Editing GSM (section)

=== Base-station subsystem ===
{{Main|Base station subsystem}}
[[File:Kathrein antenna.jpg|thumb|150px|GSM [[cell site]] antennas in the [[Deutsches Museum]], [[Munich]], [[Germany]]]]

GSM utilizes a [[cellular network]], meaning that [[cell phone]]s connect to it by searching for cells in the immediate vicinity. There are five different cell sizes in a GSM network:

* [[macrocell|macro]]
* [[microcell|micro]]
* [[Picocell|pico]]
* [[femtocell|femto]], and 
* umbrella cells

The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the [[base station|base-station]] [[antenna (electronics)|antenna]] is installed on a mast or a building above average rooftop level. 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 for use in residential or [[small business|small-business]] environments and connect to a [[telecommunications service provider]]'s network via a [[broadband internet|broadband-internet]] connection. Umbrella cells are used to cover shadowed regions of smaller cells and to fill in gaps in coverage between those cells.

Cell horizontal radius varies – depending on antenna height, [[antenna gain]], and [[propagation (disambiguation)|propagation]] conditions – from a couple of hundred meters to several tens of kilometers. The longest distance the GSM specification supports in practical use is {{convert|35|km|mi|0}}. There are also several implementations of the concept of an extended cell,<ref>[http://www.allbusiness.com/electronics/computer-electronics-manufacturing/6838169-1.html Motorola Demonstrates Long Range GSM Capability – 300% More Coverage With New Extended Cell]. {{webarchive |url= https://web.archive.org/web/20120219003334/http://www.allbusiness.com/electronics/computer-electronics-manufacturing/6838169-1.html |date= 19 February 2012 }}</ref> 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 [[Cellular repeater|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 when significant call capacity is needed indoors, as in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of radio signals from any nearby cell.

==== GSM carrier frequencies ====
{{Main|GSM frequency bands}}
GSM networks operate in a number of different [[carrier frequency]] ranges (separated into [[GSM frequency ranges]] for 2G and [[UMTS frequency bands]] for 3G), with most [[2G]] GSM networks operating in the 900&nbsp;MHz or 1800&nbsp;MHz bands. Where these bands were already allocated, the 850&nbsp;MHz and 1900&nbsp;MHz bands were used instead (for example in Canada and the United States). In rare cases the 400 and 450&nbsp;MHz frequency bands are assigned in some countries because they were previously used for first-generation systems.

For comparison, most [[3G]] networks in Europe operate in the 2100&nbsp;MHz frequency band. For more information on worldwide GSM frequency usage, see [[GSM frequency bands]].

Regardless of the frequency selected by an operator, it is divided into [[time-division multiplexing|timeslots]] for individual phones. This allows eight full-rate or sixteen half-rate speech channels per [[radio frequency]]. These eight radio timeslots (or [[burst transmission|burst]] periods) are grouped into a [[Time-division multiple access|TDMA]] frame. Half-rate channels use alternate frames in the same timeslot. The channel data rate for all {{nowrap|8 channels}} is {{nowrap|270.833&nbsp;kbit/s,}} and the frame duration is {{nowrap|4.615 ms.}}<ref>{{Cite web |title=GSM Frame Structure: Multiframe Superframe Hyperframe » Electronics Notes |url=https://www.electronics-notes.com/articles/connectivity/2g-gsm/frame-structure-superframe-hyperframe-multiframe.php |access-date=2023-02-09 |website=www.electronics-notes.com}}</ref> TDMA noise is interference that can be heard on speakers near a GSM phone using TDMA, audible as a buzzing sound.<ref>{{Cite web |date=May 2013 |title=AN-1496 Noise, TDMA Noise, and Suppression Techniques |url=https://www.ti.com/lit/an/snaa033d/snaa033d.pdf |website=Texas Instruments |series=Application Report SNAA033D |edition=Revised}}</ref>
 
The transmission power in the handset is limited to a maximum of 2 watts in {{nowrap|GSM 850/900}} and {{nowrap|1 watt}} in {{nowrap|GSM 1800/1900}}.

==== Voice codecs ====
GSM has used a variety of voice [[codec]]s to squeeze 3.1&nbsp;kHz audio into between 7 and 13&nbsp;kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called [[Half Rate]] (6.5&nbsp;kbit/s) and [[Full Rate]] (13&nbsp;kbit/s). 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. GSM was further enhanced in 1997<ref>
{{cite web
 | url= http://www.3gpp.org/ftp/Specs/archive/06_series/06.51/0651-401.zip
 | publisher= ETSI |title= GSM 06.51 version 4.0.1
 | date = December 1997
 | format= ZIP |accessdate= 5 September 2007
}}
</ref>
with the [[enhanced full rate]] (EFR) codec, a 12.2&nbsp;kbit/s codec that uses a full-rate channel. Finally, with the development of [[UMTS]], EFR was refactored into a variable-rate codec called [[Adaptive Multi-Rate|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.