GSM - Global System of Mobile Communication

Information on the structure of the GSM mobile communications system.
Focus is mainly on the AIR interface as this is of highest interest from the Mobile Station perspective.

There's no warranty or guarantee implied that the information provided is correct and up to date. You might want to check with ETSI for latest updates on the standard and further details.

The figure below shows a complete GSM - PLMN (Public Land Mobile Network).
By clicking on the different parts you will receive further information on the desired field.

GSM - Mobile Station (MS)

The mobile station contains a complete GSM system.
It consists of several layers as described by the OSI model (Open System for Interconnection).
The physical layer is called Layer 1 (L1).
The Data Link Layer presents a Link Access Protocol on D-Channel modified for mobile systems.
Network Connection Layer is represented by the Radio Resource (RR) module, which is part of Layer 3.
Also the Transport Protocol is part of Layer 3. In MS it is called Mobility Management (MM).
The Session Management is done by the Connection Management (CM) module, also part of Layer 3.

GSM - AIR Interface

The signal transmission via the AIR Interface is most interesting for mobile manufactures.
It is defined by ETSI (European Telecommunication Standardization Institute).
The frequency range from 880 MHz to 915 MHz in uplink direction (i.e.,from MS to BTS) and from 925 MHz to 960 MHz in downlink direction (i.e., from BTS to MS) contains 174 channels in case of E-GSM (s.a. table below).
A channel spacing of 200 kHz is used.
Within a GSM cell, only one frequency is used. For minimizing interference, the same bearer frequency is not repeated in any neighbouring cell.
The distance between two related uplink and downlink channels is 45 MHz. (In case of GSM 1800 it is 95 MHz.)
To serve different mobile stations in a cell the channels are divided into time bursts.
That means a FDD - TDMA (Frequency Division Duplex, Time Division Multiple Access) combination is used in GSM systems.

Standard Frequency Range in Max. Number of
Frequency Channels
Channel Spacing Duplex Spacing Number of
Uplink Downlink
E-GSM 900 880-915 MHz 925-960 MHz 174 200 kHz 45 MHz 8
P-GSM 900 890-915 MHz 935-960 MHz 124 200 kHz 45 MHz 8
GSM 1800 1710-1785 MHz 1805-1880 MHz 374 200 kHz 95 MHz 8
GSM-R 876-880 MHz 921-925 MHz 20 200 kHz 45 MHz 8

Problems of Multipath Propagation

Normally, radiated signals do not only reach its destination on a direct way i.e. by line of sight but also by reflection. At the receiver a vector sum is present as a superposition of various amplitudes and phases. This is the normal situation for radio propagation in urban areas. Several fading effects can be distinguished with different impacts on radio transmission asking for different measures to cope with.

Slow Fading

This fading type represents downlink signal attenuation caused by obstacles or natural topography, e.g. hills. To cope with it and with the distance related attenuation following the 1/r2 rule, adaptive power control is used for the uplink and downlink RF carrier. (An exception is the cell beacon carrier, that is always sending on maximum power.) Constant monitoring of the downlink path by the MS and its reports to the BTS increase or decrease the needed power. Thus interference is also reduced to a minimum level. The BTS performs the same action for the uplink path.

Selective Fading

It is caused by different propagation times due to reflection associated with distances between 1 and 5 km. As GSM uses a bit rate of 270.833 kbps with a bit duration of 3.69 μs a corresponding distance of approximately 1.1 km is done during bit transmission. This is reality in mountain and water regions.
If any signals are reflected by objects thus creating a bypass of 1.1 km the individual bits will arrive at different times and interfere in the receiver making it difficult to interpret them. GSM specified a so-called Viterbi Equalizer to cope with Inter Symbol Interference (ISI) of up to 4 bit times.
By knowing the fixed bit pattern of the training sequence of every received burst the system is able to calculate the inverse function of a filter corresponding to the very actual situation on the air interface. Each time the system models the air interface it passes the fixed 26 bit pattern through it and compares it with the received information.
Once the output matches with the received information, the system created a filter that represents with a certain accuracy the present air interface situation. Thus the useful bits of a burst can be reconstructed and processed in the following parts.
The Viterbi algorithm is chosen as it best guesses and eliminates unlikely combinations.

