Project Report On GSM

 GSM

       1. INTRODUCTION

The first generations of cellular phones were analog, but the current generation is digital, using packet radio. Digital transmission has several advantages over analog for mobile communication. First, voice, data and fax, can be integrated in to a single system. Second, as better speech compression algorithms are discovered, less bandwidth will be needed per channel. Third, error correcting codes can be used to improve transmission quality. Finally, digital signals can be encrypted for security.

Although it might have been nice if the whole world had adopted the same digital standard, such is not the case. The US system, IS-54, and the Japanese system, JDC, have been designed to be compatible with each country’s existing analog system, so each AMPS channel could be used either for analog or digital communication.

In contrast the European digital system, GSM (global system for mobile communication) has been designed from scratch as a fully digital system, without any compromises for the sake of backward compatibility. GSM is currently in use in over 100 countries, inside and outside Europe, and thus serves as an example of digital cellular radio.GSM was originally designed for use in the 900 MHz band. Later, frequencies were allocated at 1800 MHz, and the second system, closely patterned on GSM, was setup there. The later is called DCS 1800, but it is essentially GSM.

A GSM system has up to a maximum of 200 full duplex channels per cell. Each cell consists of a downlink frequency (from base station to mobile station) and uplink frequency (from mobile station to base station). Each frequency band is 200 KHz wide.

1. History of GSM

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 also 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 Group Special Mobile (GSM) to study and develop a pan-European public land mobile system. The proposed system had to meet certain criteria:

1. Good subjective speech quality
2. Low terminal and service cost
3. Support for international roaming
4. Ability to support handheld terminals
5. Support for range of new services and facilities
6. Spectral efficiency
7. 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 mid-1991, and by 1993, there were 36 GSM networks in 22 countries. Although standardized in Europe, GSM is not only a European standard. Over 200 GSM networks (including DCS1800 and PCS1900) are operational in 110 countries around the world. In the beginning of 1994, there were 1.3 million subscribers worldwide which had grown to more than 55 million by October 1997. With North America making a delayed entry into the GSM field with a derivative of GSM called PCS1900, GSM systems exist on every continent, and the acronym GSM now aptly stands for Global System for Mobile communications.

The developers of GSM chose an unproven (at the time) digital system, as opposed to the then-standard 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 over 8000 pages of GSM recommendations try to allow flexibility and competitive innovation among suppliers, but provide enough standardization to guarantee proper interworking between the components of the system. This is done by providing functional and interface descriptions for each of the functional entities defined in the system.

2. Services provided by GSM

From the beginning, the planners of GSM wanted ISDN compatibility in terms of the services offered and the control signaling used. However, radio transmission limitations, in terms of bandwidth and cost, do not allow the standard ISDN B-channel bit rate of 64 kbps to be practically achieved. Using the ITU-T definitions, telecommunication services can be divided into bearer services, teleservices, and supplementary services. The most basic teleservice supported by GSM is telephony. As with all other communications, speech is digitally encoded and transmitted through the GSM network as a digital stream. There is also an emergency service, where the nearest emergency-service provider is notified by dialing three digits (similar to 911). A variety of data services is offered. GSM users can send and receive data, at rates up to 9600 bps, to users on POTS (Plain Old Telephone Service), ISDN, Packet Switched Public Data Networks, and Circuit Switched Public Data Networks using a variety of access methods and protocols, such as X.25 or X.32. Since GSM is a digital network, a modem is not required between the user and GSM network, although an audio modem is required inside the GSM network to interwork with POTS.

Other data services include Group 3 facsimile, as described in ITU-T recommendation T.30, which is supported by use of an appropriate fax adaptor. A unique feature of GSM, not found in older analog systems, is the Short Message Service (SMS). SMS is a bidirectional service for short alphanumeric (up to 160 bytes) messages. Messages are transported in a store-and-forward fashion. For point-to-point 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 cell-broadcast mode, for sending messages such as traffic updates or news updates. Messages can also be stored in the SIM card for later retrieval .Supplementary services are provided on top of teleservices or bearer services. In the current (Phase I) specifications, they include several forms of call forward (such as call forwarding when the mobile subscriber is unreachable by the network), and call barring of outgoing or incoming calls, for example when roaming in another country.

