Global System for Mobile Communications

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The Global System for Mobile Communications (GSM), formerly known as "Groupe Spécial Mobile" (French), is a world-wide standard for digital wireless mobile phones. The standard was originated by CEPT and further developed by ETSI as a standard for European mobile phones, with the intention of developing an open, non-proprietary standard for adoption world-wide. It has been remarkably successful, with more than one billion people using GSM phones as of early 2004.

The GSM groups (Groupe Special Mobile 1, 2, 3 and 4) were founded during the year 1985. The technical fundamentals of the GSM-system were defined 1987. In 1989 ETSI takes control and 1990 the first GSM specification is born (over 6000 pages of text). Commercial operation starts in 1991.

GSM's main competitor, CDMA2000, is used primarily in the United States, although it is seeing increased worldwide adoption as a 3G standard. The ubiquity of the GSM standard makes intra-nation roaming very common, with international roaming frequently enabled by "roaming agreements" between operators. GSM differs from its predecessors most significantly in that both signalling and speech channels are digital. It has also been designed for a moderate level of security. GSM employs time division multiple access between stations on a frequency duplex pair of radio channels, with slow frequency hopping between channels. GSM uses also SDMA and FDMA

Table of contents

1 GSM versions
2 Voice Calls
3 Data Calls
4 GPRS
5 WAP
6 GSM Services
7 The GSM Radio Network

7.1 Subscriber Identity Module

8 Using Mobile phones on aircraft
9 Related topics
10 External links

GSM versions

GSM exists in four main versions, based on the band used: GSM-900, GSM-1800, GSM-850 and GSM-1900. GSM-900 (900 MHz) and GSM-1800 (1.8 GHz) are used in most of the world, excluding the United States and Canada. The United States and Canada use GSM-850 and GSM-1900 (1.9 GHz) instead, since in the U.S. the 900 and 1800 bands were already allocated.

GSM-900 uses 890-915MHz to send information and 935-960MHz to receive information.
GSM-1800 uses 1710-1785MHz to send information and 1805-1880 to receive information.

Please note, however, that GSM-1800 is known as DCS (Digital Cellular System) and it is commonly used in PCN-networks (Personal Communications Network). Also, GSM-1900 is known as PCS1900 (Personal Communications Service). All the different systems fall in to the general category called GSM. PCS-1900 standards have been integrated and harmonized with that set of GSM/DCS Specifications, resulting in a specification for GSM/DCS/PCS based on the Release 98 Series of GSM Specifications.

In some countries the GSM-900 band has been extended to cover a larger frequency range. The extended GSM, E-GSM, uses frequency range 880MHz - 915 MHz (uplink) and 925MHz - 960MHz (downlink). The GSM specifications also describe railways GSM, R-GSM, which uses frequency range 876Mhz - 915Mhz (uplink) and 921MHz - 960 MHz (downlink). All these specifications are known as GSM-900.

In Europe and other areas outside North America the GSM system initially used a frequency of 900 MHz, shortly afterwards the PCN network used the 1800 MHz frequency, nowadays the PCN networks are considered part of the GSM system and many phones are dual-band operating on 900/1800 MHz.

Voice Calls

In GSM, a call is dedicated either as voice or data. A voice call uses a GSM specific codec to transmit the audio over a 9600 bit/s digital link to the base station.

The speech codecs used in GSM are called Half-Rate (HR), Full-Rate (FR), Enhanced Full-Rate (EFR) and Adaptive Multirate (AMR). All codecs except AMR operate with a fixed date rate and error correction level.

Data Calls

A data call lets the user use the phone as a modem with 9600 bit/s bandwidth (some networks may also handle 14400 bit/s). All newer GSM phones can be controlled by a standardised hayes AT command set through a serial cable or a wireless link (irDA or bluetooth). The AT commands can control everything in the phone from ring tones to data compression algorithms. An extension to the GSM data capabilities, high-speed circuit-switched data (HSCSD), allows data transmission speeds up to 43.3 kbit/s by allocating several data channels into one logical link. Realistic bandwidth is usually about 30 kbit/s when standing still. Expect 10 kbit/s when moving.

GPRS

A GSM extension, called GPRS, allows packet switched data transmission. GPRS has been called 2.5G as it is viewed as a stepping stone toward pure 3G systems like UMTS/W-CDMA or similar.

GPRS is backward compatible with GSM. This eases the migration path for a GSM operator, who can gradually upgrade the infrastructure to GPRS as the market expands.

Packet switched data under GPRS is achieved by allocating unused cell bandwidth to transmit data. As dedicated voice (or data) channels are setup by phones, the bandwidth available for packet switched data shrinks. A consequence of this is that packet switched data has a poor bitrate in busy cells. The theoretical limit for packet switched data is approx. 170 kbit/s. A realistic bitrate is 30-70 kbit/s. A change to the radio part of GPRS called EDGE allows higher bit rates of between 20 and 200 kbit/s. The maximum data rates are achieved only by allocation more than one time slot in the TDMA frame. the network. Also, the higher the data rate, the lower the error correction capability. Generally, the connection speed drops logarithmically with distance from the base station. This is not an issue in heavily populated areas with high cell density, but may become an issue in sparsely populated/rural areas.

