Is a National Roaming SIM an Option for Critical Broadband?


Goodmill specialises in Critical Broadband solutions. We advocate for multi-radio, multi-modem solutions such as our flagship w24h-S router.

There are other candidate solutions in the marketplace and community, none of which we believe is optimal for professional & critical communications; but one of them we regard as not only sub-optimal, but wholly inadequate – National Roaming SIM cards.

What is a National Roaming SIM?

For those who aren’t familiar, it’s usually the case that within one country, mobile network operators issue SIM cards which prohibit one another’s networks. This makes sense, of course. Network Operator 1 doesn’t want your phone attaching to Network 2 and making calls there; they want you always on their own network. Operator 2 (and 3, and 4, etc.) are therefore prohibited networks in the SIM cards which Operator 1 issues.

When you roam to another country, however, your SIM card is happy to attach to any network it finds and can access.

The National Roaming SIM approach to critical communications is one wherein the government and regulator persuade network operators to issue SIM cards to police, fire, ambulance, etc., which do not prohibit other networks within the country.

The intention is that a router or device equipped with such a SIM card will be able to use any network which is available, anywhere the first responder goes.

At first glance this seems like a great idea, but in fact it’s overly simplistic and ignores crucial nuances of how a mobile radio network functions.

National Roaming SIM is a really bad solution when communication is truly critical

All modern mobile radio networks, including 2G/3G/4G/5G networks, implement complex functionality which is intended to allow the radio (modem, phone, router, etc.) to find, attach to, and traverse the network, maintaining calls when necessary, even as it moves from base station to base station. Generally referred to as mobility functionality, these complex interactions between the phone/modem and network are responsible for making your experience as a user appear seamless and elegant. Switching on, getting service quickly, and traveling down the highway at 120km/h whilst making a call.

You’re not directly aware of it, but your phone is in regular communication with the network even when you’re not using it. The phone and the network are frequently exchanging messages:

  • I’m still here even though I’ve been silent (periodic registration, for the technically inclined)
  • I’m moving to this base station now (cell reselection procedure)
  • I need you to move to this base station (directed/commanded cell reselection)
  • Here’s a list of nearby base stations to check in case you need to move (neighbour cell broadcast)
  • And many others

This kind of ongoing dialogue between your phone and the network enables the phone to stay connected and minimise periods of outage.

One of the most important messages is the “Here’s a list of nearby base stations to check in case you need to move”. This message is crucial to making your phone’s life easier, and without it your phone has a really hard job to do. Sometimes too hard.

At any time, the phone might need to move base station; for example, as you drive and the base station it’s using becomes poor or out of reach. These messages give assistance information to the phone in advance, so that when it needs to move it’s already aware of where it can go next and can make the jump elegantly and seamlessly.

The essential problem with a National Roaming SIM is that these mobility functionalities and dialogues are constrained within a given single network. There is no mechanism for Network 1 to give assistance information about Network 2.

Without this information, when the phone loses coverage, it must go back to square one; it must start a search for a new connection from scratch; bluntly scanning the radio spectrum looking for candidate signals, attempting to synchronise to them, attempting to decode their broadcast channels, attempting to gain access, etc. This is a brute force procedure, and if the SIM card is configured to use any network the search space is huge all the frequency bands, all the technologies, all the networks, etc. It’s very time-consuming.

Test Drive

We wanted to test how time-consuming. Consider the drive results below.

This is a drive test route, driven three times with the same setup, each time with a different SIM card. Network 1 and Network 2 are normal “high street” SIM cards, for two UK mobile networks. They are ‘locked’ to only those networks.
The “Roaming SIM” is just that – a SIM card issued by a foreign mobile operator for use in the UK as a national roamer. It can use any UK network.

The route was chosen because there is a known area of poor coverage for all networks, around the lower half of the drive on the maps below.

The data presented is simple:

  • green - modem is connected to a network
  • red - modem is not connected to any network and is searching

Network #1

Network #2