                         

Worldwide GSM Networks in Service

Countries with GSM service

Countries without GSM service

4.  Architecture of the GSM network

A GSM network is composed of several functional entities, whose functions and interfaces are specified. Figure 1 shows the layout of a generic GSM network. The GSM network can be divided into three broad parts. The Mobile Station is carried by the subscriber. The Base Station Subsystem controls the radio link with the Mobile Station. The Network Subsystem, the main part of which is the Mobile services Switching Center (MSC), performs the switching of calls between the mobile users, and between mobile and fixed network users. The MSC also handles the mobility management operations. Not shown is the Operations and Maintenance Center, which oversees the proper operation and setup of the network. The Mobile Station and the Base Station Subsystem communicate across the Um interface, also known as the air interface or radio link. The Base Station Subsystem communicates with the Mobile services Switching Center across the A interface.

Figure 1. General architecture of a GSM network

4.1 Mobile Station

The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to subscribed services irrespective of a specific terminal. By inserting the SIM card into another GSM terminal, the user is able to receive calls at that terminal, make calls from that terminal, and 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) used to identify the subscriber to the system, a secret key for authentication, and other information. The IMEI and the IMSI are independent, thereby allowing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number.

4.2 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 standardized Abis interface, allowing operation between components made by different suppliers.

The Base Transceiver Station houses the radio transceivers that define a cell and handles the radio-link protocols with the Mobile Station. In a large urban area, there will potentially be a large number of BTSs deployed, thus 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 radio-channel setup, frequency hopping, and handovers, as described below. The BSC is the connection between the mobile station and the Mobile service Switching Center (MSC).

4.3 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 additionally 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 conjunction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the fixed networks (such as the PSTN or ISDN). Signaling between functional entities in the Network Subsystem uses Signaling System Number 7 (SS7), used for trunk signaling 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 call-routing and roaming capabilities of GSM.

1. Home Location Register (HLR)

A Home Location Register (HLR) is a database that contains semi-permanent mobile subscriber information for a wireless carriers' entire subscriber base. HLR subscriber information includes the International Mobile Subscriber Identity (IMSI), service subscription information, location information (the identity of the currently serving Visitor Location Register (VLR) to enable the routing of mobile-terminated calls), service restrictions and supplementary services information.

The HLR handles SS7 transactions with both Mobile Switching Centers (MSCs) and VLR nodes, which either request information from the HLR or update the information contained within the HLR. The HLR also initiates transactions with VLRs to complete incoming calls and to update subscriber data.

Traditional wireless network design is based on the utilization of a single Home Location Register (HLR) for each wireless network, but growth considerations are prompting carriers to consider multiple HLR topologies. . The location of the mobile is typically in the form of the signaling address of the VLR associated with the mobile station. The actual routing procedure will be described later. There is logically one HLR per GSM network, although it may be implemented as a distributed database.

2. Visitor Location Register (VLR)

A Visitor Location Register (VLR) is a database which contains temporary information concerning the mobile subscribers that are currently located in a given MSC serving area, but whose Home Location Register (HLR) is elsewhere.

When a mobile subscriber roams away from his home location and into a remote location, SS7 messages are used to obtain information about the subscriber from the HLR, and to create a temporary record for the subscriber in the VLR. There is usually one VLR per MSC.

The Visitor Location Register (VLR) 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, all manufacturers of switching equipment to date implement the VLR together with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, thus simplifying the signaling 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 (AuC) 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 encryption over the radio channel.

3. Adding a Second HLR to the GSM Network

As a GSM wireless carrier's subscriber base grows, it will eventually become necessary to add a second HLR to their network. This requirement might be prompted by a service subscription record storage capacity issue, or perhaps an SS7 message processing performance issue. It might possibly be prompted by a need to increase the overall network reliability.

Typically, when new subscribers are brought into service, the second HLR will be populated with blocks of IMSI numbers that are allocated when new MSE equipment is ordered. As the following example shows, this grouping of IMSI numbers within a single HLR simplifies the routing translations that are required within the SS7 network for VLR to HLR Location Update Request transactions. Global Title Translation (GTT) tables will contain single translation records that translate an entire range of IMSIs numbers into an HLR address. Even if some individual records are moved between the HLRs, as shown in the example, the treatment of IMSIs as blocks results in a significant simplification of the Global Translation tables.

