A GSM phone is a type of mobile phone that uses the Global System for Mobile Communications to send and receive phone calls. This is one of the two main standards for mobile communication in the world, with the other being Code Division Multiple Access (CDMA). Generally speaking, a GSM phone is likely to be a good choice for someone who travels outside the US a lot, changes carriers often, lives in a rural area, or needs to transfer a lot of data-heavy information.
Advantages and Disadvantages
One of the benefits of a GSM phone is the ease with which users can change carriers or handsets. This is because of the phone's Subscriber Identification Module (SIM) card, which is a removable smart card that carries the user's contacts, calendar information, and personal identification information. A user can move it between mobile phones and still receive voice calls to the same number, and it prevents users from needing to re-enter personal information each time they buy a new phone. Additionally, if a person changes mobile services, then he or she can put the new SIM card in an old handset.
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GSM phones also tend to have clearer signals when making calls indoors, and can use cellular repeaters, which boost and rebroadcast calls for better signal strength. They also use less power, and so can be used longer before needing to be recharged. Disadvantages include a tendency for GSM signals to interfere with some electronic devices, such as radios and speakers, though this usually only happens with 2G phones. Additionally, many of the carriers in the US use CDMA, so it's often difficult to use a GSM phone in the US.
GSM vs. CDMA
GSM and CDMA differ primarily in terms of where they're used, the type of smart card they contain, and roaming. In pre-4G phones, there are also differences in terms of data transfer speeds. GSM is used in most places throughout the world, except for North America and some parts of Asia, where CDMA is more common. Though both contain a Universal Integrated Circuit Card (UICC), GSM phones use a SIM card, while CDMA phones have a CDMA2000 Subscriber Identity Module (CSIM) card. Though there are some UICC cards that are designed to work with both GSM and CDMA phones, SIM cards and CSIM cards are not interchangeable.
Both work pretty well in urban areas, but GSM tends to be better for both domestic and international roaming, since GSM carriers are more common throughout the world, and contract with other carriers to let users use their network when they're in the area. In 3G phones, GSM can have faster data transfer speeds as well, though this isn't a concern with 4G phones, since both it and CDMA both work with the main 4G standards.
GSM phones use a technology called Time Division Multiple Access (TDMA) to transmit calls. This splits transmission time on a single communication channel into segments called frames, each of which is then split into eight time slots. These slots are assigned to different users whose phones transmit during their appointed time slot in rapid succession, many times each second. This allows multiple people to use the same channel simultaneously, though it has the disadvantage of lower total bandwidth available per communications channel due to the technical challenges of keeping the phones sharing the channel properly synchronized.