What is a GSM network and how does it work?

GSM is one of the most widely deployed cellular technologies globally. Together with another technology, D-AMPS, it represents the start of the digital era in mobile communications. GSM stands for Global System for Mobile Communications, and it was introduced in 1991 in Europe initially before it made its way to the rest of the world. Mobile networks had already started their journey in the 1980s through various analogue technologies; however, they were very decentralised and operated nationally. So there was a need for a standard that could work at a more global level to allow users of one network within a country to continue using mobile services in other countries on compatible mobile networks.

GSM (Global System for Mobile Communications) is a 2G digital cellular technology standard that offers voice calls, SMS and mobile data. It employs the FDD duplex scheme and a combination of FDMA and TDMA for radio access. GSM can operate at 900 MHz, 1800 MHz, 850 MHz and 1900 MHz frequency bands.

The journey for GSM began in the early 1980s when the world had only seen the first generation of mobile networks that were based on analogue technology standards, including AMPS, NMT, TACS and C-Netz. At that time, GSM was initiated as a European standard rather than a global one by a coordinating body, CEPT (European Conference of Postal and Telecommunications Administrations), that set up a group called Groupe Speciale Mobile (GSM). This group was created to establish a European wireless telecommunication standard based on the 900 MHz frequency band. Today GSM stands for Global System for Mobile Communications and is not limited to Europe but has reached all parts of the world and is truly global.

Duplex and Multiple Access Schemes

When a mobile phone communicates with the mobile network, the communication takes place in two directions. The communication from the mobile phone to the mobile network is called the uplink, and the communication from the network back to the phone is called the downlink. GSM networks employ separate frequency bands for the uplink and downlink and require a pair of frequency channels to communicate with each mobile phone. This methodology is called Frequency Division Duplex or FDD, where uplink and downlink communications take place on separate frequency bands. GSM networks enable multiple access by applying a combination of FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) to the available frequency band. The frequency bands for uplink and downlink are divided into frequency channels of 200 kHz each. These 200 kHz channels are then grouped in pairs called ARFCN (Absolute Radio Frequency Channel Number), where each pair has one frequency range for uplink and one for downlink. TDMA is then applied by further splitting each 200 kHz frequency channel within an ARFCN into eight timeslots. You can check out our dedicated post on GSM frequencies to learn about the specific details for GSM ARFCNs.

GSM frequency bands

While one of the requirements at the start of the GSM journey was to use the 900 MHz frequency band, it is no longer the limitation. GSM networks have since then expanded and can operate in various frequency bands. Generally, there are two key tracks for the GSM frequency bands: 900/1800 MHz and 850/1900 MHz. The 900 MHz / 1800 MHz band is primarily used in most parts of Europe, Asia, Africa, the Middle East and Australia. The 850 MHz / 1900 MHz band is used in North and South America, including the US, Canada, Mexico and other countries. Please look at our dedicated post to find out how these frequencies are allocated to uplink and downlink in the GSM networks.

What GSM network looks like

The GSM networks are a combination of multiple subsystems. These subsystems mainly comprise the radio network and the core network. The user device in GSM is called a Mobile Station, abbreviated as MS. MS is connected to the Base Station Subsystem (BSS), the radio network. The radio network then connects to the Network Switching Subsystem (NSS), the mobile core network. Finally, the core network connects to other landline and mobile networks like PSTN (Public Switched Telephone Network), PLMN (Public Land Mobile Network) and ISDN (Integrated Services Digital Network).

Mobile Station

The Mobile Station (MS) is the mobile phone or cell phone. There are two parts to the mobile station: the hardware/software of the device and the SIM card. The phone itself has the capability to operate on the GSM technology and associated frequencies, whereas the SIM (Subscriber Identity Module) has the user data. Once the SIM is inserted into the phone, it can register on the mobile network and communicate through assigned uplink and downlink frequencies. The phone does not need to be on a call to communicate because the signalling continues even in the idle mode as long as the mobile phone is switched on and in the vicinity of the registered mobile network.

Base Station Subsystem

The Base Station Subsystem (BSS) represents the mobile radio network for GSM. BSS consists of BTS (Base Transceiver Station), BSC (Base Station Controller), and separate transcoders depending on the network vendor. BTS is the cellular tower also referred to as Base Station (BS), which the mobile phone (MS) communicates with. The BTS has radio units installed to transmit and receive mobile signals. In GSM networks, reusing frequency channels (ARFCNs) needs to be done very carefully to avoid co-channel and adjacent-channel interference. Generally, one BTS can have multiple sectors, and each sector is a cell that requires an ARFCN. The BSC manages the handover and power control capabilities for numerous BTSs.