Much more complicated SS7 message routing Global Title Translations are required for Routing Information Request transactions between the MSCs distributed over the entire wireless carrier serving area and the two or more HLRs. MSC Routing Information Requests are routed to the appropriate HLR based on the dialed MSISDN and not the IMSI. Unlike the IMSI numbers, the MSISDN numbers can not easily be arranged in groups to reside within a single HLR and therefore, the MSC must contain an MSISDN to HLR address association record for every mobile subscriber homed on each of the MSCs. As the example illustrates, the MSC routing tables quickly grow much more extensive than the STP tables. The network administration becomes increasingly complex and prone to error.

4.7 Example: Simple Network with two MSCs and two HLRs 

The example illustrates the issues relating to GSM network routing table administration with multiple HLRs. A simple GSM network is shown, with the various routing tables  following:                                                                          

HLR Datafill

HLR #1 is populated with IMSI Range 310-68-4451000 to 310-68-4451005 and is populated with service subscribers from two different MSCs.

HLR #1 

IMSI	MSISDN	Other Subscriber Data
310-68-4451000	813-567-1234	~~~~~~~~~~~~
310-68-4451001	813-567-4355	~~~~~~~~~~~~
310-68-4451002	813-567-8479	~~~~~~~~~~~~
310-68-4451003	415-457-0238	~~~~~~~~~~~~
310-68-4451004	415-457-2332	~~~~~~~~~~~~
310-68-4451005	415-387-6325	~~~~~~~~~~~~
310-68-5568099	415-387-8884	~~~~~~~~~~~~

New HLR#2 is populated with IMSI Range 310-68-5568095 to 310-68-5568100 and is populated with new service subscribers from the same two MSCs. One subscriber has been moved from HLR #2 to HLR #1 (IMSI = 310-68-5568099).

HLR #2

IMSI	MSISDN	Other Subscriber Data
310-68-5568095	415-457-1235	~~~~~~~~~~~~
310-68-5568096	415-387-4444	~~~~~~~~~~~~
310-68-5568097	415-457-1236	~~~~~~~~~~~~
310-68-5568098	415-457-4444	~~~~~~~~~~~~
310-68-5568100	813-567-0055	~~~~~~~~~~~~

STP Datafill 

The STPs route SS7 messages to these HLRs based on the IMSI numbers which are usually provisioned in blocks. In this case, the STPs (which have identical GTT tables) are provisioned to route one block of IMSIs to the each HLR. Note that individual records can be moved between HLRs with the addition of another record in the routing table which specifies the individual IMSI. Individual records take precedence over IMSI block entries.

STP #1, #2 

IMSI	HLR
310-68-4451XXX	1
310-68-5568XXX	2
310-68-5568099	1

MSC Datafill 

When a GSM subscriber receives a phone call, the call attempt messages are routed to the subscriber's MSC, based on the dialed numbers (the MSISDN). The MSC is provisioned with routing tables which relate each MSISDN to an HLR. Note that the MSISDN numbers cannot be assigned in convenient blocks like the IMSI numbers.

MSC #1 

MSISDN	HLR
813-567-1234	1
813-567-4355	1
813-567-8479	1
813-567-0055	2

MSC #2 

MSISDN	HLR
415-457-1235	2
415-457-1236	2
415-387-8884	1

5. Mobile Communications

The use of mobile radio-telephones has seen an enormous boost in the 1980s and 1990s. Previous to this time, citizen band (CB) radio had served a limited market. However, the bandwidth assignation for CB radio was very limited and rapidly saturated. Even in the U.S., a total of only 40 10 KHz channels were available around 27MHz. The use of digital mobile telephones has a number of advantages over CB radio:

1. Access to national and international telephone system.
2. Privacy of communication.
3. Data independent transmission.
4. An infinitely extendable number of channels.

Mobile communications are usually allocated bands in the 50MHz to 1GHz band. At these frequencies the effects of scattering and shadowing are significant. Lower frequencies would improve this performance, but HF bandwidth is not available for this purpose. The primary problems associated with mobile communication at these frequencies are:

1. Maintaining transmission in the fading circumstances of mobile communication.
2. The extensive investigation of propagation characteristics required prior to installation.

Mobile communication work by limiting transmitter powers. This restricts the range of communication to a small region. Outside this region, other transmitters can operate independently. Each region is termed a cell. These cells are often represented in diagrams as hexagons.

