Global System for Mobile Communication (GSM) Part-IIa

During the early 1980s, analog cellular telephone systems were experiencing rapid growth in Europe, particularly in Scandinavia and the United Kingdom, but also in France and Germany. Each country developed its own system, which was incompatible with everyone else's in equipment and operation. This was an undesirable situation, because not only was the mobile equipment limited to operation within national boundaries, which in a unified Europe were increasingly unimportant, but there was a very limited market for each type of equipment, so economies of scale, and the subsequent savings, could not be realized.

The Europeans realized this early on, and in 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study group called the Groupe Spécial Mobile (GSM) to study and develop a pan­European public land mobile system. The proposed system had to meet certain criteria:

• good subjective speech quality


• low terminal and service cost


• support for international roaming


• ability to support handhald terminals


• support for range of new services and facilities


• spectral efficiency, and ISDN compatibility

In 1989, GSM responsibility was transferred to the European Telecommunication Standards Institute (ETSI), and phase I of the GSM specifications were published in 1990. Commercial service was started in mid­1991, and by 1993 there were 36 GSM networks in 22 countries, with 25 additional countries having already selected or considering GSM . This is not only a European standard - South Africa, Australia, and many Middle and Far East countries have chosen GSM. By the beginning of 1994, there were 1.3 million subscribers worldwide . The acronym GSM now (aptly) stands for Global System for Mobile telecommunications.

The developers of GSM chose an unproven (at the time) digital system, as opposed to the then­standard analog cellular systems like AMPS in the United States and TACS in the United Kingdom. They had faith that advancements in compression algorithms and digital signal processors would allow the fulfillment of the original criteria and the continual improvement of the system in terms of quality and cost. The 8000 pages of the GSM recommendations try to allow flexibility and competitive innovation among suppliers, but provide enough guidelines to guarantee the proper interworking between the components of the system. This is done in part by providing descriptions of the interfaces and functions of each of the functional entities defined in the system.

GSM -> Services Provided by GSM

From the beginning, the planners of GSM wanted ISDN compatibility in services offered and control signalling used. The radio link imposed some limitations, however, since the standard ISDN bit rate of 64 kbps could not be practically achieved.

Using the ITU­T definitions, telecommunication services can be divided into bearer services, teleservices, and supplementary services. The digital nature of GSM allows data, both synchronous and asynchronous, to be transported as a bearer service to or from an ISDN terminal. Data can use either the transparent service, which has a fixed delay but no guarantee of data integrity, or a non­transparent service, which guarantees data integrity through an Automatic Repeat Request (ARQ) mechanism, but with a variable delay. The data rates supported by GSM are 300 bps, 600 bps, 1200 bps, 2400 bps, and 9600 bps .

The most basic teleservice supported by GSM is telephony. There is an emergency service, where the nearest emergency­service provider is notified by dialling three digits (similar to 911).

Group 3 fax, an analog method described in ITU­T recommendation T.30, is also supported by use of an appropriate fax adaptor. A unique feature of GSM compared to older analog systems is the Short Message Service (SMS). SMS is a bidirectional service for sending short alphanumeric (up to 160 bytes) messages in a store­and­forward fashion. For point­to­point SMS, a message can be sent to another subscriber to the service, and an acknowledgement of receipt is provided to the sender. SMS can also be used in a cell­broadcast mode, for sending messages such as traffic updates or news updates. Messages can be stored in the SIM card for later retrieval .

Supplementary services are provided on top of teleservices or bearer services, and include features such as caller identification, call forwarding, call waiting, multi­party conversations, and barring of outgoing (international) calls, among others.

Mobile Station

The mobile station (MS) consists of the physical equipment, such as the radio transceiver, display and digital signal processors, and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to all subscribed services irrespective of both the location of the terminal and the use of a specific terminal. By inserting the SIM card into another GSM cellular phone, the user is able to receive calls at that phone, make calls from that phone, or receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI), identifying the subscriber, a secret key for authentication, and other user information. The IMEI and the IMSI are independent, thereby providing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number.

Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). These communicate across the specified A­bis interface, allowing (as in the rest of the system) operation between components made by different suppliers.

The Base Transceiver Station houses the radio tranceivers that define a cell and handles the radio­link protocols with the Mobile Station. In a large urban area, there will potentially be a large number of BTSs deployed. The requirements for a BTS are ruggedness, reliability, portability, and minimum cost.

