GSM stands for Global System for Mobile Communications and it is the most widely deployed standard for the second generation of mobile networks or 2G. GSM network uses a combination of two access techniques Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA).
The combination of FDMA and TDMA allows a mobile operator to first divide the frequency band available to them into smaller chunks of frequency channels, and then break the channels further into time-slots. These time-slots are responsible for accommodating voice calls through circuit-switching.
What frequencies GSM network uses
GSM networks employ Frequency Division Duplex (FDD) which means that the uplink and the downlink communications use separate frequency bands. More about FDD and GSM frequencies in our post What frequency does GSM use? When GSM networks were originally launched, the GSM frequency band was from 890 MHz to 915 MHz for the uplink and 935 MHz to 960 MHz for the downlink. That original frequency band is known as Primary GSM band or P-GSM. The primary GSM band later saw an extension in order to accommodate more frequency channels. As a result, an additional 10 MHz was added to the uplink frequency band as well as the downlink frequency band. This extension is known as Extended GSM or E-GSM, where 880 MHz to 915 MHz is for the uplink and 925 MHz to 960 MHz for the downlink.
The frequency channels in the GSM networks operate in pairs (uplink/downlink) and are allocated numbers or IDs known as Absolute Radio Frequency Numbers (ARFCN). Each ARFCN in both P-GSM and E-GSM has a channel bandwidth of 200 kHz. More about ARFCNs in the GSM frequency post we referred to earlier.
Enhancements to GSM networks
Due to the use of circuit-switching, GSM networks offered carrier-grade voice and SMS services. However, the mobile data or mobile internet part was not properly there. The GSM networks had the capability to offer limited mobile data but the circuit-switched technique was not efficient for that.
The need for mobile data led to further enhancements in the GSM networks which paved way for the packet-switching technique. General Packet Radio Service (GPRS) and Enhanced Data rates for Global Evolution (EDGE) were then introduced into the GSM networks for fulfilling the data needs. GPRS was the first enhancement which could enable peak downlink speeds of up to 171.2 kbps for downloads. EDGE followed GPRS and enabled peak downlink speeds of up to 384 kbps. In the GSM world, GPRS is referred to as 2.5 G as it came soon after GSM but before UMTS (3G). Similarly, EDGE is referred to as 2.75 G as it was launched to enhance GPRS before the 3G UMTS networks.
What GSM network looks like
In GSM networks, the mobile phone communicates wirelessly with the cellular towers known as the base stations or base transceiver stations (BTS). BTS is controlled by another network entity called the Base Station Controller (BSC). Both BTS and BSC are part of a system called Base Station Subsystem (BSS) which is responsible for managing the radio network resources with GSM networks. As part of BSS, BSC has the intelligence to manage mobility resources and controls tasks such as handover and frequency channel allocation. One BSC usually controls a number of base transceiver stations (BTS). BSS is connected to the mobile core network through MSC (Mobile Switching Centre) for circuit-switched services and SGSN (Serving GPRS Support Node) for packet-switched services. The mobile core network is then connected to external networks such as PSTN (Public Switched Telephone Network) and the internet.
Other inter-related technologies
While GSM was the most widely deployed 2G standard, it wasn’t the only 2G standard. The second generation of mobile networks replaced the earlier 1G networks, which in some cases meant upgrading what already existed. One such technology was D-AMPS (Digital Advanced Mobile Phone System). D-AMPS was the digital upgrade path for the earlier 1G analogue technology AMPS (Advanced Mobile Phone System). D-AMPS used the same frequency band (824 MHz to 894MHz) as AMPS did. Like GSM, it also used a combination of FDMA and TDMA to move into the digital world of 2G. More about D-AMPS in our post Is D-AMPS different from AMPS?
The other 2G technology was Interim Standard 1995 or IS-95. It was the first-ever CDMA standard used for mobile networks. The commercial name for IS-95 is cdmaOne. There have been two versions of IS-95: IS-95 A and IS-95 B. IS-95 used carrier frequencies with a bandwidth of 1.25 MHz and could also accommodate data. IS-95 A enabled peak data rates of up to 14.4 kbps which improved further to 115 kbps with IS-95 B. You can find more about IS-95 in our post What is the difference between cdmaOne and CDMA2000?