first. Control-plane steering is optional for operators, and it can’t be combined with steering by rejection.

No steering, light steering and the rest

The terms are not standardised, so ask what a provider actually does. In general use:

• no steering means no preference list is used and no registration is refused for steering reasons, so the device follows the TS 23.122 order alone;
• light steering means a preferred list on the SIM, updated over the air, with no refusals;
• hard or network steering means registrations on unwanted networks are refused.

Without steering the device still does not simply take the strongest signal. After its last network, its home network and any lists, it tries networks whose signal meets the high-quality threshold in random order, then the rest in order of signal strength. Once registered, it stays until it loses service or a periodic search finds a higher-priority network.

Practical points for IoT

• Use automatic network selection unless you have a specific reason not to. Manual selection fixes the device to one network.
• Watch the forbidden list. A device that has been refused with a permanent cause can avoid a network long after the reason has gone. Clearing it, or a SIM refresh, restores normal behaviour.
• Expect longer connection times where steering refuses registrations, and allow for them in power and retry budgets.
• For LTE-M and NB-IoT, check which networks have roaming for that technology. A network can be preferred for 4G and still offer no LTE-M roaming.