 Figure:   Use of cells to provide geographical coverage for mobile phone service

 Figure:   Frequency re-use in cells

Within each cell, the user communicates with a transmitter within the cell. As the mobile approaches a cell boundary, the signal strength fades, and the user is passed on to a transmitter from the new cell. Each cell is equipped with cell-site(s) that transmit/receive to/from the mobile within the cell. Within a single cell, a number of channels are available. These channels are (usually) separated by frequency. Then a mobile initiates a call, it is assigned an idle channel within the current cell by the mobile-services switching centre (MSC). He/she uses the channel within the cell until he/she reaches the boundary. He/she is then allocated a new idle channel within the next cell.

For example, the American advanced mobile phone service (AMPS) makes use of a 40MHz channel in the 800 - 900MHz band. This band is split into a 20MHz transmit and 20MHz receive bandwidth. These bands are split into 666 two-way channels, each having a bandwidth of 30 KHz. These channels are subdivided into 21 sets of channels, arranged in 7 groups of 3. The nominally hexagonal pattern contains 7 cells, a central one and its 6 nearest neighbours. Each cell is assigned a different group in such a way that at least two cells lie between it and the next block using that set. With a total of 666 channels, it is possible to assign three sets of 31 channels per cell.

The great strength of this type of network is the ease with which more channels may be introduced. As demand rises, one simply reduces the cell size. Then the same number of channels is available in a smaller area, increasing the total number of channels per unit area. In a well planned system, the density of cells would reflect the user density.

AMPS is a first generation mobile phone system. It uses analogue modulation. It is one of six incompatible first generation systems that exist around the world. Currently, second generation systems are being introduced. These are digital in nature. One aim of the second generation mobile systems was to try and develop one global standard, allowing use of the same mobile phone anywhere in the world. However, there are currently three digital standards in use, so this seems unlikely. The pan-European standard is known as GSM (Groupe Special Mobile), and is now available in the UK. The services planned for the GSM are similar to those for ISDN (e.g. call forwarding, charge advice, etc. ). Full GSM will have 200KHz physical channels offering 270Kb/s. Currently, one physical channel is split between 8 users, each having use of 13Kb/s (the rest is used for channel overhead). The aims of the GSM system were:

1. Good speech quality
2. Low terminal cost
3. Low service cost
4. International roaming
5. Ability to support hand-held portables
6. A range of new services and facilities (ISDN!)

The heart of the mobile telephone network is the MSC. Its task is to acknowledge the paging of the user, assign him/her a channel, broadcast his/her dialed request, return the call. In addition it automatically monitors the signal strength of both transmitter and receiver, and allocates new channels as required. This latter process, known as hand-off, is completely hidden to the user, although is a major technical problem. In addition, the MSC is responsible for charging the call. The decision making ability of the MSC relies to a great extent on modern digital technology. It is the maturity of this technology that has permitted the rapid growth of mobile communications.

 Figure:   Hand-off between cells

The principle problem with mobile communication is the variation in signal strength as the communicating parties move. This variation is due to the varying interference of scattered radiation -- fading. Fading causes rapid variation in signal strength. The normal solution to fading, increasing the transmitter power, is not available in mobile communication where transmitter power is limited.

The installation of a mobile telephone system requires a large initial effort in determining the propagation behaviour in the area covered by the network. Propagation planning, by a mixture of observation and computer simulation, is necessary if the system is to work properly. At UHF and VHF frequencies, the effects of obstructions is significant. Some of the effects that need to be considered are:

1. Free space loss. This significantly increases in urban environments. Studies have indicated that a relationship is more often followed than a law.
2. Effect of street orientation. Streets have a significant waveguide effect. Variations of up to 20dB have been measured in urban environments as a result of street direction.
3. Effects of foliage. Propagation in rural areas is significantly effected by the presence of leaves. Variations of 18dB between summer and winter have been observed in forested areas.
4. Effect of tunnels. Tunnels can introduce signal attenuation of up to 30dB according to the tunnel length and frequency.

1. Radio link aspects

The International Telecommunication Union (ITU), which manages the international allocation of radio spectrum (among many 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.

1. Multiple access and channel structure

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 Frequency-Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. One or more carrier frequencies are assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame.

Channels are defined by the number and position of their corresponding burst periods. All these definitions are cyclic, and the entire pattern repeats approximately every 3 hours. Channels can be divided into dedicated channels, which are allocated to a mobile station, and common channels, which are used by mobile stations in idle mode.

