What is GSM? A simple overview of the GSM Communication Standard

In the past few posts in this wireless and mobile communication course, we have seen various concepts relating to wireless communication like handoffs and channel interference. In this post and a couple more that follow this, we shall be having a look at various mobile communication standards that are prominent in the field, starting with GSM.

What is GSM?

GSM, which stands for Global System for Mobile communication, is a mobile communication standard that is used to transmit mobile voice and data services. It was developed as a replacement for the first generation (1G) analog networks and paved the way for the second generation (2G) networks.

• The concept of GSM emerged from a cell-based radio system being developed in Bell Laboratories in the 1970s.
• In 1982, the European Conference of Postal and Telecommunications Administrations (CEPT) set up a “Groupe Spécial Mobile” (GSM) committee to widen the standards of the existing analog network system.
• Five years later, in 1987, 13 European countries came together to sign a memorandum of understanding in Copenhagen to develop and deploy a standard cellular telephone system across Europe.
• The European Telecommunications Standards Institute (ETSI) improved on GSM as a standard in the late 1980s, and they deployed it for the first time as a commercial service in Finland in 1991.
• The United States of America adopted GSM in 1995, replacing its predecessor, the Advanced Mobile Phone System (AMPS).
• By 2000, GSM spread to other parts of the world and became the most widely used standard for mobile communication.

Popular GSM carriers:

In India:

In India, as of May 2020, four giants dominate the entire wireless mobile communication services market:

• Reliance Jio
• Airtel
• Vodafone  Idea
• BSNL

Others, such as Aircel, Hutch, and Tata Docomo, either ran out of business by being bankrupt or were bought by or merged with the four giants.

Out of these, only Airtel, Vodafone Idea, and BSNL provide GSM services such as GPRS and EDGE. This is the reason why Reliance Jio subscribers face issues when they move to an area with poor connectivity, and never see the ‘E’ (which stands for EDGE) or ‘G’ (which stands for GPRS) symbols next to the network connectivity icon.

In the US:

Similar to the situation in India, four giants dominate the entire wireless mobile communication services market, and all of them provide GSM services:

• AT&T Mobility
• Verizon Wireless
• T-mobile US
• Sprint Corporation

There are other minor GSM service providers as well, such as Cellular One Nation and DTC Wireless, in the US market.

GSM Architecture

There are four major components of the GSM network architecture.

Base Station Subsystem

The Base Station Subsystem (BSS) comprises the Base Transceiver Station (BTS), which is responsible for providing connectivity within a cell, and the Base Station Controller (BSC), which takes care of switching between multiple BTS.

Network Switching Subsystem

Also known as the Core network, the Network Switching Subsystem (NSS) consists of the Mobile Switching Centre and its associated components. The main responsibility of the NSS is to switch between multiple channels and to ensure mobility management (to be able to track the subscribers). They’re generally owned and managed by the service providers themselves and further connect to an interconnected network known as the Public Switched Telephone Network (PSTN).

GPRS core network

The GPRS core network is the central part of the general packet radio service (GPRS), which allows GSM mobile networks to transmit data over the internet. It provides mobility management, session management, and transport for the IP packets (that contain the data, in the form of images, videos, or text, to be transmitted over the internet).

This network is also responsible for the billing of subscribers by keeping a note of their data, call history, and usage.

Operations Support System (OSS)

The Operations Support System (OSS) is used by service provides to take care of their cellular network. It mainly performs the following functions:

• Network Inventory – keeping track of all the components in the network.
• Service Provisioning – preparing a network to allow it to provide a new service for the customers.
• Network Configuration – the process of making sure that the performance and functionalities of the components are never compromised.
• Fault Management – a set of operations that detect, isolate, and manage malfunctions in the network.

GSM Subscriber Services

Teleservices

Teleservices, or telephony services, refer to the ability of the GSM service provider to transfer data. The ‘data” could be in the form of:

• Voice calls – This includes the ability to transmit audio data in real-time, which is done at a rate of 13 kbps.
• Facsimile – More commonly known as “faxing,” this involves sending over a copy of a piece of text and reproducing it as close as possible to the source.
• Short Message Service (SMS) – This involves sending and receiving short text messages over the mobile network itself, without the need of being connected to the internet.

