I presume that the main reason phone operators require a SIM card to get access to the network is due to security and/or authentication purposes.

Just wondering why this is? What does a SIM card do that a phone can't? I imagine a SIM card is just data storage right for either some data or some algorithm that generates some data. And at the end of the day, you're just sending the network the right data to authenticate to them you are the owner of the account. Why can't this data just be generated and sent from an app on the phone?

Wouldn't it make more sense? Phone makers won't need to put in a SIM card slot, and phone networks suddenly become much more accessible because users don't need to organize to have a physical thing sent to them for every device they want connected to the network.

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    I think it's mostly because of history and retro-compatibility, so old handsets would still be usable. And also because the inconvenience of a physical SIM card is better at forcing you to buy separate plans for multiple devices, where as an username/password would be shared easily and only requires to turn your current device off and turn the new one on, without swapping anything.
    – user42178
    Feb 20 '15 at 1:04
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    Basically you propose to always have a sim in the phone when you buy it (if you think about it, you can think of such phones as sim free). It exists. To remove the sim card means to still have the data in the phone, it's just a flat design easier to maintain and diagnose for operators/phone manufacturers.
    – Aki
    Feb 20 '15 at 5:21
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    @Aki I don't know what stickman thought, but (from my limited knowledge about SIM cards / mobile networks) I would guess that you could get the identification keys sent via Email / post and load an app of the provider to connect to their network. No need to have a built-in SIM and seemingly no need for physical SIM cards. Mar 6 '15 at 16:18
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    This seems off topic, but has a bounty and cannot be closed.
    – Eric G
    Mar 6 '15 at 16:24
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    @Aki: I don't understand your argument. Smartphones always have persistant storage. It does not need a SIM card for it. Changing the operator without a SIM should work in concept just like logging in to your Google Account with your phone. This isn't a problem either. The advantage of not having a physical SIM card are: You don't need to have that in hardware. This makes the phone more robust, you can use that space for other stuff (e.g. more storage), every phone could have any number of operators (you want 3 different operators? no problem), SIM card can't break. Mar 9 '15 at 8:50

The SIM card, initially, is the root for the legal value. The operator wants to bill the user, and does not want users refusing to pay and claiming that they did not made these three-hours calls to Tanzania. Within the legal context of European countries, where GSM was designed, the SIM card was convenient: it could serve as a basis for legal proofs, under the idea that the SIM card cannot be cloned, and thus if the secret key contained in the SIM card was used then the actual card was used, and the owner could then be formally held responsible for the calls, and pay for it (and three hours to Tanzania imply a non-negligible amount of money).

It is plausible that the SIM card concept was pushed by some French lobbying, the "smart card" being a French speciality (and, at that time, was still patented by some French companies).

Another facet of the issue is that in Europe, you can unlock your phone. Operators are legally mandated to send phone owners an "unlock" code, no more than six months after buying the phone, so that users may then reuse the same hardware phone with another operator. There again, the SIM card is convenient. Compare with SIM-less phones in North America: there is no SIM, but the phone is hardwired (through its firmware) to a specific operator, and you cannot easily (or at all) repurpose the phone to another operator.

The SIM card thus incarnates the separation between the phone hardware and the contract with the operator.

So the "why ?" is, as usual, mostly a matter of History; GSM phones need a SIM card because GSM phones have always needed a SIM card. Yet there are at least semi-rational reasons why GSM phones have SIM cards, and SIM-less phones are not necessarily "better" than phones with SIM.

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    "three hours to Tanzania imply a non-negligible amount of money" - What about for local Tanzanians? I should hope they get some sort of resident discount. ;-)
    – Iszi
    Mar 6 '15 at 18:52
  • There aren't any SIM-less phones in North America anymore, as far as I know. Apr 9 '16 at 6:23
  • @MichaelHampton I haven't bought service in a while, but even just a couple of years ago in North America I had a phone service that used IEMI numbers to activate/deactivate phones-- no SIM card required. While I'm pretty sure that SIM-less phones are on the way out, I believe that most burner phones probably still don't have SIM cards, as they are locked to the carrier and they can just use IEMI numbers to enable/disable phones.
    – phyrfox
    Apr 9 '16 at 12:00
  • @phyrfox You're probably talking about IMEI numbers. And that's just an identifier. It very well may use a SIM card. Apr 10 '16 at 0:11

Be careful not to equate the SIM card with a password (something you know) rather than a token or smartcard (something you have). When you are dealing with a password, you are creating the secret. In the case of the phone connection, you are actually giving the provider back their secret, you do not set it. If you buy a pre-paid sim, often you can just slip it in and you don't need to setup an account; if you were to sign-up through some app or on phone mechanism you would likely need an account and other overhead in many cases.

That said, there is no practical reason to require a sim card. CDMA phones do not require a SIM card, but (per Unlocking 101: Difference Between CDMA and GSM)

GSM phones can be unlocked using a passcode that the manufacturer has (or that can be generated using a keygen similar to a keygen for a hacked cd rom for your computer). Once that passcode is entered the phone will allow the use of any SIM card in the SIM card slot.

CDMA, on the other hand, doesn’t have this unlocking passcode feature since it doesn’t have the GSM standard locking mechanism. Instead, CDMA phones must literally be physically flashed with different firmware that has the new CDMA carrier’s information in it for the phone to access that new network (in other words, to unlock a CDMA phone you need to literally plug it in and use a flashing software to flash the carrier you want to use’s information onto the phone).

You could certainly have a setup similar to wi-fi connections with a captive portal. But the way a cell provider network and a wifi network operate, the segregation of traffic, quality guarantees, etc. are different.

There are also practical reasons to have a sim card. The sim card can store information, contacts, etc. More practically, remember there was a point in time where cell phones didn't have apps, internet connectivity, etc. Even today, to require an app based solution would require a lot of work and brings in potential software issues. The SIM solution seems to be less prone to user error or user frustration. It also simplifies support processes. Probably most important is that this is the way it was, and to implement the changes would be costly for providers to replace all of the infrastructure, train call center and sales reps, etc with minimal overall advantage to most customers, who often don't care. You see the same argument come up frequently for cable TV vs CableCard and set-top-box.

While you state this as a problem:

much more accessible because users don't need to organize to have a physical thing sent to them for every device

there may be others who really like the idea of just slipping out a card and moving it between phones, for some this may make more sense.


I have heard that the SIM cards were encouraged or required by European antitrust concerns.

So individuals had the right to move cellular providers.

You would need to research a bit more from the relevant history to find out or sure.


Back in the day, where on phone memory, capability and widespread usb usage was scarce, the sim card was meant to be a store for various user related data: The imsi, contacts, java card functionality. The idea was to give the user a way to transfer their identity across phones without having to find a way to do it cellularly.

As it stands, over the top services provide a much more compelling way of interacting with data, so much so, the reliance on the other functionality of SIM cards starts to pale in comparison, at the expense of physical data phone portability.

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