1. Traffic channels

A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined using a 26-frame multiframe, or group of 26 TDMA frames. The length of a 26-frame multiframe is 120 ms, which is how the length of a burst period is defined (120 ms divided by 26 frames divided by 8 burst periods per frame). Out of the 26 frames, 24 are used for traffic, 1 is used for the Slow Associated Control Channel (SACCH) and 1 is currently unused (see Figure 2). TCHs for the uplink and downlink are separated in time by 3 burst periods, so that the mobile station does not have to transmit and receive simultaneously, thus simplifying the electronics.

In addition to these full-rate TCHs, there are also half-rate TCHs defined, although they are not yet implemented. Half-rate TCHs will effectively double the capacity of a system once half-rate speech coders are specified (i.e., speech coding at around 7 kbps, instead of 13 kbps). Eighth-rate TCHs are also specified, and are used for signalling. In the recommendations, they are called Stand-alone Dedicated Control Channels (SDCCH).

2. Control channels

Common channels can be accessed both by idle mode and dedicated mode mobiles. The common channels are used by idle mode mobiles to exchange the signaling information required to change to dedicated mode. Mobiles already in dedicated mode monitor the surrounding base stations for handover and other information. The common channels are defined within a 51-frame multiframe, so that dedicated mobiles using the 26-frame multiframe TCH structure can still monitor control channels. The common channels include:

Broadcast Control Channel (BCCH)

Continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequency-hopping sequences.

Frequency Correction Channel (FCCH) and Synchronization Channel (SCH)

Used to synchronize the mobile to the time slot structure of a cell by defining the boundaries of burst periods, and the time slot numbering. Every cell in a GSM network broadcasts exactly one FCCH and one SCH, which are by definition on time slot number 0 (within a TDMA frame).

Random Access Channel (RACH)

Slotted Aloha channel used by the mobile to request access to the network.

Paging Channel (PCH)

Used to alert the mobile station of an incoming call.

Access Grant Channel (AGCH)

Used to allocate an SDCCH to a mobile for signaling (in order to obtain a dedicated channel), following a request on the RACH.

3. Burst structure

There are four different types of bursts used for transmission in GSM. The normal burst is used to carry data and most signaling. It has a total length of 156.25 bits, made up of two 57 bit information bits, a 26 bit training sequence used for equalization, 1 stealing bit for each information block (used for FACCH), 3 tail bits at each end, and an 8.25 bit guard sequence. The 156.25 bits are transmitted in 0.577 ms, giving a gross bit rate of 270.833 kbps.

The F burst, used on the FCCH, and the S burst, used on the SCH, have the same length as a normal burst, but a different internal structure, which differentiates them from normal bursts (thus allowing synchronization). The access burst is shorter than the normal burst, and is used only on the RACH.

2. Speech coding

GSM is a digital system, so speech which is inherently analog, has to be digitized. The method employed by ISDN, and by current telephone systems for multiplexing voice lines over high speed trunks and optical fiber lines, is Pulse Coded Modulation (PCM). The output stream from PCM is 64 kbps, too high a rate to be feasible over a radio link. The 64 kbps signal, although simple to implement, contains much redundancy. The GSM group studied several speech coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay, and power consumption once implemented) before arriving at the choice of a Regular Pulse Excited -- Linear Predictive Coder (RPE--LPC) with a Long Term Predictor loop. Basically, information from previous samples, which does not change very quickly, is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps. This is the so-called Full-Rate speech coding. Recently, an Enhanced Full-Rate (EFR) speech coding algorithm has been implemented by some North American GSM1900 operators. This is said to provide improved speech quality using the existing 13 kbps bit rate.

3. Channel coding and modulation

Because of natural and man-made electromagnetic interference, the encoded speech or data signal transmitted over the radio interface must be protected from errors. GSM uses convolutional encoding and block interleaving to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks will be described below.

From subjective testing, it was found that some bits of this block were more important for perceived speech quality than others. The bits are thus divided into three classes:

1. Class Ia 50 bits - most sensitive to bit errors
2. Class Ib 132 bits - moderately sensitive to bit errors
3. Class II 78 bits - least sensitive to bit errors

Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightly attenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder of constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remaining Class II bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.

To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples.