The transfer speed depends also on the channel encoding used. The best encoding scheme (CS-4) is available near the BTS while the worst encoding scheme (CS-1) is used when the MS is further away from the BTS.

Using the CS-4 it is possible to achieve a speed of 21,4 kb/s per time slot. However by using this scheme the cell coverage is 25% from the normal. CS-1 can achieve a speed of 9.05 kb/s per time slot and has 98% of the normal coverage.

GPRS packet switched data is packet based. When IP is used, each phone can have one (or more?) IP addresses allocated. GPRS will store and forward the IP packets to the phone during cell handover (when you move from one cell to another). TCP's inability to differ between radio noise induced pauses and network congestion makes the protocol unsuitable for GPRS (or any radio based IP traffic). A radio noise induced pause will make TCP (unnecessarily) throttle back its transmission speed.

From the user's point of view, GPRS is a wireless extension of data networks. It can access multiple types of data networks, such as IP based networks like the public Internet, private intranet, both IPv4 and IPv6 protocols, and X.25 based networks.

GPRS upgrades GSM data services providing:

Point-to-point (PTP) service: internetworking with the Internet (IP protocols) and X.25 networks.
Point-to-multipoint (PTM) service: point-to-multipoint multicast and point-to-multipoint group calls.
Short Message Service (SMS): bearer for SMS.
Anonymous service: anonymous access to predefined services.
Future enhancements: flexible to add new functions, such as more capacity, more users, new accesses, new protocols, new radio networks.

WAP

WAP and its transmission layer protocol, WTP, use UDP/IP to solve the problem of TCP's inadequacy for high packet loss networks . Application developers creating a new mobile IP based protocol can

do the same (not recommended), or
implement a TCP proxy relay service for the wireless link, or
utilise the experimental TCP extensions such as Selective ACK that allow TCP to operate better in the presence of non-congestion packet loss, or
use a new protocol such as SCTP.

GSM Services

GSM has been defined with the main purpose of voice services. Operators also offer data services at speeds of 9.6 and 14.4 kbit/s. Although a cellular network can never be regarded as complete, today the operators can less and less compete on coverage area or quality of the network. However, in these days the data services start to play a big role in operator business. The High Speed Circuit Switch Data Service (HSCSD) offers higher rate data services (see above), and GPRS offers yet higher bit rates for these services. Short message service, real-time messaging (USSD) and a number of value added services are also specified in the GSM system.

The GSM Radio Network

The GSM network consists of cells and cells can be named after their size. Basically there are 4 different cellsizes - Macro, micro, pico and umbrellacells. The coverage area of each cell is different in different environments. Macro cells can be regarded as cells where the base station antenna is installed in a mast or a building above the average roof top level. However, micro cells are cells where the antenna height is under the average roof top level and they are typically used in urban areas. The picocells are small cells whose diameter is a few dozen metres and are mainly used inside. On the other hand, umbrellacells are used to cover shadow regions of smaller cells and share their load. These cells are usually build ontop of tall buildings etc.

The cell radius can vary depending on the antenna height, antenna gain and propagation conditions from couple of hundred meters to several tens of kilometers. Because of the timeslot (time period allocated to one call) overlap that occurs when calls are maintained at large Handset-Basestation separations, practically 35 km is the longest distance GSM specification supports, though the specifications define an extended cell, where the cell radius could be double or even more. This is done by utilizing 2 timeslots per user, so the call has a better chance hitting the right timeslot. Indoor coverage is also supported by GSM.

Indoor coverage can be built by using power splitters to deliver an RF signal from the antenna outdoors to a separate indoor antenna distribution system. When all the capacity of the cell is needed indoors, e.g. in shopping centers or airports etc., the indoor coverage can be built by using antennas only inside the building. In suburban areas the indoor coverage is usually provided by the inbuilding penetration of radio signal, not by a separate indoor antenna system.

Subscriber Identity Module

One of GSM's key features is the Subscriber Identity Module, commonly known as a SIM card. The SIM is a detachable smartcard containing the user's subscription information and phonebook. This allows the user to retain his information while switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM; this practice is known as SIM locking, and is illegal in some countries. In the US most operators do lock the mobiles they sell. This is done because, usually, they subsidize the price of the mobile phone with revenue from subscriptions and try to avoid subsidizing competitor's mobiles.

Using Mobile phones on aircraft

Besides being prohibited by many local regulations, cellular networks in general and GSM ones in particular, are not designed to handle mobile equipment moving at high speeds. When a connection is active, call or packet handovers would happen so often as to introduce many issues with signal processing, network subscriber handling, etc. It would be next to impossible for a cellular network to handle a plane full of mobile subscribers even if not all have an active connection at the time. Also from an altitude, more cells are visible to the mobile. This means that the phone could try to establish contact with a far away cell, which would in turn ask the phone to increase its output power. This could interfere with aircraft and other electronic equipment. But it is worrying that the equipment on planes may be at risk from interference.