今天(existence today)几乎所有的蜂窝电话都使用SIM 卡(SIM card)。“SIM”是用户身份模块的缩写。(Subscriber Identity Module.)没有它，您将无法拨打或接听电话。您也不能通过蜂窝连接使用任何互联网数据。
您可能已经知道这一点，但是为什么(why )没有这小块金属和塑料(metal and plastic)，您的手机会如此无用？SIM(Just)卡(SIM card)到底是干什么用的？
SIM卡长什么样(What a SIM Card Looks Like)
SIM卡(SIM Card)很容易识别。这是一张带有一组独特金属触点的塑料卡。在SIM 卡(SIM card)内，您会发现一个集成电路，它通过上述金属触点与手机进行通信。
SIM卡有多种形状和尺寸。事实上，总共有四种尺寸。最大的被称为“全尺寸”，实际上已经过时了。全尺寸SIM卡的尺寸为 3.37 英寸 x 2.125 英寸，基本上与智能门禁卡相当。
原因很有趣。在移动电话技术(phone technology)的初期，多人共享一部电话。这个想法是，当你想使用它时，你可以将巨大的SIM 卡插入手机。(SIM card)由于卡片在钱包中的时间比在手机中的时间长，因此大塑料机身(plastic body)非常合理。
如今，您购买的大多数 SIM 卡都具有所有三种常见尺寸，塑料主体(plastic body)的碎片会脱落，直到您达到您想要的尺寸。如今，Mini-SIM 尺寸相当少见，主要用于传统设备或基于旧设计的低成本设备。
Micro-SIM 尺寸(Micro-SIM size)非常流行，大多数主流手机都使用它。Nano - SIM尺寸几乎消除了SIM触点周围的所有塑料机身(plastic body)，基本上是旗舰和中高端智能手机的标准，这些智能手机的内部空间非常宝贵。SIM卡是物理键控的，因此不可能以错误的方式插入它们。
这里有一个提示：当你拿到一张新的SIM 卡(SIM card)时，不要扔掉 Mini- 和 Micro- SIM 卡(SIM)框。如果您需要将SIM移动到另一部手机，您可能仍然需要它们！
SIM卡有什么作用？(What Does a SIM Card Do?)
您的服务提供商(service provider)拥有的蜂窝网络就是一个网络。他们需要控制谁可以访问该网络，既是出于安全原因，也是为了使整个工作正常(thing work)进行！
那么什么是SIM卡(SIM card)呢？您的SIM 卡(SIM card)是一张访问卡(access card)。它包含您的凭据。所以从这个意义上说，它就像一张存储您的用户名和密码(username and password)的卡片。您使用SIM 卡(SIM card)“登录”到蜂窝网络。您的电话号码(phone number)是您唯一的网络身份(network identity)。
无论您走到哪里，只要您在属于您的服务提供商(service provider)的蜂窝塔的范围内，就可以联系到您并联系到其他人。
SIM卡上存储了什么？(What’s Stored On a SIM?)
SIM 卡(SIM card)上有网络需要验证您的帐户的信息。其中之一是ICCID或集成电路卡标识符(Integrated Circuit Card Identifier)。这是特定物理SIM 卡(SIM card)独有的数字。
SIM 卡(SIM)上的下一个重要数据位称为身份验证密钥(authentication key)。这是用于对发送到蜂窝网络的数据进行签名的加密密钥。他们的密钥通过加密函数运行，然后与使用存储在运营商数据库中的密钥副本完成的另一个计算进行比较。如果它们匹配，您就可以进入网络。
SIM 卡(SIM)上还有内存，您可以使用它来存储您的联系人和短信。然而，这是非常有限的，大多数SIM 卡(SIMs)的空间在 64k 到 256k 之间。如今，大多数人最好将他们的联系人存储在手机自己的内存中，并使用iCloud或他们的 Google 帐户等备份。
合同和预付 SIM 卡(Contract and Prepaid SIM Cards)
蜂窝提供商通常使用两种模型向客户销售其服务。您可以签订固定合同协议(contract agreement)，也可以成为预付费客户。
在这两种情况下，您都会获得一张SIM 卡(SIM card)。卡片本身并没有什么不同。相反，提供者如何处理使它们与众不同的帐户。您可以购买现成的预付费SIM 卡。(SIM card)尽管在许多国家/地区，您必须使用政府身份证和居住证明进行购买登记。(government ID and proof)
合同 SIM(Contract SIM)卡根据您的合同分配数据、通话时间和其他资源。(talk time)超额费用会添加到您的帐单中，并在结算周期(billing cycle)结束时从您的帐户中扣除。使用预付卡，您通常会在SIM 卡(SIM)上加载货币金额，或者预先购买通话时间或移动数据。(purchase talk time)
根据您居住的地方，某些合约手机可能仅被锁定到该提供商的SIM 卡。(SIM card)通常这是一种以大幅折扣购买手机的方法，但如果您需要使用另一张SIM 卡(SIM card)，也许在旅行时，您会发现它不起作用！为避免这种情况，请务必寻找以“解锁(unlocked)”形式出售的手机。
切换 SIM 卡(Switching SIM Cards)
从一部手机中取出 SIM 卡并将其移至另一部手机非常轻松。鉴于两部手机使用相同的SIM 卡(SIM card)尺寸。如果另一部手机使用更大的尺寸，则必须使用适配器使其适合更大的插槽。如果您要使用的SIM 卡对于(SIM card)目标手机(target phone)来说太大，您有两种选择。
首先是将SIM缩小到合适的尺寸。有些人很勇敢，只用一把剪刀和某种模板就可以做到这一点。如果您不小心切入了SIM 卡(SIM card)的电路，游戏就结束了，您需要一张新的。最安全的方法是使用专用的SIM切割工具。即便如此，您也冒着毁坏卡片的风险，所以走这条路只有您自己承担风险。
