Global System for Mobile Communication aka GSM is defined as the digital network aiding mobile connectivity. The GSM network is used by people from different parts of the world and Europe, on a whole. Of the three variants, GSM, TDMA and CDMA, the former stands as the most popularly used network. Though GSM is seen to use one of the variants of TDMA, it’s functioning is pretty different from that of the other two.
As per the GSM Association, nearly 80% of mobile phone users across the globe use GSM as their prime network for placing wireless calls.
GSM Timeline
History of GSM dates back to the time of 1982. It was then that the European Conference of Postal and Telecommunications Administrations (CEPT) created the Groupe Spécial Mobile (GSM) which now is named as the Global System for Mobile Communication.
Besides, the main purpose of the creation of GSM was to design a mobile technology supporting the pan-European network.
Further in 1991, after its development using TDMA as the base, did we see the actual implementation and usage of the network, on a commercial level. Since then, GSM has been in continuous use and growth.
GSM Features
Features provided by the GSM network include:
- Phone call encryption,
- Data networking,
- Caller ID,
- Call forwarding,
- Call waiting,
- SMS, and
- Conferencing.
Type of GSM Phones & Carriers
GSM mobile phones have different variants with the top ones used widely being:
- T-Mobile
- AT&T
- Indigo Wireless
- Pine Cellular
- TerreStar
Basically, GSM was built to aid 2g connectivity in mobile phones. Earlier, the network incorporated a circuit switching network but with time and need, it had to collaborate with GPRS, implementing the packet switching protocol. The frequency of operation of the GSM network falls within the range of 900-1800 MHz.
Talking about the technologies supporting the overall functioning of GSM mobiles, top contributions are given through:
- GPRS
- HSDPA, being the GSM 2g network
- EDGE
GSM Architecture
As we all know, GSM is all about driving 2g connectivity in mobile phones, the architecture is a bit different than those of other network providers. The network on a whole has been segmented into three parts:
- Radio Subsystem
- Mobile Station
- Base Station
- Operation Subsystem
- Network and Switching Subsystem
What Is A Mobile Station?
Mobile station basically refers to the mobile device which is equipped with a processor, transceiver, and a display unit. Every portable unit of mobile station further entails the storage of unique identity in a separate module, namely SIM (Subscriber Identification Module). Now, as a mobile user, you might very well be aware of what a SIM is. Inserted in the form of a small microchip, a SIM card contains information related to the mobile station database.
Components Of A Base Station
Part of the radio subsystem, the base station subsystem is, responsible for drawing a connection between the network and the mobile station using the OTA (over the air) interface.
Every base station comprises of the following:
Base Transceiver Station
The Base Transceiver Station is the one that leads to the connection driven between the mobile station and the cellular network using the mechanism of over the air connectivity. On the basis of the structure of morph, load, and the subscriber’s behavior, the base transceiver has various configurations.
Standard Configuration
Calls for a separate cell identity for every BTS and multiple BTS combines to be regarded as one location.
Umbrella Cell Configuration
Deems the presence of one BTS working at a higher altitude having greater transmission power. Also, this high pitched BTS acts as the umbrella shielding all other lower-powered Base Transmitter Stations.
Collocated Configuration
Suggests the collocation of multiple BTSs at one single site. However, the antennas cover only those areas that are either at 120 degrees or at 180 degrees. Hence, this can be thought of as the network of neighboring radio cells that lays total coverage of the entire service area.
Base Station Controller
This controls the operation of one or more BTS and terminologically regarded as the power controller. Furthermore, an abis-interface is used to draw a connection between the BTS and the BSC. Besides that, BSC comprises a database maintaining the records of BTS status, radio quality, and the terrestrial resources along with BTS software.
Transcoding Rate & Adaptation Unit
This can be regarded as being sandwiched between the BSC and mobile switch controller, used to either compress or decompress speech.
Network Switching Subsystem
This controls the entire database functions, right from authentication to encryption and features of roaming needed to place a call. In summary, this aids connectivity between two mobile stations.
Further, it has three different components:
Mobile Switching Center
Named as the most important element of GSM, the mobile switching center lays interfaces to Public Switched Telephone Network (PSTN) so as to draft connectivity with the landline or any other Mobile Switching Centre (MSC) or between mobile phones.
Home Location Register
Comprises data related to the subscribers.
Virtual Location Register
Similar to the home location, the virtual location register holds dynamic data related to every subscriber.
How Does GSM Work?
GSM as a network comprises of TDMA and FDMA. Firstly, Frequency division multiple access is about dividing the frequency bands in different frequency bands. On the other hand, TDMA is all about allowing a similar channel of frequency to different subscribers. Hence, every user would have their own time slot to use a specific frequency.
So, when it comes to GSM, each of the subdivided carrier frequency is further segmented into separate slots of time using the techniques of TDMA. Each frame of TDMA would last for around 4.164 milliseconds (ms) and would then have 8-time slots. Each physical channel or time slot in the given time frame would last for 577 microseconds. The data is then transmitted in the predetermined slot of time in the form of bursts.
This on whole accounts for the overall functioning of the GSM network. Though there are plenty of technologies emerging, GSM still stands as one of the most preferred technology nation-wide.
