What is GSM and CDMA
Global System for Mobile Communications (GSM) and Code Division Multiple Access (CDMA) are two competing standards in cellular service. They both have derivatives for use with 3G phones known as Universal Mobile Telecommunications System (UMTS) and CDMA2000, respectively. The major difference between the two technologies is how they turn voice data into radio waves and how the carrier connects to the phone. Other differences include the coverage area, the data transfer speeds, and the type of hardware used.
Some areas and countries only have one technology available, so users should be sure to review coverage maps before buying a phone. Generally speaking, CDMA is most commonly found in North America and some parts of Asia, while GSM is found in most other places. Some carriers do offer international or "world" phones that can work with both, but if the phone is going to be used predominantly in one area, it may make sense just to get one that's tied to one type.
Data Transfer Speed
Both technologies can be used with 3G standard phones, but 3G GSM speeds can be faster than 3G CDMA speeds, which can make a big difference for those who use their phones for social networking, email and streaming video. The fastest 3G standard used with CDMA2000 is EV-DO Rev B., which has downstream data rates of about 15.67 Megabits per second (Mbit/s). The fastest standard available with UMTS is HSPA+, with downstream speeds of up to 28 Mbit/s.
GSM phones and CDMA phones also use different types of smart cards known as Universal Integrated Circuit Cards (UICC). These are small removable cards that can be used to store information like a contact list and activate, interchange, and upgrade phones without carrier intervention as long as the phone is unlocked. This means that the carrier makes it possible for phone to work even if the end user removes and changes the card. Though both types can be sold locked, this is more common with CDMA phones. UICCs can be programmed to work with either GSM or CDMA and their derivatives, or with both. Those that only work with GSM phones are called Subscriber Identity Module (SIM) cards and those that only work with CDMA phones are called CDMA2000 Subscriber Identity Module (CSIM) cards. There are also a few types of UICCs that are programmed to work with GSM, UMTS, CDMA, and CDMA2000, including CSIM/USIM cards and Removable User Identity Module (R-UIM) cards.
Generally speaking, both networks have fairly concentrated coverage in major cities and along major highways. GSM carriers, however, have roaming contracts with other GSM carriers, allowing wider coverage of more rural areas, generally speaking, often without roaming charges to the customer. CDMA networks may not cover rural areas as well, and though they may contract with GSM cells for roaming in more rural areas, the charge to the customer can be significantly higher.
Some GSM and CDMA carriers offer international roaming, which means users can still use their phones when traveling abroad. To work internationally, the phone has to be a quad-band phone, which means that it works with frequencies of 850, 900, 1800, and 1900 MHz. Additionally, the phone does have to be unlocked and the user does have to be trying to use a network that exists in the country, which is sometimes more difficult to do with CDMA since fewer countries have CDMA networks. If the phone is unlocked and there is a network present, though, then users can buy a UICC with minutes and a local number in the country in which they're traveling to avoid paying international rates.
The distinction between the two technologies is largely irrelevant in 4G phones, since both can be used with the both of the main standards used with 4G phones, Long Term Evolution (LTE) and Worldwide Interoperability for Microwave Access (WiMax). Since GSM and CDMA work by means of radio waves, but 4G technologies work by means of an IP network, the radio signals from both have to be translated into electronic data for use on the network or the phone has to switch back to a 2G or 3G network to make voice calls.
What is a SIM card?
A Subscriber Identity Module (SIM) card is a portable memory chip used mostly in cell phones that operate on the Global System for Mobile Communications (GSM) network. These cards hold the personal information of the account holder, including his or her phone number, address book, text messages, and other data. When a user wants to change phones, he or she can usually easily remove the card from one handset and insert it into another. SIM cards are convenient and popular with many users, and are a key part of developing cell phone technology.
Activating a SIM Card
Since all of a user's data is tied to the SIM card, only it needs to be activated when the person opens an account with a cell phone service provider (also called a carrier). Each card has a unique number printed on the microchip, which the carrier needs to activate it. In most cases, the phone's owner can go either to the carrier's website and enter this number in the appropriate tool or call the service provider directly from another phone to get it turned on. SIM cards are tied to a particular carrier and can only be used with a service plan from that carrier.
