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Is a National Roaming SIM an Option for Critical Broadband?

23.09.2021

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

Map Description automatically generated

Roaming SIM

It’s obvious to see that the Roaming SIM performance is terrible. For approximately half the drive the modem is not connected and is searching for a network to connect to. This is an almost continuous 15-minute outage even though we know from the other SIM data that there are at least two perfectly serviceable networks available!

The problem which the phone faces is worsened when it is moving, as it was in this test, and as it would be in a real scenario. The search procedure is slow and cumbersome, and by the time the phone has compiled a list of candidate networks & cells to attach to, it’s moved already, and the picture has changed.

There are other subtleties which don’t work well for a roaming SIM too. For example, because the roaming SIM attaches to a network and will simply stay on that network until the bitter end, i.e., until it loses service completely, this approach compares very unfavourably to a multi-channel approach. The roaming SIM will frequently be in a location where its selected network is not the best network and will be offering a degraded level of service compared to the alternative. This will directly translate to degraded user experience and service experience with certain applications being essentially out of coverage.

Can’t this performance be improved?

Maybe, but not enough, we believe.

Critics of our work here might say that it’s possible to improve the situation by carefully selecting which networks are whitelisted and blacklisted in the roaming SIM card. Maybe certain technologies and bands can be similarly allowed & disallowed, etc. All with the intention of “reducing the search space” for the modem when it needs to find a network to attach to.

We will accept such propositions. The situation may be improvable. Maybe the extremely unacceptable results we have presented here could be improved to simply unacceptable.

But we believe that it cannot be improved to critical grade.

The complex mobility behaviour of a network exists for a reason. The specifiers of the technology standards went to great lengths to create mechanisms to make mobility elegant and seamless; but only within one network.

When transitioning between networks those mechanisms don’t exist, and “elegant and seamless” will not be achieved without a multichannel router.

We know, and our tests show, that National Roaming SIM cards are not a viable alternative to providing Critical Grade Broadband

Goodmill Systems multinetwork broadband solution is designed and built from the ground-up for the needs and expectations of the most demanding critical users.

We are already delivering uptime and reliability benefits to police, fire, ambulance, military, and business-critical users in many countries. This is the future of critical broadband communications.

Goodmill Systems can help you to rollout your mobile broadband services with confidence because we know that for critical users there is no substitute for being "Always Online".