Flat Fading

This fading type is caused by vector sums of signals reflected by nearby objects. Superposition may be beneficial but in the more likely case can also weaken the amplitude to a minimum or even zero. This effect is strictly related to the used wavelength, i.e. approx. 34 cm in GSM 900 and 17 cm in GSM 1800.
Every half wavelength the sum might be at its maximum, every half the minimum. Thus fading dips might occur approx. every 17 or 8.5 cm decreasing the incoming energy beyond the receiver sensitivity (loss of signal).
This phenomenon is also related to the travel speed of the MS. The faster it is the less the MS will camp on the location with the critical conditions.
RF related procedures to minimize losses are Frequency Hopping and BTS RX Antenna Diversity.
Signal processing procedures are Channel Coding and Interleaving.

GSM - Terrestrial Interfaces

GSM - A Interface

This fully standardized interface is located between TRAU and MSC, i.e., between BSS and NSS. The network operator thus is enabled to link subsystems of different suppliers.
The basic frame structure is a PCM 30 frame at 2,048 kbit/s. Timeslot 0 (TS0) is used for synchronization (Frame Alignment Signal, Not Frame Alignment Signal, Cyclic Redundancy Check, Double Frame Check).
In even frames FAS is used to synchronize the line (C0011011, with C is the result of the CRC4 check).
In odd frames NFAS informs about alarms (X10NYYYY, with X contains the MFAS (R) or the report of the CRC4 check (E), N is a minor alarm of the distant part and Y is reserved for national alarm specifications).
After synchronizing on FAS the system looks for the Multiframe Alignment Signal (R1-R6, contained in the first bit of the NFAS) that is always "001011".

Timeslot 16 (TS16) is used for SS7 signaling. The remaining 30 timeslots are used to carry Traffic Channels (TCH), that are A-law coded with 8 kHz and 8 bits. This leads to 64 kbit/s/user.

GSM - Ater Interface

This supplier dependent interface is physically located between BSC and TC (TRAU). Due to the GSM specialties in speech coding and data rate adaptation it can carry up to 4 times more TCH on a physical 2,048 kbit/s link than the A interface. Thus enables the network operator to economize BSS transmission links if TC is installed at MSC site. A PCM 30 with 2,048 kbit/s is used for this link. The 32 timeslots are subdivided into 4 * 16 kbit/s subtimeslots (fullrate) or 8 * 8 kbit/s subtimeslots (halfrate). The High Density Bipolar Code 3rd order (HDB3) is used as line code. Timeslot 0 is used for synchronization.

GSM - Abis Interface

The interface is located between BTS and BSC. It is not completely specified by ETSI because it contains supplier specific Operation and Maintenance (OM) information to control the connected BTS. A LAPD protocol is used.

GSM - Base Station System (BSS)

Base Station System

GSM - Base Transceiver Station (BTS)

The Base Transceiver Station controls the Transceivers (TRX). It is connected to the Mobile Station via the AIR Interfaces. Modulation, encryption, output power control, protocol for signaling and the protection of user information is done here. Up to 3 antennas (2 RX, 1 TX) with different antenna characteristics are generating an omnidirectional or a sectorized cell. The BTS throughconnects calls, signaling and O&M information to the BSC via the Abis Interface.

GSM - Base Station Controller (BSC)

Radio Resource Management is one of the tasks of the BSC. It controls the available traffic channels (TCHs) and the maximum BTS / MS power. Intra BSC handover are performed by the BSC. Calls and signaling are throughconnected to the TC (TRAU) via the Ater Interface. O&M information is throughconnected to the OMC via a X.25 Interface.