One of the biggest advantages of SIM cards is that they can easily be removed from one mobile phone and used in any other compatible phone to make a call. This means that, if the user wants to buy a new handset, he or she can activate it quickly by inserting his or her old SIM card. The user's phone number and personal information is carried on the card, so there's no need to do anything else to transfer this information. Most phone applications (apps) are stored in the phone's memory or secure digital (SD) card, however, so they will not be transferred to the new handset.Some cell phone carriers lock their handsets; this means they will only work with SIM cards from that carrier. This is especially common in the US, where service providers sell the handsets at a discount in exchange for the consumer signing a long-term contract for service. Handsets can be unlocked with the right code, however, although the exact process varies by manufacturer and model. Any SIM card can be used with an unlocked phone. Phones without a card, or with one that is not compatible, can typically only be used for emergency calls.
Pre-paid SIM cards are also available, allowing a phone that's locked to the pre-paid carrier's network or an unlocked phone to be used without a long-term contract. This type of card is useful for people who don't want to be tied to one carrier, or who want to try out a service provider before committing. It's especially useful for international travelers, who can purchase a local SIM card to use their phone abroad. This allows the user to keep the same phone — with the apps and other settings that are stored in it — without paying international roaming charges from the carrier back home. It also gives the traveler a local phone number, making it less expensive for people to call the traveler from within the same country.
SIM Cards Sizes
SIM cards are made in three different sizes to accommodate different devices. Most phones use mini-SIM or micro-SIM cards, which are quite small — the mini is 25 mm by 15 mm (0.98 in by 0.59), and the micro is 15 mm by 12 mm (0.59 in by 0.47 in). Full-sized cards are much larger, 85.6 mm by 53.98 mm (3.37 in by 2.13 in), and are too big for most phones. All cards are only 0.76 mm (0.03 in) thick, however, and the microchip contacts are in the same arrangement. This means that, with the proper adapter, the smaller cards can be used in devices designed for larger ones.
A SIM card offers security for both the user's data and his or her calls. The cards can be locked, meaning that only someone who has the correct personal identification number (PIN) can use the card. If the phone is stolen, the thief cannot use a locked SIM or get any information off of it without the PIN.In addition, the card has a secret authentication code and an encryption key that protect the phone's transmissions. Although it is possible to "clone" a cell phone on the GSM networkand thus steal service, it's much more difficult than it is to clone a phone on the competing Code Division Multiple Access (CDMA) network. Because of the way the encryption information is transmitted to the carrier, it's usually necessary to have physical access to the SIM card in order to copy it.
Other Cell Phone Technologies
Phones that operate on the CDMA network do not use SIM cards; instead, most save the phone number and other identifying information in the handset itself. While this can be less convenient for users, CDMA is most common in the US, where handsets are usually heavily subsidized and users may have less incentive to switch phones frequently. In addition, some carriers, mostly in Asia, do use their own removable card format, called a Re-Useable Identification Module (RUIM).As technology changes, however, more cell phone providers are upgrading to 3GPP Long Term Evolution (LTE), which is based on GSM technology. This means that some sort of SIM card is likely to be required for devices that use this network
What is an unlocked phone?
Phones That Can Be Unlocked
Many modern mobile phones include a tiny card in them called a SIM card. This card identifies a mobile phone as a certain phone number, holds contact data, and stores other data. A mobile phone user can move their SIM card from one phone to another and still keep the same number and other information. These SIM cards are used in phones on a network that utilizes Global System for Mobile communication (GSM), which is used by many European providers and some in the US.
A locked phone essentially has software on it that prevents it from working with a SIM card that has any other carrier’s information coded into it. For example, a mobile phone locked by AT&T® can only function with a SIM card that is affiliated with that network. With an unlocked phone, however, a person can use any provider that uses the GSM standard, including whichever company offers the best rate or the highest quality services.
Unlocking a Phone
There are a few different ways to turn a locked device into an unlocked phone. Some mobile providers unlock phones for their customers, as long as there is no longer a contract tying that user to the provider. There are also third-party companies that can be contacted and provide codes or software used to unlock one. Either method is usually quite inexpensive, though prices can vary depending on the type of unlocked phone and the service used.
Benefits And Drawbacks
An unlocked phone gives a user more options, allowing him or her to choose a GSM service provider and swap out SIM cards more easily, even when traveling in a foreign country. Another advantage is that an unlocked phone is worth more for resale, since someone can use it with more providers. Unlocking a phone is typically legal, though local laws should be consulted by anyone considering unlocking one. The ToS provided by most companies usually forbids unlocking while under contract, and doing so can result in fines by the provider.
Why Phones Are Locked
One of the ways that cell phone companies attract new customers is to offer free or inexpensive phones when the customer signs a contract. In addition to charging a high termination fee if customers cancel their service before the contract is up, cell phone companies protect themselves by locking the phones they sell to their network. This keeps them from losing money on customers who want to buy a phone at a low price and then activate it with another network for a better rate.