Recall that each time-slot burst is transmitted at a gross bit rate of 270.833 kbps. This digital signal is modulated onto the analog carrier frequency using Gaussian-filtered Minimum Shift Keying (GMSK). GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions. The complexity of the transmitter is related to power consumption, which should be minimized for the mobile station. The spurious radio emissions, outside of the allotted bandwidth, must be strictly controlled so as to limit adjacent channel interference, and allow for the co-existence of GSM and the older analog systems (at least for the time being).

4. Multipath equalization

At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extract the desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal. This known signal is the 26-bit training sequence transmitted in the middle of every time-slot burst. The actual implementation of the equalizer is not specified in the GSM specifications.

6.5 Discontinuous transmission

Minimizing co-channel interference is a goal in any cellular system, since it allows better service for a given cell size, or the use of smaller cells, thus increasing the overall capacity of the system. Discontinuous transmission (DTX) is a method that takes advantage of the fact that a person speaks less that 40 percent of the time in normal conversation by turning the transmitter off during silence periods. An added benefit of DTX is that power is conserved at the mobile unit.

The most important component of DTX is, of course, Voice Activity Detection. It must distinguish between voice and noise inputs, a task that is not as trivial as it appears, considering background noise. If a voice signal is misinterpreted as noise, the transmitter is turned off and a very annoying effect called clipping is heard at the receiving end. If, on the other hand, noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased. Another factor to consider is that when the transmitter is turned off, there is total silence heard at the receiving end, due to the digital nature of GSM. To assure the receiver that the connection is not dead, comfort noise is created at the receiving end by trying to match the characteristics of the transmitting end's background noise.

1. Discontinuous reception

Another method used to conserve power at the mobile station is discontinuous reception. The paging channel, used by the base station to signal an incoming call, is structured into sub-channels. Each mobile station needs to listen only to its own sub-channel. In the time between successive paging sub-channels, the mobile can go into sleep mode, when almost no power is used.

2. Power control

There are five classes of mobile stations defined, according to their peak transmitter power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize co-channel interference and to conserve power, both the mobiles and the Base Transceiver Stations operate at the lowest power level that will maintain an acceptable signal quality. Power levels can be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm (20 milliwatts).

2. Network aspects

Ensuring the transmission of voice or data of a given quality over the radio link is only part of the function of a cellular mobile network. A GSM mobile can seamlessly roam nationally and internationally, which requires that registration, authentication, call routing and location updating functions exist and are standardized in GSM networks. In addition, the fact that the geographical area covered by the network is divided into cells necessitates the implementation of a handover mechanism. These functions are performed by the Network Subsystem, mainly using the Mobile Application Part (MAP) built on top of the Signalling System No. 7 protocol.

Figure 3. Signaling protocol structure in GSM

The signaling protocol in GSM is structured into three general layers depending on the interface, as shown in Figure 3. Layer 1 is the physical layer, which uses the channel structures discussed above over the air interface. Layer 2 is the data link layer. Across the Um interface, the data link layer is a modified version of the LAPD protocol used in ISDN, called LAPDm. Across the A interface, the Message Transfer Part layer 2 of Signaling System Number 7 is used. Layer 3 of the GSM signaling protocol is itself divided into 3 sub layers.

Radio Resources Management

Controls the setup, maintenance, and termination of radio and fixed channels, including handovers.

Mobility Management

Manages the location updating and registration procedures, as well as security and authentication.

Connection Management

Handles general call control, similar to CCITT Recommendation Q.931, and manages Supplementary Services and the Short Message Service.

Signaling between the different entities in the fixed part of the network, such as between the HLR and VLR, is accomplished through the Mobile Application Part (MAP). MAP is built on top of the Transaction Capabilities Application Part (TCAP, the top layer of Signaling System Number 7. The specification of the MAP is quite complex, and at over 500 pages, it is one of the longest documents in the GSM recommendations .

1. Radio resources management

The radio resources management (RR) layer oversees the establishment of a link, both radio and fixed, between the mobile station and the MSC. The main functional components involved are the mobile station, and the Base Station Subsystem, as well as the MSC. The RR layer is concerned with the management of an RR-session which is the time that a mobile is in dedicated mode, as well as the configuration of radio channels including the allocation of dedicated channels.

