What is GSM architecture

GSM network

In Germany, the GSM standard is used in the digital, circuit-switched cellular networks "D" and "E". The GSM network has been in regular operation since 1993 and is characterized by a large network structure in which small cells are integrated. An automatic handover takes place between the cells. Due to the limited range of the transmission frequencies of the base stations, the reception area is divided into a honeycomb plan with hexagonal cells.


The three subsystems of the GSM network

A GSM network consists of three subsystems: the Base Station Subsystem (BSS), formerly known as the Radio Subsystem (RSS), the Network Subsystem (NSS), also known as the Switching Subsystem (SSS), and the Operations and Maintenance Subsystem (OMS). The base station subsystem essentially consists of the radio transmission systems and the mobile station (MS). The components of the BSS system include the Base Station Controller (BSC) and the Base Transceiver Station (BTS). The network subsystem or switching subsystem takes on the switching functions and consists of the components mobile services switching center (MSC) and the gateway for external networks, the gateway MSC. The switching subsystem also includes several databases for the location directory (HLR), the visitor directory (VLR), the device directory (EIR) and the authentication center (AC).

The third subsystem of a GSM network is the operation and maintenance subsystem (OMS) in which the control and monitoring of the system takes place and whose task it is to maintain undisturbed operation.

Mediation and public network

The exchanges themselves are connected to other mobile services exchanges via connection-oriented lines using an SS7 network. It goes without saying that the transition to the public network is possible from all exchanges. In addition to the channel for the transparent full duplex transmission of user data, the Traffic Channel (TCH), and the assigned signaling channel, the Dedicated Control Channel (DCCH) with circuit-switched dial-up connection, each mobile radio transmitter also has a common control channel, the Common Control Channel (CCCH) , and a broadcast control channel, the Broadcast Control Channel (BCCH). All four, the useful channel and the three control channels, are fed to a TDMA multiplexer via an error correction system, the output of which is connected to the modulation stage. After multiplexing, the TDMA time slots are modulated onto a carrier frequency by means of GMSK modulation, Gaussian Minimum Shift Keying (GMSK).

Frequency ranges and data rates

GSM works in the 900 MHz range or in the 1.8 GHz range, specifically in the 900 MHz range on two separate radio channels, each 25 MHz wide. In the range from 890 MHz to 915 MHz, the mobile end devices transmit to the base station (uplink) on one of 124 channels, each 200 kHz wide; in the range from 935 MHz to 960 MHz, the base station sends to the end devices (downlink).

With DCS 1800 the bands are 75 MHz wide and 372 channels are available. The GSM services are divided into basic services, which in turn are divided into teleservices and bearer services. Teleservices support all functions of the layer model and offer a high level of compatibility with all fixed network services.

The GSM network offers a high level of security through various processes and systems. The personal subscriber key is stored in the authentication center for authentication. The keys are only handed over to the visitor directory (VLR) in the form of derived values. As an encryption measure, all data is transmitted in coded form. Further security measures are the frequency hopping procedure with cyclical change of the transmission frequencies and the identification and blocking of end devices.