2G GSM: What is a GSM network and how does GSM work?

GSM stands for Global System for Mobile Communications, and it is one of the most widely deployed second-generation (2G) cellular technologies globally. GSM was initially introduced in 1991 in Europe before it made its way to the rest of the world.

GSM or Global System for Mobile Communications is a second-generation (2G) digital technology standard that allows mobile networks to securely deliver voice calls, text messages (SMS) and mobile data. GSM employs FDMA and TDMA and supports 900 MHz, 1800 MHz, 850 MHz and 1900 MHz frequency bands.

While GSM is not the only second-generation cellular technology standard, it has been more widely deployed than other standards, including IS-95 (Interim Standard 1995 or cdmaOne) and Digital AMPS (D-AMPS).

Background of GSM networks

Mobile networks started their journey in the early 1980s in a very decentralised way. A range of analogue technology standards was used in different countries to introduce the first generation of mobile networks. However, there was a need for a global standard that could allow users of one network within a country to continue using mobile services in other countries on compatible mobile networks.

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 by a coordinating body, CEPT (European Conference of Postal and Telecommunications Administrations), that set up a group called Groupe Speciale Mobile (GSM).

Groupe Speciale Mobile was created to establish a European wireless telecommunication standard based on the 900 MHz frequency band. Today the acronym GSM represents Global System for Mobile Communications and is not limited to Europe but has reached all parts of the world.

GSM initially provided voice calls, texts and limited mobile data

When GSM networks were originally launched, they only had the circuit-switched capability that delivered voice calls and text messages (SMS). Later, circuit-switched data (CSD) was introduced that offered limited mobile data. Circuit-switching is not an efficient technique which led to the introduction of packet-switching in GSM.

The circuit-switching capability in GSM networks requires dedicated timeslots for the entire duration of a voice call or data session. The voice calling and SMS in GSM still use circuit-switching, but the mobile data part is handled differently. Since a data session does not require a user to send and receive during a session constantly, a new approach was needed, which led to the introduction of GPRS.

GPRS introduced packet-switched mobile data in GSM

GPRS or General Packet Radio Service was introduced in GSM networks in the year 2000 to deliver mobile internet efficiently. The circuit-switched data (CSD) technique and its enhancement High-Speed Circuit Switched Data (HSCSD) were operationally costly as they required permanent engagement of network resources during a data session.

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.

GSM network components: Mobile Station, BSS, NSS and OSS/BSS

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).

— A high-level network diagram for GSM and GPRS —

Mobile Station – MS

The mobile phone or cell phone in GSM is called Mobile Station or MS. The mobile phone communicates with the mobile network through cell towers that are part of the Base Station Subsystem (BSS). The mobile signals between MS and BSS are transmitted at radio frequencies specific to GSM networks.

Mobile Station (MS) consists of two parts: the hardware/software of the device and the SIM card. The phone itself can 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 – BSS

The Base Station Subsystem or BSS in GSM represents the mobile radio network and primarily consists of Base Transceiver Station (BTS) and Base Station Controllers (BSC). Depending on the network vendor, BSS may also include transcoders.

BTS is the cell tower that a mobile phone communicates with. The BTS has radio units that transmit and receive mobile signals at specified radio frequencies. The frequency channels in GSM are called ARFCN or Absolute Radio Frequency Channel Numbers. One BTS can have multiple sectors, and each sector is a cell that is assigned an ARFCN.

Since frequency spectrum is a scarce resource, the frequency channels are re-used where possible to maximise the utilisation. In GSM networks, the re-use of the frequency channels must be planned carefully to avoid co-channel and adjacent channel interference.

In GSM, the BSS has another entity called the Base Station Controller which has responsibilities to manage or control multiple Base Transceiver Stations (BTSs). BSC is responsible for the mobility management, handovers and power control capabilities for a number of BTSs.

Network Switching Subsystem – NSS

Network Switching Subsystem or NSS in GSM represents the mobile core network that 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 (Public Switched Telephone Network) and ISDN (Integrated Services Digital Network).

When General Packet Radio Services (GPRS) was introduced in the GSM networks in 2000, two new entities, Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN) were added to the mobile core network. SGSN and GGSN enable the packet-switching capability in GSM networks.

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 and Business Support Subsystems – OSS & BSS

While we don’t always see OSS and BSS in the network architecture diagrams, they play an important role in managing the day-to-day operations and maintenance of the network as well as business functions. OSS supports network operations tasks such as performance and fault etc. whereas BSS supports customer-facing tasks including orders, billing, charging etc.

Within OSS, a key entity that is linked to NSS and BSC is the Operations and Maintenance Centre (OMC), which provides operational support to network components within the radio and core networks. OMC monitors and controls network elements to ensure the best Quality of Service (QoS).

I have written a dedicated post on OSS and BSS systems in mobile networks.

GSM supports 900, 1800, 850 and 1900 MHz frequency bands

While one of the original 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.

The 900 MHz frequency band in GSM ranges from 880 to 960 MHz, whereas the 1800 MHz band ranges from 1710 to 1880 MHz. On the other hand, the 850 MHz frequency band ranges from 824 to 894 MHz, whereas the 1900 MHz band ranges from 1850 to 1990 MHz.

I have written a dedicated post on GSM frequencies that can provide you with all the details you need.

GSM is based on FDD and uses TDMA and FDMA for air interface

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 occur 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.

GSM uses various identifiers: 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.

If you are looking for more details on these identifiers, I have written a dedicated post on IMSI, IMEI, MSISDN and ICCID numbers.

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.

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 extra support, especially when preparing for a new job, studying a new topic, or 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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