A general rule of thumb is that if your economic model has no value judgements, it's positive economics, and if it does have value judgements it's normative economics, since to tell someone what he ought to do, you first have to judge what is best for him.
A subscriber identity module or subscriber identification module (SIM) is an integrated circuit that is intended to securely store the international mobile subscriber identity (IMSI) number and its related key, which are used to identify and authenticate subscribers on mobile telephony devices (such as mobile phones and computers).
GSM phones use subscriber identity module (SIM) cards. A SIM card is a small card that's inserted into the phone. It contains all your contacts and settings, and it's linked to your account. You can take the SIM card out, put it into another phone, and if someone calls your number, the new phone will ring.he SIM card won't work in other phones, and the phone won't work with other SIM cards. If you live in North America, you can usually determine whether your phone is GSM or CDMA based on the service provider you use. T-Mobile and AT&T use GSM, while Verizon and Sprint use CDMA.