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GSM (Groupe Spéciale Mobile)

The most successful mobile digital communications system in today’s world is GSM, with networks in over 130 countries and more than 100 million users worldwide. Back at the start of the 1980’s Europe was facing a big problem, there were many existing analogue mobile networks, which were based on similar standards, for example NMT 450, however they were all running on slightly different carrier frequencies. To avoid this problem in the second generation mobile phone system, the Groupe Spéciale Mobile (GSM) was created in 1982. Now the system developed by this group is known as global system for mobile communications (GSM).

The primary goal of GSM was to provide a mobile phone system that would allow it’s users to use their mobile phone in any European country i.e. Roaming. This system would have to provide voice services comparable with ISDN and other PSTN systems. The initial specification details were over 5000 pages, with the new services in particular data services there are even more specification details.

GSM is a typical second generation system, replacing the old analogue first generation system, however it still does not offer worldwide high data rates but will be offered in the new third generation system UMTS. GSM was initially deployed in Europe using the 890 – 915 MHz band for the uplinks and the 935 – 960 MHz band for the downlinks. This version is commonly known as GSM 900, and later version GSM 1800 (1710 – 1785MHz uplink, 1805 – 1880MHz downlink) which commonly known as DCS (Digital Cellular System)

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GSM Bearer Services

GSM specifies different mechanisms for data transmission for data transmission for the original GSM allowed for data rates up 9600 bit/s for non-voice services. Bearer services allow for both transparent and non-transparent, synchronous or asynchronous data transmission. Transparent bearer services use only the functions of the physical layer (layer 1 ISO/OSI reference model) to transmit …

GSM Cell Handover

GSM systems require a procedure known as a Handover to maintain the continuity of the call. This is because a single cell does not cover the whole service area e.g. a whole city or country. However a single cell has a maximum service area of approximately 23 miles (35 km) for each antenna (Tripathi, et …

GSM Encryption

In the GSM system there is a large amount of user related information transmitted over the air interface and since this is the weakest part of GSM system it uses encryption to ensure privacy. After the user has been authenticated, the MS and the BTS can start using encryption by applying the cipher key Kc. …

GSM Localisation and Calling

One of the main features of GSM system is the automatic, worldwide localisation of it’s users. The GSM system always knows where a user is currently located, and the same phone number is valid worldwide. To have this ability the GSM system performs periodic location updates, even if the user does not use the MS, …

GSM Logical Channels and Frame Hierarchy

GSM specifies two basic groups of logical channel, they are the Traffic Channels and the Control Channels : Traffic Channels (TCH): The GSM network uses a TCH to transmit user data (e.g. Voice). There are two basic TCHs and they are defined as Full Rate TCH (TCH/F) and Half Rate TCH (TCH/H). A TCH/F has …

GSM Mobile Services

The GSM system permits the integration of different voice and data services and the inter-working with existing networks. Services make a network flexible to users, i.e. they can choose one network over another. GSM has three different categories of services, bearer, tele and supplementary services And these are described in the following sections. Figure 3 …

GSM Network and Switching SubSysetm

At the centre of any GSM system there is the network and switching subsystem (NSS) that connects the GSM network with the public land network (i.e. a PSTN), performs the handovers between BSS’s, comprises functions for worldwide localization of users and supports charging, accounting and roaming of users between different networks and in different countries. …

GSM Operation SubSystem

The GSM system is broken up in to three parts, the first two parts have already been discussed, the third part of the GSM system is the operational subsystem (OSS). The OSS contains all the functions necessary for network operation and maintenance. The OSS possesses network entities of its own and accesses other entities via …

GSM Protocols

Figure 7 shows the architecture of protocols used within the GSM system, with signalling protocols, interfaces as well as the entities already shown in Figure 5. Again the main area of focus is in the Um interface, this is because the other interfaces occur between entities in a fixed network. The physical layer, Layer 1 …

GSM Radio Air Interface

One of the most interesting interfaces in the GSM network is the Um, the Radio Air Interface because it comprises many of the mechanisms used for multiplexing and media access. GSM utilises SDMA (Space Division Multiple Access) using cells with BTS and assigns an MS to a BTS. What’s more, FDD (Frequency Division Duplex) is …

GSM Radio SubSystem (RSS)

As suggested by the name, the radio subsystem is comprised of all the radio specific elements, i.e. the mobile stations (MS) and the base station subsystem (BSS). The connection between the RSS and the NSS (shown in figure x.ii) via the A interface (solid lines) and the connection to the OSS via the O interface …

GSM Security

The GSM system has several security services for security, these security services use confidential information that is stored in the AuC and in the customers SIM (Subscriber Identity Module) chip. The SIM chip may be plugged into any MS, however for the SIM chip to allow access to the MS the user must enter a …

GSM Subscriber Authentication

As I have already stated before any user can access the GSM system they must be authenticated as a valid user and this authentication is done by use of the PIN on the SIM. The SIM stores the users Individual Authentication Key Ki, the user identification IMSI and used the A3 algorithm for identification. The …

GSM Summary & Key Points

With the increase in demand for the need for communications whist away from a fixed line, GSM specifications were developed in order to provide greater capacity, improved quality and significantly greater security. Also the need to be able to use your mobile phone in other countries became of paramount importance. GSM provided significantly greater capacity …

GSM System Architecture

As with all telecommunications systems, GSM has a hierarchical and complex system architecture comprising of many entities, interfaces and acronyms figure 4 shows a simplified overview of the GSM system as specified in the ETSI (TS 101.622). GSM systems consist of three subsystems, the radio subsystem (RSS), the network and switching subsystem (NSS), and the …

GSM Tele Services

GSM is mainly focused on voice tele services and these comprise of encrypted voice transmission, message services, and basic data communication with terminals as known from the PSTN or ISDN (e.g. fax). However as the main service is telephony, the primary goal of GSM was to provide high-quality digital voice transmission, offering at least the …

Supplementary Services

Further to bearer and tele services, GSM network operators can also offer supplementary services. These services offer enhancements to the standard telephony service and may differ from operator to operator, though typical services available to the user are caller location identifier (CLI), call forwarding, or redirection.

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