An RR-session is always initiated by a mobile station through the access procedure, either for an outgoing call, or in response to a paging message. The details of the access and paging procedures, such as when a dedicated channel is actually assigned to the mobile, and the paging sub-channel structure, are handled in the RR layer. In addition, it handles the management of radio features such as power control, discontinuous transmission and reception, and timing advance.

1. Handover

In a cellular network, the radio and fixed links required are not permanently allocated for the duration of a call. Handover, or handoff as it is called in North America, is the switching of an on-going call to a different channel or cell. The execution and measurements required for handover form one of basic functions of the RR layer.

There are four different types of handover in the GSM system, which involve transferring a call between:

1. Channels (time slots) in the same cell
2. Cells (Base Transceiver Stations) under the control of the same Base Station Controller (BSC),
3. Cells under the control of different BSCs, but belonging to the same Mobile services Switching Center (MSC), and
4. Cells under the control of different MSCs.

The first two types of handover, called internal handovers, involve only one Base Station Controller (BSC). To save signaling bandwidth, they are managed by the BSC without involving the Mobile services Switching Center (MSC), except to notify it at the completion of the handover. The last two types of handover, called external handovers, are handled by the MSCs involved. An important aspect of GSM is that the original MSC, the anchor MSC, remains responsible for most call-related functions, with the exception of subsequent inter-BSC handovers under the control of the new MSC, called the relay MSC.

Handovers can be initiated by either the mobile or the MSC (as a means of traffic load balancing). During its idle time slots, the mobile scans the Broadcast Control Channel of up to 16 neighboring cells, and forms a list of the six best candidates for possible handover, based on the received signal strength. This information is passed to the BSC and MSC, at least once per second, and is used by the handover algorithm.

The algorithm for when a handover decision should be taken is not specified in the GSM recommendations. There are two basic algorithms used, both closely tied in with power control. This is because the BSC usually does not know whether the poor signal quality is due to multipath fading or to the mobile having moved to another cell. This is especially true in small urban cells.

The 'minimum acceptable performance' algorithm gives precedence to power control over handover, so that when the signal degrades beyond a certain point, the power level of the mobile is increased. If further power increases do not improve the signal, then a handover is considered. This is the simpler and more common method, but it creates 'smeared' cell boundaries when a mobile transmitting at peak power goes some distance beyond its original cell boundaries into another cell.

The 'power budget' method uses handover to try to maintain or improve a certain level of signal quality at the same or lower power level. It thus gives precedence to handover over power control. It avoids the 'smeared' cell boundary problem and reduces co-channel interference, but it is quite complicated.

2. Authentication and security

Since the radio medium can be accessed by anyone, authentication of users to prove that they are who they claim to be is a very important element of a mobile network. Authentication involves two functional entities, the SIM card in the mobile, and the Authentication Center (AuC). Each subscriber is given a secret key, one copy of which is stored in the SIM card and the other in the AuC. During authentication, the AuC generates a random number that it sends to the mobile. Both the mobile and the AuC then use the random number, in conjunction with the subscriber's secret key and a ciphering algorithm called A3, to generate a signed response (SRES) that is sent back to the AuC. If the number sent by the mobile is the same as the one calculated by the AuC, the subscriber is authenticated.

The same initial random number and subscriber key are also used to compute the ciphering key using an algorithm called A8. This ciphering key, together with the TDMA frame number, use the A5 algorithm to create a 114 bit sequence that is XORed with the 114 bits of a burst (the two 57 bit blocks). Enciphering is an option for the fairly paranoid, since the signal is already coded, interleaved, and transmitted in a TDMA manner, thus providing protection from all but the most persistent and dedicated eavesdroppers.

Another level of security is performed on the mobile equipment itself, as opposed to the mobile subscriber. As mentioned earlier, each GSM terminal is identified by a unique International Mobile Equipment Identity (IMEI) number. A list of IMEIs in the network is stored in the Equipment Identity Register (EIR). The status returned in response to an IMEI query to the EIR is one of the following:

White-listed

The terminal is allowed to connect to the network.

Grey-listed

The terminal is under observation from the network for possible problems.

Black-listed

The terminal has either been reported stolen, or is not type approved (the correct type of terminal for a GSM network). The terminal is not allowed to connect to the network.

3. Call routing

Unlike routing in the fixed network, where a terminal is semi-permanently wired to a central office, a GSM user can roam nationally and even internationally. The directory number dialed to reach a mobile subscriber is called the Mobile Subscriber ISDN (MSISDN), which is defined by the E.164 numbering plan.

