GSM Tutorial: Basics, Architecture, Interfaces & Protocol Stack

Introduction

The full form of GSM is Global System for Mobile Communications. It’s also known as 2G or Second Generation technology. GSM is globally accepted for mobile communication, supporting voice, text messaging, and data services through its networks. GSM ensures interoperability and seamless roaming across different countries.

There are various GSM standards such as GSM900, EGSM900, GSM1800, and GSM 1900 based on RF carrier frequency bands and bandwidths. This GSM tutorial covers GSM basics, architecture with interfaces, frame structure, channel & burst types, protocol stack layers, and its unique benefits.

GSM Network Architecture

The GSM system consists of a Mobile Station, Base Station Subsystem, and Network and Operation Subsystem. This is illustrated in the block diagram below.

Mobile Station

The Mobile Station is the GSM mobile phone equipment, housing the DSP, RF chip, and SIM (Subscriber Identity Module). The SIM is sufficient to avail the service of the GSM network. It contains all subscriber-related information, the network with which the subscriber is subscribed, and encryption-related information.

Base Station Subsystem

The Base Station Subsystem houses the Base Transceiver Station (BTS) and Base Station Controller (BSC). This subsystem handles radio control-related functions and provides the GSM air interface for GSM mobile phones to connect with the GSM network.

To provide GSM service, a region/city is divided into various cells. The cell size is usually about 100m to about 35 km. BTS coverage is limited to this cell. Many BTSs cover the entire region. All these BTSs are interfaced with one BSC in various ways (mesh, star, etc.).

The BSC handles radio frequency assignments to the mobile phones and takes care of handoff within the BSS, i.e., between one BTS and another.

Network Subsystem (NSS)

This subsystem provides an interface between the cellular system and the circuit-switched telephone network (PSTN). It performs switching and operation & maintenance-related functions. The NSS takes care of call processing functions such as call setup, switching, tear down, and handover between BSCs. It also manages security and authentication-related functions.

There are various network elements in this subsystem, as mentioned in the GSM network architecture. They are explained below. These are basically database elements.

• HLR (Home Location Register): Stores permanent and temporary subscriber-related information.
• VLR (Visitor Location Register): Stores visitor subscriber-related information about its facilities, the network it is subscribed to, and its home location, etc.
• AUC (Authentication Center): Used to authenticate activities in the system. It holds encryption (A5 key) and authentication keys (A3 key) in both HLR and VLR.
• EIR (Equipment Identification Register): Helps in security by keeping track of the equipment type available in the Mobile Station or Terminal.

GSM Network Interfaces

• Air interface: Between Mobile Station and BTS
• Abis interface: Between BTS and BSC
• A interface: Between BSC and MSC
• SS7 interface: Between MSC and PSTN

GSM System Specifications

• Access Method: TDMA/FDMA
• Uplink frequency band: 890 to 915 MHz
• Downlink frequency band: 935 to 960 MHz
• System Bandwidth: 200 KHz
• No. of frequency channels or ARFCN (Absolute Radio Frequency Channel Number): 124
• Users per channel: 8
• Frame duration: 4.615ms
• Spectral efficiency: 1.35 b/s/Hz
• Data rate per user: 33.6 kbps (270.833 kbps Gross data rate for 8 users/8users)

GSM Frame Structure

The GSM frame structure is designated as hyperframe, superframe, multiframe, and frame.

The minimum unit, being a frame (or TDMA frame), is made of 8 time slots.

One GSM hyperframe is composed of 2048 superframes.

Each GSM superframe is composed of multiframes (either 26 or 51 as described below).

Each GSM multiframe is composed of frames (either 51 or 26 based on the multiframe type).

Each frame is composed of 8 time slots.

Hence, there will be a total of 2715648 TDMA frames available in GSM, and the same cycle continues.

As shown in the GSM hyperframe diagram, there are two variants to the GSM multiframe structure:

1. 26 frame multiframe: Called a traffic multiframe, composed of 26 bursts in a duration of 120ms. Out of these, 24 are used for traffic, one for SACCH, and one is not used.
2. 51 frame multiframe: Called a control multiframe, composed of 51 bursts in a duration of 235.4 ms. This type of multiframe is divided into logical channels. These logical channels are time-scheduled by the BTS. They always occur at the beacon frequency in time slot 0. It may also take up other time slots if required by the system (for example, 2, 4, 6).