另一种选择是切换SIM 卡(SIM card)。当然，如果你刚出去买了一张新的SIM 卡(SIM card)，它会关联一个不同的电话号码！(phone number)每个服务提供商(service provider)可能有不同的程序将您的电话号码(phone number)从一张SIM 卡(SIM)转移到另一张 SIM 卡。
但是，一般来说，它应该只需要一个电话并验证(phone call and verification)您的身份。如果提供商有实体店，您甚至应该可以选择购买新SIM 卡(SIM)，同时禁用旧 SIM 卡。
如今，使用发送到特定SIM(SIM)卡的短信作为双因素身份验证的形式很流行。所以现在我们有一种被称为SIM 交换欺诈的(SIM-swap fraud)东西。犯罪分子想方设法将您的电话号码(phone number)转移到他们控制的SIM 卡上。(SIM)SIM复制(SIM duplication)是另一种方法。这当然是需要注意的事情。
SIM卡后的生活(Life After SIM Cards)
SIM卡，至少是离散的SIM卡，不再需要了。我们已经看到eSIM或嵌入式 SIM(embedded SIM )卡的兴起。这是直接内置在手机中的SIM 卡。(SIM card)此卡是可编程的，这意味着您可以轻松更改提供商或一次使用多个提供商。
在一些国家，“双卡”手机非常流行。有了这些，您可以拥有两个电话号码，例如，一个供个人使用。您还可以拥有一张专用的数据SIM 卡(SIM card)，价格可能更优惠。
eSIM 使这一点变得多余，并将访问和身份验证(access and authentication)转变为类似于固网宽带使用(broadband usage)的东西。您只需输入用户名和密码(username and password)即可访问给定的服务提供商(service provider)。
在撰写本文时，目前很少有手机嵌入SIM 卡(SIM card)，但随着行业采用这一新标准，这种情况将发生变化。你的下一部手机很可能根本没有地方放SIM 卡(SIM card)，这将是一个时代的结束，但肯定是一个更好的举措。
What Is a SIM Card Used For?
Just about еvеry cellular telephone in existence today mаkes use of a ЅIM card. “SIM” is ѕhort for Subscriber Identity Module. Without it, you can’t make or receive phone calls. You also can’t use any internet data over a cellular connection.
You probably already knew this, but why is your phone so useless without this small piece of metal and plastic? Just what is a SIM card actually used for?
What a SIM Card Looks Like
A SIM Card is pretty easy to identify. It’s a plastic card with a distinct set of metal contacts. Inside the SIM card you’ll find an integrated circuit, which communicates with the phone through the aforementioned metal contacts.
SIM cards come in a variety of shapes and sizes. In fact, there are four sizes in total. The largest is known as “full-size” and is effectively obsolete. Measuring 3.37 in by 2.125 in, full-size SIM cards were basically the size of smart access cards.
The reason for this is quite interesting. At the outset of mobile phone technology, multiple people would share a single phone. The idea was that you would insert your enormous SIM card into the phone when you wanted to use it. Since the card would spend more time in your wallet than in a phone, the large plastic body made perfect sense.
These days most SIM cards you’ll buy have all three of the common sizes, with pieces of the plastic body breaking away until you reach the size you want. The Mini-SIM size is pretty uncommon these days and is mostly used in legacy devices or low-cost devices based on older designs.