Data Services

The Data Services, also known as the Bearer Services, revolve around services that help in connecting the user to the internet, by providing packet transmission capabilities. They are generally divided into two, based on the connection that the data services use:

• Circuit Switched Data – For a continuous connection, GSM circuit-switched data is the reserving of a continuous path of transmission resources from a sender to a receiver. This means that the data transmission path is always available even if there is no data to send.
• Packet Switched Data – GSM packet-switched data is the sending of data that is divided into small packets that can take different paths through a packet data network, which provides a “bursty” connection. The paths are dynamically allocated and are not reserved, like in the case of Circuit Switched Data.

Additional Services

They are the services that are provided in addition to the telephony and data services. They include call identification, call forwarding, call waiting and holding, multi-party conversations, and call barring.

GSM Specifications

Speech Signal Processing

While making a call, the speech data cannot be sent as it is over the channel. It has to be encoded into some form of digital data that can be transmitted. To convert the audio data into bits, signal processing happens behind the scenes, steps of which are given below:

• Speech Coding – The voice signal of the speaker is sampled every 20 ms, and 260 bits are provided for each of these 20 ms samples, resulting in a net bit rate of 13 kbps. GSM takes advantage of the fact that most conversations, as a whole, are filled with silences, which can be filtered out with a Voice Activity Detector (VAD), and codes only those samples that contain speech.
• Channel Coding – After speech coding, the resultant 260 bits for every 20 ms are ordered into groups for error protection. The grouping is done based on their significance in contributing to speech quality.
  • The most important 50 bits, called type 1a bits, have 3 CRC (Cyclic-redundancy check code) bits added to them to detect the presence of errors at the receiver end.
  • The next 132 bits, known as the type 1b bits, are added to the first 53 bits and reordered. Then four additional bits are added to obtain a data block of length 189.
  • Then, for error protection, the 189 bits are passed through a rate 1/2 convolutional encoder, which outputs a block twice the input size (378 bits).
  • The least import 78 bits, called the type 2 bits, are not provided with any error protection and are appended as it is to the existing 378 bits to get the final frame of size 456 bits.
• Interleaving – The 456 encoded bits are further broken down into eight 57 bit blocks, and they are sent over consecutive time slots.
• Burst Formatting – It is the process of adding binary data to the blocks to help in synchronization and equalization of the received signal. The added bits result in a larger bit rate.
• Ciphering – Ciphering modifies the contents of the eight interleaved blocks through the use of encryption techniques, which are known only to the particular mobile station and BTS. This is done to ensure security.
• Modulation – GSM employs Gaussian Minimum Shift Keying (GMSK) to modulate the ciphered signal on to the phase of the carrier signal.

GSM Channels

In telecommunication, a channel is a path over which data can be transmitted. These channels are mainly split into two types, which are discussed below:

Physical Channel

Physical channels refer to a combination of time slots and carrier frequency that must be established between the receiver (like the MS) and transmitter (like the BTS) for communication. The physical channel is one that could be established over the air or a physical medium like optic fiber cables.

Logical Channel

The Logical Channels constitute the data being sent in the physical channel. In other words, the data being sent is mapped onto the physical channel by defining several logical channels.

The logical channels are further divided into two, which are given below.

Traffic Channels (TCH)

Traffic Channels (TCH) are used to carry encoded speech or user data. There are only two types of Traffic Channels:

• Full Rate Traffic Channels (TCH/F) – carries information at a gross bit rate of 22.8 kbps.
• Half Rate Traffic Channels (TCH/H) – carries information at a gross bit rate of 11.4 kbps.

Channels dedicated to encoded speech are defined for both TCH/F and TCH/H.

Data channels can operate at rates different from the default bit rate. For Half Rate Channels, the bit rates could be 2.4 kbps (TCH/H2.4) or 4.8 kbps (TCH/H4.8). For Full Rate Channels, in addition to TCH/F2.4 and TCH/F4.8, the data can also be transmitted at 9.6 kbps (TCH/F9.6).

Control Channels (CCH)

The Control Channels (CCH) are used to carry synchronization and signaling data between the BTS and the MS. The CCH is broadly divided into three categories, which are given below.

Broadcast Channels (BCH)

They are used to broadcast synchronization and general network information to all the Mobile Stations in the cell. As the transmission only takes place from the BTS to the MS, BCH is a downlink channel.

There are three types of Broadcast Channels:

• Frequency Correction Channel (FCCH) – Used in frequency correction/ synchronization with the MS. The frequency mentioned here is the frequency that the MS must transmit at for the physical channel with the BTS to be established.
• Synchronization Channel (SCH) – The SCH is used to synchronize the MS time-wise with the BTS.
• Broadcast Control Channel (BCCH) – The BCCH is used to broadcast control information to all the MS. The control information could include the control channel configuration used at the BTS and a number of parameters that are used by the MS when accessing the BTS.