Network Switching Subsystem

Network Switching Subsystem or NSS is essentially the core part of the mobile network, which consists of Mobile Switching Centre (MSC), Home Location Register (HLR), Visitor Location Register (VLR), Authentication Centre (AuC) and Equipment ID Register (EIR). MSC is like a telephone exchange that performs circuit switching to enable voice calls and SMS in the GSM network. Through the Gateway MSC (GMSC), the MSC connects the mobile phone users to external networks such as PSTN. When GPRS was introduced in 2000, a packet-switched part was added to the GSM network architecture through two new entities Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). One of the most important entities within NSS is the HLR which contains information about mobile users and their subscription status. The VLR is a distributed location register that keeps track of the location of a mobile phone. The VLR is connected to specific MSCs and is in constant communication with the HLR to continuously update the location and service status of all connected mobile phones. That way, if mobile phone user A wants to call mobile phone user B, the mobile network always knows where to find user B and if user A has enough credit/allowance in their account to make a call.

Operations Support Subsystem

The primary network entity within Operations Support Subsystem is the Operations and Maintenance Centre (OMC) that is linked to the NSS and BSC to provide operational support for network entities within the radio and core networks. OMC monitors and controls network elements to ensure the best Quality of Service (QoS).

IMSI, TMSI, MSISDN and MSRN

GSM networks use a range of numbers or unique codes to identify the mobile users and dispatch the right services to them. IMSI stands for International Mobile Subscriber Identity and is a 15 digit code assigned to each SIM card. For each IMSI, the mobile network can generate a temporary code called Temporary Mobile Subscriber Identity (TMSI) to conceal the permanent identity (IMSI) for security purposes. MSISDN stands for Mobile Station International Subscriber Directory Number, and it is the full mobile number for a particular SIM with all prefixes. Finally, MSRN stands for Mobile Subscriber Roaming Number, and it is a temporary mobile number assigned to a mobile station when it is not on the home network (roaming) so that any calls or communication can be directed to it.

Enhancements to GSM networks

As we briefly touched upon earlier, General Packet Radio Service or GPRS was introduced in the GSM networks around the year 2000. GSM networks had the circuit-switched data capability; however, circuit-switched data was not efficient as it required permanent allocation of network resources for the entire duration of data sessions. GPRS was a game-changer as it was based on packet-switched technology, which was more efficient. GPRS later evolved to Enhanced GPRS through the EDGE (Enhanced Data for Global Evolution). GPRS can enable peak downlink speeds of up to 171.2 kbps for downloads, whereas GPRS (EDGE) can enable peak downlink speeds of up to 384 kbps. In the GSM world, GPRS and EDGE are referred to as 2.5G and 2.75 G, respectively. Check out our dedicated post to learn about the average data speeds for EDGE and GPRS.

Other inter-related technologies

While GSM was the most widely deployed 2G standard, it wasn’t the only second-generation technology. When GSM was introduced in Europe, a different technology, D-AMPS (Digital Advanced Mobile Phone System), was launched in the US. D-AMPS was the digital upgrade path for the earlier first-generation (1G) analogue technology AMPS (Advanced Mobile Phone System). D-AMPS adopted the same frequency band (824 MHz to 894MHz) as AMPS. Like GSM, it also employed a combination of FDMA and TDMA to move into the digital world of 2G.

Another 2G technology introduced in the mid-1990s was Interim Standard 1995 or IS-95. IS-95 was the first-ever CDMA standard used for mobile communications. 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.

Conclusion

GSM stands for Global System for Mobile Communications. It is a second-generation (2G) digital cellular technology that allows mobile users to make and receive voice calls, send and receive text messages and use mobile data services. GSM can operate at various frequency bands, including but not limited to 900 MHz & 1800 MHz and 850 MHz & 1900 MHz bands. It employs Frequency Division Duplex (FDD) as a duplex scheme and a combination of FDMA and TDMA for enabling multiple access.

Here are some helpful downloads

Thank you for reading this post, I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need some extra support, especially when preparing for a new job, studying a new topic, or maybe just buying a new phone. Whatever you are trying to do, here are some downloads that can help you:

Students & fresh graduates: If you are just starting, the complexity of the cellular industry can be a bit overwhelming. But don’t worry, I have created this FREE ebook so you can familiarise yourself with the basics like 3G, 4G etc. As a next step, check out the latest edition of the same ebook with more details on 4G & 5G networks with diagrams. You can then read Mobile Networks Made Easy, which explains the network nodes, e.g., BTS, MSC, GGSN etc.

Professionals: If you are an experienced professional but new to mobile communications, it may seem hard to compete with someone who has a decade of experience in the cellular industry. But not everyone who works in this industry is always up to date on the bigger picture and the challenges considering how quickly the industry evolves. The bigger picture comes from experience, which is why I’ve carefully put together a few slides to get you started in no time. So if you work in sales, marketing, product, project or any other area of business where you need a high-level view, Introduction to Mobile Communications can give you a quick start. Also, here are some templates to help you prepare your own slides on the product overview and product roadmap.

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