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Comment by jeroenhd | Hacker News Reader

SIM cards don't connect to networks, the phone modem can just disable support for such protocols. That'd probably be illegal, though, in case you're trying to call emergency services and don't have 5G reception.

Some Android phones have a setting to at least disable 2G and you can easily configure them to a "preference" of only 5G. I believe iPhones have a 2G toggle as well if you enable lockdown mode.

It'll be years before you can reliably get rid of 4G without losing coverage, though.

I don't know about any such settings on mobile platforms such as watches, though. I also doubt cars have a setting for this (maybe if you use one of those Chinese Android-tablet-with-a-car-skin systems?).

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> SIM cards don't connect to networks

SIM cards have hundreds of various configuration knobs influencing what a (compliant) baseband does, so I wouldn’t be surprised if there was one that does just that.

That said, some knobs are frustratingly missing, though – why is manually entering an APN a thing, but the default SMSC can be stored on the SIM?

That's true, of course, but SIMs can be reprogrammed by the carrier on a whim. Plus, there are handover features that command the modem to downgrade the connection from the network side, and who knows if the modem will listen to the SIM's config if the network commands it to do something.

I haven't needed to enter APNs in years, there are standards to provision those by SMS if they're missing and most of them are pre-configured in the phone's OS.

I think limiting this at the modem side will be more effective than reprogramming the SIM card, but the specifications are open enough that you could take a look at a SIM's contents by throwing it in a reader.

You could also look at the code and blobs dealing with eSIMs, as they provide the same features but often come packaged in the form of software.

Check your local laws before you start messing with SIM cards, though, altering certain identifiers can be a crime.

In terms of existing examples, there's a few equivalent (or at least similar) fields defined as SIM files - for example, the FPLMN (forbidden PLMN) list of networks your phone shouldn't attempt to attach to.

You're right that this needs limited at the modem - but the main user accessible method of configuring the modem is the phone UI. As this setting is one which needs network support, and is likely to disconnect a user who misconfigured this, a SIM file for permitted RAT (radio access technology) types would make sense, as SIM files are under the responsibility of the operator.

Where this would get complex is edge cases, like under roaming scenarios, where your home network can't predict what might be available, and your handset may need to permit downgrading to a technology not permitted on the home network.

The toggle in Android to disable 2G seems a start towards a user accessible setting for this, which selects what the modem is willing to join, but it's certainly far from a user friendly way to enable and disable particular technologies.

> Check your local laws before you start messing with SIM cards, though, altering certain identifiers can be a crime.

Generally the contents of specific important Elementary Files (EF) are protected by requiring you to have an ADM code to read/write.

> I haven't needed to enter APNs in years, there are standards to provision those by SMS if they're missing and most of them are pre-configured in the phone's OS.

You might need to enter an APN if you have a B2B contract with the operator, where they'll route all traffic from your device(s) through a VPN directly to you. Besides that and static addresses, I am not aware of any other prevalent use-case for changing an APN.

> SIM cards have hundreds of various configuration knobs influencing what a (compliant) baseband does, so I wouldn’t be surprised if there was one that does just that.

There is EF-UST (USIM Service Table) but it doesn't explicitly allow/deny radio access technologies.

The wording your usage here seems to suggest that the phones can be configured to not connect to 2G networks. This is false if you live in the USA. The phone will not connect to 2G networks regardless of any setting. There have not been any to connect to for a while now. The only thing out there that is 2G any longer is malicious actors.

It should come as no small surprise that phones in the US markets ship with a feature that is a de-facto backdoor.

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