Common Control Channels (CCCH)

The common control channels are used by an MS during the paging and access procedures. Common Control Channels are of four types:

• Paging Channel (PCH) – The PCH is used by the BTS when it wants to contact the MS, the reason for which could be an incoming message or call.
• Random Access Control Channel (RACH) – The RACH comes into play when the MS wishes to access the network, most probably when it initiates a call or sends out a text.
• Access Grant Control Channel (AGCH) – The AGCH is used to instruct the MS to operate in a particular physical channel, and also to specify details of the dedicated channel (which we shall see next) for further communication.
• Cell Broadcast Channel (CBCH) – The CBCH is used to broadcast messages to all MS in a cell.

Of the four, RACH is an uplink channel, while the other three are downlink channels.

Dedicated Control Channels (DCCH)

The Dedicated (and Associated) Control Channels are used to carry signaling information between the MS and the BTS. There are two types of Dedicated Control Channels:

• Standalone Dedicated Control Channel (SDCCH) – The SDCCH is used to inform the MS about which physical channel is to be used for TCH. Additionally, the SDCCH also carries information for call forwarding.
• Associated Control Channel (ACCH) – The ACCH is generally associated with the SDCCH or TCH and is used to carry out information with the process being carried out in the channel it’s associated with. The ACCH is further subdivided into two:
  • Slow Associated Control Channel (SACCH) – The SACCH is used by the BTS to transmit power control and timing information to the MS and is used by the MS to transmit signal strength indicator and link quality report to the BTS. SACCH messages are sent once every 480 ms.
  • Fast Associated Control Channel (FACCH) – The FACCH generally is used in place of a TCH, and is said to steal bursts from TCH and insert information of its own. It is used to carry out user authentication. A complete FACCH message may be transmitted every 20 ms.

Frame Structure

• In GSM, a frequency band of 25 MHz is allocated for transmission, which is further divided into smaller bands, which are 200 kHz wide, giving us a total of 125 bands, and each band houses an RF carrier.
• One carrier band of these bands is used as a guard channel to separate the GSM bands from the other bands, resulting in 124 channels that can be used for communication.
• Each of these RF bands is further divided into 8-time slots. The division of the 25 MHz into channels is known as FDMA and the further division into time slots as TDMA.
• The splitting of the band into 8-time slots means that eight different conversations can be carried out on the same band at the same time, with each time slot having a duration of 577 microseconds, which is also known as a burst.
• These 8-time slots together constitute the most basic component of the GSM frame structure, which is called a frame.

The next component is the multiframe, which is of two types:

• Traffic multiframes – The Traffic Channel frames are organized into multiframes consisting of 26 bursts taking 120 ms. Out of these 26 bursts, 24 are reserved for traffic, one for SACCH and the other one is left empty.
• Control multiframes – They comprise of 51 bursts and last for 235.4 ms, and are divided into logic channels, which are time-scheduled and primarily used for frequency correction and synchronization.

Superframes are a collection of multiframes that are to last for 1326 bursts or 6.12 seconds. Hence, they consist of either 51 traffic multiframes or 26 control multiframes.

Finally, 2048 superframes collectively form a hyperframe, which has a duration of roughly 3 hours and 28 minutes.

The following image gives an idea of the frame structure.

Having the GSM frame structure enables the data to be organized logically so that the system can handle the data correctly. It also provides the basis for the various physical channels used within GSM and is at the heart of the overall system.

Types of handover in GSM

As we saw in the post on types of handoffs in mobile communication, a handoff occurs when the MS moves away from the jurisdiction of a BTS/BSC/MSC and enters into another. In GSM, handovers are generally classified into four major categories:

• Intra-BTS Handover – This occurs either to optimize the load in a cell or when a change in frequency or slot (the channel) is requested by the MS due to interference or some other issue. Here, the MS remains connected to the same BTS throughout the process.
• Inter-BTS, Intra-BSC Handover – This occurs when the MS moves out of the cell but stays within the coverage of the same BSC. In this case, the BSC performs the handover and assigns a slot and frequency for the MS with the BTS of the cell it had entered, before releasing the old BTS from communication.
• Inter-BSC, Intra-MSC Handover – This occurs when the MS moves out of the cells under a BSC, into another. Here the handover happens between the BSCs, in addition to between the BTSs. The MSC handles the handover.
• Inter-MSC Handover – This form of handover occurs when changing between networks. The two MSCs involved negotiate with each other to control the handover.