GSM - Transcoding and Rate Adaptation Unit (TC / TRAU)

The TRAU module handles the main GSM problem that is transmission of high data rates and restricted radio resources.
User data rate is reduced by two means, speech transcoding and data rate adaptation.
Speech transcoding means the replacement of the original speech by description parameters. It is done by the LPC-LTP-RPE codec which is a digital signal processor as a part of the transcoder. Therefor the Fullrate Speech signal is converted and segmented. Then the energy is analyzed by a Linear Predictive Coding (LPC) and the basic frequency and harmonics (color of the voice) is analyzed by a Longterm Prediction (LTP) and Regular Pulse Excitation (RPE). That leads to 260 bit / 20 ms. That means from 64 kbit/s original signal to 13 kbit/s.
Enhanced full rate codec reduces to 12.2 kbit/s plus 0.8 kbit/s additional CRC information. The half rate codec will use only 5.6 kbit/s thus enabling network operators doubling capacity on the air interface without any additional hardware.

The data rate adaptation is done from 64 kbit/s (V.110 format) to 16 kbit/s and vice versa (RA2 adaptation).

A TRAU frame is used to carry the resulting information in both cases.

A voice activity detector is used to detect speech information. In cases of break the background noise is analyzed, transferred to the TC that automatically reproduces the background as "comfort noise" and the transmitter is switched off.

GSM - Network Switching Subsystem (NSS)

Network Switching Subsystem

GSM - Mobile Services Switching Center (MSC)

The Mobility and Call Management of one or more BSS is task of the MSC. Also the adaptation to the 64 kbps V.110 format for data users by the Interworking Function (IWF) is performed here. A toll ticket generation takes place here.

GSM - Gateway MSC (GMSC)

Additionally, the MSC is the gateway to a fixed PTT network or another PLMN (Public Land Mobile Network). It is then called Gateway MSC.

GSM - Home Location Register (HLR)

This is a permanent storage of subscriber data. Ordered services and access profile are can be found here. And the actual address of the MSC / VLR is stored here when the subscriber is roaming.

GSM - Visitor Location Register (VLR)

This volatile storage contains subscriber data needed to control calls. Information on the MS location (Location Area) can be found here. It is used in roaming cases.

GSM - Authentication Center (AuC /AC)

This block checks the subscriber's permission to access the network. Therefor data is compared with the information stored on the SIM card. The ciphering parameters for the MS to BTS encryption are provided by the AC (AuC).

GSM - Equipment Identification Register (EIR)

This unit is a world wide data base located in Dublin, Ireland.
It can be used to prevent use of stolen mobiles by checking the IMEI (International Mobile Equipment Identity).
Also it is possible to restrict terminals causing interference.
This identity number is implemented by the manufacturer of the mobile.
Most service providers do not make use of the possibilities of this data base.

Additional GSM Components

GSM - Billing Center (BC)

Release and blocking of the SIM card is done here.

GSM - Short Message Service Center (SMS-C)

This is a manual operated access point for Short Message Services (SMS). The transfer of SMS to the MS is done by this Service Center. Messages are stored and forwarded if the MS is not available. Also Cell Broadcast messages are possible (e.g. cell specific information to all Mobile Stations within this cell).

GSM - Voice Mail System (VMS)

A digital mailbox within the network is represented by this module. It can be used for speech and fax calls.

GSM - Operation & Maintenance System (OMS)

GSM - Operation and Maintenance Center (OMC)

Central control of one or more BSS is possible here. This is used for remote fault management, remote configuration and software management and remote performance management.

GSM - Network Management Center (NMC)

This tool is used for centralized network planning and optimization.

Mobile Station (MS) AIR Interface Base Transceiver Station (BTS) Abis Interface Base Station Controller (BSC) Ater Interface Transcoding and Rate Adaptation Unit (TRAU) A Interface Mobile Services Switching Center (MSC) Billing Center (BC) Home Location Register (HLR) Authentication Center (AuC) Equipment Identification Register (EIR) Gateway MSC (GMSC) Visitor Location Register (VLR) Network Switching Subsystem (NSS) Base Station System (BSS) Network Management Center (NMC) Operation and Maintenance Center (OMC) PSTN, PTT, ISDN Short Message Service Center (SMSC)