The Micro-SIM size is very popular and most mainstream phones use this. The Nano-SIM size eliminates virtually all plastic body around the SIM contacts and is essentially the standard for flagship and mid-high-end smartphones, where internal space is at an extreme premium. SIM cards are physically keyed to make it impossible to insert them the wrong way around.
Here’s a tip: don’t throw away the Mini- and Micro-SIM frames when you get a new SIM card. If you ever need to move the SIM to another phone you may still need them!
What Does a SIM Card Do?
The cellular network owned by your service provider is just that – a network. They need to control who has access to that network both for security reasons and to make the whole thing work!
So what is a SIM card? Your SIM card is an access card. It contains your credentials. So in that sense it’s like a card that stores your username and password. You “log in” to the cellular network with the SIM card. Your phone number is your unique network identity.
No matter where you go, as long as you are within range of a cellular tower belonging to your service provider, you can be reached and reach others.
What’s Stored On a SIM?
The SIM card has information on it that the network needs to authenticate your account. One of these is the ICCID or Integrated Circuit Card Identifier. This is a number that’s unique to that specific physical SIM card.
The next important bit of data on the SIM is known as the authentication key. This is a cryptographic key used to sign data sent to the cellular network. They key is run through a cryptographic function and then compared to another calculation done with a copy of the key stored in the carrier’s database. If they match, you’re let onto the network.
The SIM also has memory on it, that you can use to store your contacts and text messages. This is pretty limited however, with most SIMs sporting between 64k and 256k of space. These days most people are better off storing their contacts on the phone’s own memory, along with a backup using something like iCloud or their Google account.
Contract and Prepaid SIM Cards
There are generally two models cellular providers use to sell their services to customers. You can sign a fixed contract agreement, or you can be a prepaid customer.
In both cases, you’ll get a SIM card. The cards themselves are no different from each other. Rather, it’s how the provider handles the account that sets them apart. You can buy prepaid SIM cards off the shelf. Although in many countries you must register the purchase, using government ID and proof of residence.
Contract SIM cards are allocated data, talk time and other resources according to your contract. Overages are added to your bill and at the end of the billing cycle the money gets deducted from your account. With prepaid cards, you’ll usually load a monetary amount onto the SIM or otherwise purchase talk time or mobile data upfront.
Depending on where you live, certain contract handsets may be locked to SIM cards only from that provider. Often this is a way to get a phone for a substantial discount, but if you ever need to use another SIM card, perhaps when travelling, you’ll find that it doesn’t work! To avoid that sort of situation, be sure to look for phones sold as “unlocked”.
Switching SIM Cards
Taking a SIM from one phone and moving it to another is pretty painless. Given that both phones use the same SIM card size. If the other phone uses a larger size, you’ll have to use an adapter to make it fit the larger slot. If the SIM card you want to use is too large for the target phone, you have two choices.
The first is to cut the SIM down to size. Some people are brave enough to do this using nothing but a pair of scissors and some sort of template. If you accidentally cut into the circuity of the SIM card, it’s game over and you’ll need a new one. The safest way to do this is by using a dedicated SIM cutting tool. Even then, you run the risk of destroying the card, so take this route only at your own risk.
The other option is to switch SIM cards. Of course, if you just went out and bought a new SIM card, it would have a different phone number associated with it! Each service provider may have a different procedure to move your phone number from one SIM to another.
In general however, it should simply take a phone call and verification of your identity. If the provider has physical stores, you should even have the option of going in for a new SIM, with your old one disabled.
These days it’s popular to use text messages sent to specific SIM cards as a form of two-factor authentication. So now we have something known as SIM-swap fraud. Where criminals find ways to have your phone number transferred to a SIM they control. SIM duplication is another method. It’s certainly something to watch out for.
Life After SIM Cards
SIM cards, at least discrete SIM cards, aren’t really necessary anymore. We are already seeing the rise of the eSIM or embedded SIM card. This is a SIM card built directly into the phone. This card is programmable, which means you can easily change providers or use multiple providers at once.
In some countries, “dual-SIM” phones have been very popular. With these you could have two phone numbers, one for personal use, for example. You could also have a dedicated data SIM card, perhaps with better rates.
eSIMs make this redundant and turns access and authentication into something similar to fixed-line broadband usage. Where you simply enter a username and password to access a given service provider.
Few current phones have embedded SIM cards at the time of writing, but that’s set to change as the industry adopts this new standard. You next phone might very well have no place to put a SIM card at all, which would be the end of an era, but certainly a move for the better.