Working of Intra-MSC handover in GSM

To know what happens behind the scenes in performing a handover, we consider an Inter-BSC, Intra-MSC Handover, assuming that the MS is travelling from one BSC to another, both of which are within the same MSC.

• The MS continuously sends measurement reports to the BTS, which contains information regarding the signal strength it receives from different BTS and traffic volume. The BTS forwards these reports to the BSC.
• As soon as the signal strength goes below a threshold, the BSC readies up to initiate a handover.
• In our case, the BTS, which is closest to the MS (which can be found from the measurement reports), is under another BSC, and hence the old BSC requests the corresponding MSC to take care of the handover.
• The MSC sends a request to the new BSC and instructs it to allocate a channel for the MS.
• Once the channel is set up and acknowledged by the new BSC, the resources of the old BSC are released, and the user can communicate once again.

Security in GSM

GSM is easily the most widely used and one of the oldest mobile communication standards in the world. To keep user data safe while accessing the network, GSM has a few standardized security methods up its sleeve to ward off eavesdroppers and other miscreants. GSM maintains end-to-end security by retaining the confidentiality of calls and anonymity of the GSM subscriber.

Mobile Station Authentication

• The GSM network authenticates the identity of the subscriber through the use of a challenge-response mechanism.
• The BTS sends a random number to the MS, which then generates a signed response based on the encryption of the random number that was sent using the A3 Algorithm, with a unique key called the individual subscriber authentication key that is assigned to the MS and sends it back to the BTS.
• Upon the receipt of the response, the BTS performs the calculation again to verify if the user is authorized to access the network.
• If the value calculated by the BTS doesn’t match the MS, the connection is immediately terminated.

An additional safety measure in a Mobile Station device is the secret PIN (Personal Identification Number) that can be used in case the SIM card gets stolen.

Data Confidentiality

Voice and Signalling data sent from the MS is encrypted before transmission to evade eavesdroppers. After successful authentication, the A8 algorithm is employed to generate a key from the same random number sent initially by the BTS to generate a new key that is then used to encrypt all transmissions.

Subscriber Identity Confidentiality

To provide User Anonymity, a temporary identity is assigned to every MS called the Temporary Mobile Subscriber Identity (TMSI). The TMSI is issued by the BTS, and to the BTS, the MS is known by its TMSI. Once outside a cell, a new TMSI is assigned at every location update (LUP).

Advantages of GSM

Some of the advantages of GSM are:

• GSM has been around for a long time, and hence GSM devices are widely available and supported across the world.
• GSM is generally quite cost-effective.
• The fact that GSM Base Stations are available across the world helps in providing seamless wireless connectivity. This helps users to avail data and voice services without any disruption, and hence, international roaming is not a concern.
• The phone works based on the SIM card and is quite easy to switch over from one phone to another.
• GSM signals do not deteriorate much in the presence of obstacles and are often the only means of access to the network in remote locations.

Disadvantages of GSM

Some of the disadvantages of GSM are:

• Many of the GSM technologies are patented by Qualcomm, and hence licenses need to be obtained from them.
• GSM provides limited data rate capability. The user must employ advanced versions of GSM for higher data rates.
• GSM uses the FTDMA access scheme. Here multiple users share the same frequency range and hence can lead to interference when more users are on the network at the same time.
• GSM sends data in bursts, which can interfere with certain electronic apparatuses. It is because of this fact that airplanes, petrol bunks, and hospitals urge people to prevent the use of GSM-based devices.
• GSM consumes a lot of the space available in the spectrum and is not compensated by the quality of the services provided, making service providers move towards other standards such as LTE.

Future of GSM

With the entry of LTE (4G) in the market and its ability to be able to offer higher data rates and better voice quality, GSM’s future was under threat. Many service providers across the world have started decommissioning their GSM networks to reallocate the spectrum to the existing LTE and the upcoming 5G technology standards. The first service provider to do so was Telstra in Austalia in 2016, followed by AT&T in the United States in 2017. In April 2017, Singapore became the first country to shut down its 2G services entirely.

In India, Jio is the only service provider to not provide GSM services to its customers. As of now, the other three giants, Airtel, Vodafone-Idea, and BSNL, still provide GSM services as they are the only means of communication for people living in remote areas. But, with rapid urbanization and development, towers are being set up everywhere, making GSM redundant and unnecessary as a service.

Difference between GSM and EDGE

Though EDGE is an enhancement of GSM itself, there are a couple of factors that set the two apart.

Difference between GSM and AMPS