The Base Station Controller manages the radio resources for one or more BTSs. It handles radio­channel setup, frequency hopping, and handovers, as described below. The BSC is the connection between the mobile and the Mobile service Switching Center (MSC). The BSC also translates the 13 kbps voice channel used over the radio link to the standard 64 kbps channel used by the Public Switched Telephone Network or ISDN.

Network Subsystem

The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and in addition provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. These services are provided in conjuction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the public fixed network (PSTN or ISDN), and signalling between functional entities uses the ITU­T Signalling System Number 7 (SS7), used in ISDN and widely used in current public networks.

The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call­routing and (possibly international) roaming capabilities of GSM. The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile. The current location of the mobile is in the form of a Mobile Station Roaming Number (MSRN) which is a regular ISDN number used to route a call to the MSC where the mobile is currently located. There is logically one HLR per GSM network, although it may be implemented as a distributed database.

The Visitor Location Register contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. Although each functional entity can be implemented as an independent unit, most manufacturers of switching equipment implement one VLR together with one MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, simplifying the signalling required. Note that the MSC contains no information about particular mobile stations - this information is stored in the location registers.

The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and ciphering of the radio channel.

GSM -> Radio Links

• structure


• Speech coding


• Channel coding and modulation
  
• Multipath equalization
  
• Frequency hopping
  
• Discontinuous transmission


• Discontinuous reception


• Power control
  

The International Telecommunication Union (ITU), which manages the international allocation of radio spectrum (among other functions) allocated the bands 890-915 MHz for the uplink (mobile station to base station) and 935-960 MHz for the downlink (base station to mobile station) for mobile networks in Europe. Since this range was already being used in the early 1980s by the analog systems of the day, the CEPT had the foresight to reserve the top 10 MHz of each band for the GSM network that was still being developed. Eventually, GSM will be allocated the entire 2x25 MHz bandwidth. Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time­ and Frequency­Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz bandwidth. One or more carrier frequencies are then assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme, into eight time slots. One time slot is used for transmission by the mobile and one for reception. They are separated in time so that the mobile unit does not receive and transmit at the same time, a fact that simplifies the electronics.

In the rest of this section, the procedure involved in digitally transmitting a voice signal in a GSM network is examined, along with some of the features, such as discontinuous transmission and reception, used to improve voice quality, reduce the mobile unit's power consumption, and increase the overall capacity of the network.

Channel structure

The structure of the most common time­slot burst is shown in Figure 2. A total of 156.25 bits is transmitted in 0.577 milliseconds, giving a gross bit rate of 270.833 kbps. There are three other types of burst structure for frame and carrier synchronization and frequency correction. The 26­bit training sequence is used for equalization, as described below. The 8.25 bit guard time allows for some propagation time delay in the arrival of bursts.

Each group of eight time slots is called a TDMA frame, which is transmitted every 4.615 ms. TDMA frames are further grouped into multiframes to carry control signals. There are two types of multiframe, containing 26 or 51 TDMA frames. The 26­frame multiframe contains 24 Traffic Channels (TCH) and two Slow Associated Control Channels (SACCH) which supervise each call in progress. The SACCH in frame 12 contains eight channels, one for each of the eight connections carried by the TCHs. The SACCH in frame 25 is not currently used, but will carry eight additional SACCH channels when half­rate traffic is implemented. A Fast Associated Control Channel (FACCH) works by stealing slots from a traffic channel to transmit power control and handover­signalling messages. The channel stealing is done by setting one of the control bits in the time slot burst.

In addition to the Associated Control Channels, there are several other control channels which (except for the Stand­alone Dedicated Control Channel) are implemented in time slot 0 of specified TDMA frames in a 51­frame multiframe, implemented on a non­hopping carrier frequency in each cell. The control channels include:

• Broadcast Control Channel (BCCH): Continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequency­hopping sequences.


• Stand­alone Dedicated Control Channel (SDCCH): Used for registration, authentication, call setup, and location updating. Implemented on a time slot, together with its SACCH, selected by the system operator.


• Common Control Channel (CCCH): Comprised of three control channels used during call origination and call paging.


• Random Access Channel (RACH): A slotted Aloha channel to request access to the network


• Paging Channel (PCH): Used to alert the mobile station of incoming call.
  
• Access Grant Channel (AGCH): Used to allocate an SDCCH to a mobile for signalling, following a request on the RACH.