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Macro cells can be regarded[ by whom? Micro cells are cells whose antenna height is under average rooftop level; they are typically deployed in urban areas. Picocells are small cells whose coverage diameter is a few dozen meters; they are mainly used indoors.
Femtocells are cells designed[ by whom? Umbrella cells are used[ by whom? Cell horizontal radius varies — depending on antenna height, antenna gain , and propagation conditions — from a couple of hundred meters to several tens of kilometers. There are also several implementations of the concept of an extended cell,  where the cell radius could be double or even more, depending on the antenna system, the type of terrain, and the timing advance.
GSM supports indoor coverage — achievable by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters — to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. Picocells are typically deployed[ by whom?
However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of radio signals from any nearby cell. Regardless of the frequency selected by an operator, it is divided into timeslots for individual phones. This allows eight full-rate or sixteen half-rate speech channels per radio frequency.
These eight radio timeslots or burst periods are grouped into a TDMA frame. Half-rate channels use alternate frames in the same timeslot. The channel data rate for all 8 channels is Voice codecs[ edit ] GSM has used a variety of voice codecs to squeeze 3.
Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate 6. These used a system based on linear predictive coding LPC. In addition to being efficient with bitrate s, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.
Finally, with the development of UMTS , EFR was refactored into a variable-rate codec called AMR-Narrowband , which is high quality and robust against interference when used on full-rate channels, or less robust but still relatively high quality when used in good radio conditions on half-rate channel.
This allows the user to retain his or her information after switching handsets. Alternatively, the user can change operators while retaining the handset simply by changing the SIM. Phone locking[ edit ] Sometimes mobile network operators restrict handsets that they sell for exclusive use in their own network. This is called SIM locking and is implemented by a software feature of the phone.
A subscriber may usually contact the provider to remove the lock for a fee, utilize private services to remove the lock, or use software and websites to unlock the handset themselves. It is possible to hack past a phone locked by a network operator. It has considered the user authentication using a pre-shared key and challenge-response , and over-the-air encryption.
However, GSM is vulnerable to different types of attack, each of them aimed at a different part of the network. The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation. GSM uses several cryptographic algorithms for security.
Since different efforts have been made in order to crack the A5 encryption algorithms. As an example, Karsten Nohl developed a number of rainbow tables static values which reduce the time needed to carry out an attack and have found new sources for known plaintext attacks. The most commonly deployed GPRS ciphers were publicly broken in They also noted that some carriers do not encrypt the data i.
Global System for Mobile Communications
The GSM System for Mobile Communications