The following are possible channel combinations in the GSM system, which the network (BTS) will adopt based on the need for traffic channels versus signaling (control) GSM channels. They are called combined and non-combined types.

Noncombined 51-frame multiframe configuration

Following are the two GSM time slot hierarchies used in combined and noncombined 51 frame multiframe configurations.

TS0

TS1

Follow the link below for the complete chart of these configurations for TS0 and TS1.

In a noncombined configuration, dedicated signaling channels are not combined with BCCH/CCCH and thus require a separate time slot (TS1). FCCH, SCH, BCCH, and CCCH channels are mapped on TS0.

Combined 51 frame multiframe configuration

Follow the link below for the complete chart of these configurations for TS0.

In a combined configuration, FCCH, SCH, BCCH, and CCCH channels are present along with SDCCH on time slot TS0. Hence, dedicated signaling channels SDCCH are combined with BCCH/CCCH on the same time slot TS0. SDCCH can also be mapped on TS1 in addition to TS0; even SDCCH can be mapped onto any other time slots also.

GSM Channels

There are two main types of GSM channels: physical channel and logical channel.

• Physical channel: Specified by a specific time slot/carrier frequency.
• Logical channel: Run over the physical channel, i.e., logical channels are time-multiplexed on physical channels; each physical channel (time slot at one particular ARFCN) will have either a 26 Frame MF (Multi-frame) or a 51 Frame MF structure described here.

Logical channels are classified into traffic channels and control channels.

• Traffic channels: Carry user data.
• Control channels: Are interspersed with traffic channels in well-specified ways. There are various control channels such as BCCH (Broadcast control channel), SCH (synchronous channel), FCCH (Frequency control channel), and DCCH (Dedicated control channel).

Traffic channel is designated as TCH.

GSM Burst Types

GSM burst types include normal burst, frequency correction burst, synchronization burst, dummy burst, and access burst. Each of these bursts is used for specific purposes.

• Normal burst: Used for all common applications, including the exchange of signaling traffic and user traffic.
• Access burst: Used by the mobile to make the first attempt to communicate with the network (i.e., BTS).

Before normal bursts and access bursts are used, the BTS in GSM uses two special bursts known as the frequency correction burst and synchronization burst in order to allow the mobile subscriber (MS) to synchronize in frequency and time.

GSM Protocol Stack

The figure depicts GSM protocol stack layers at various GSM network elements viz. MS, BTS, BSC and MSC.

GSM Physical Layer

The GSM physical layer, i.e., layer-1, sits below GSM layer-2 (LAPDm).

As shown, it consists of various modules viz. cyclic encoder, convolution encoding, interleaving, ciphering, burst formation, differential encoding, GMSK modulation, and RF upconversion.

Advantages or Benefits of GSM technology

Following are the benefits or advantages of GSM technology compared to other analog and digital mobile technologies.

1. GSM supports international roaming across most countries, offering seamless connectivity for users traveling globally. Other standards may have limited or incompatible roaming capabilities.
2. GSM uses SIM cards, allowing users to easily switch devices without changing numbers or subscriptions. This flexibility isn’t as prevalent in some other standards that rely on fixed device identification.
3. GSM introduced the Adaptive Multi-Rate (AMR) codec, which adjusts speech quality based on network conditions, offering better voice clarity compared to other early mobile standards.
4. GSM uses Time Division Multiple Access (TDMA), enabling more users to share the same frequency band, ensuring efficient use of limited spectrum resources compared to older standards like analog cellular systems.
5. GSM has the largest infrastructure deployment globally, making it more accessible and reliable in most regions compared to less popular or regional standards.

Conclusion

GSM technology has laid the foundation for subsequent generations of mobile communication technologies such as 3G, 4G LTE, 5G NR, and beyond while continuing to serve as a reliable mobile communication platform in various regions worldwide.

GSM network architecture has evolved further to include standards such as GSM-R (GSM Railway), EGPRS (Enhanced GPRS), GERAN (GSM/EDGE Radio Access Network), TACS, NMT, etc.

This GSM tutorial is very useful for beginners who would like to start learning GSM system basics and its core fundamentals.