The MSISDN is the dialable number that callers use to reach a mobile subscriber. Some phones can support multiple MSISDNs - for example, a U.S.-based MSISDN and a Canadian-based MSISDN. Callers dialing either number will reach the subscriber.  This number includes a country code and a National Destination Code which identifies the subscriber's operator. The first few digits of the remaining subscriber number may identify the subscriber's HLR within the home PLMN.

An incoming mobile terminating call is directed to the Gateway MSC (GMSC) function. The GMSC is basically a switch which is able to interrogate the subscriber's HLR to obtain routing information, and thus contains a table linking MSISDNs to their corresponding HLR. A simplification is to have a GSMC handle one specific PLMN. It should be noted that the GMSC function is distinct from the MSC function.

The routing information that is returned to the GMSC is the Mobile Station Roaming Number (MSRN), which is also defined by the E.164 numbering plan. MSRNs are related to the geographical numbering plan, and not assigned to subscribers, nor are they visible to subscribers.

The most general routing procedure begins with the GMSC querying the called subscriber's HLR for an MSRN. The HLR typically stores only the SS7 address of the subscriber's current VLR, and does not have the MSRN (see the location updating section). The HLR must therefore query the subscriber's current VLR, which will temporarily allocate an MSRN from its pool for the call. This MSRN is returned to the HLR and back to the GMSC, which can then route the call to the new MSC. At the new MSC, the IMSI corresponding to the MSRN is looked up, and the mobile is paged in its current location area (see Figure 4).

The GSM system operates on a number of frequencies around 900 MHz (CDMA operates from 824-894MHz). The pie chart below shows a typical example of the relationship of the GSM system with other broadcasters using radio frequency transmission. Television and FM radio use frequencies of about 100MHz and AM radio uses frequencies near 1MHz. The pie chart gives the relative amount of RFR emitted by various sources measured in Burwood a middle class suburb East of Melbourne and about 25km from the television transmission antennas and 0.1km from the nearest base station. Measurements of power density levels (in micro watts per square centimeter - white text) are made at a position which maximizes the exposure from the mobile phone base station. It can be seen that exposure levels are less than those from FM radio stations (100 MHz) and significantly less than levels from AM radio stations (1 MHz).

These levels are well below the former Australian Standard requirement of 0.2mW/cm2. The average exposure from a base station antenna is similar to the exposure (albeit visible rather than RF radiation) from a 2 Watt torch bulb where the light is used to illuminate an area of approximately 7 acres.

4. Conclusion and Comments

In this paper we have tried to give an overview of the GSM system. It is a standard that ensures interoperability without stifling competition and innovation among suppliers, to the benefit of the public both in terms of cost and service quality. For example, by using Very Large Scale Integration (VLSI) microprocessor technology, many functions of the mobile station can be built on one chipset, resulting in lighter, more compact and more energy-efficient terminals.

Telecommunications are evolving towards personal communication networks, whose objective can be stated as the availability of all communication services anytime, anywhere, to anyone, by a single identity number and a pocketable communication terminal. Having a multitude of incompatible systems throughout the world moves us farther away from this ideal. The economies of scale created by a unified system are enough to justify its implementation, not to mention the convenience to people of carrying just one communication terminal anywhere they go, regardless of national boundaries.

The GSM system, and its sibling systems operating at 1.8 GHz (called DCS1800) and 1.9 GHz (called GSM1900 or PCS1900, and operating in North America), are a first approach at a true personal communication system. The SIM card is a novel approach that implements personal mobility in addition to terminal mobility. Together with international roaming, and support for a variety of services such as telephony, data transfer, fax, Short Message Service, and supplementary services, GSM comes close to fulfilling the requirements for a personal communication system: close enough that it is being used as a basis for the next generation of mobile communication technology in Europe, the Universal Mobile Telecommunication System (UMTS).

Another point where GSM has shown its commitment to openness, standards and interoperability is the compatibility with the Integrated Services Digital Network (ISDN) that is evolving in most industrialized countries and Europe in particular (the so-called Euro-ISDN). GSM is also the first system to make extensive use of the Intelligent Networking concept, in which services like 800 numbers are concentrated and handled from a few centralized service centers, instead of being distributed over every switch in the country. This is the concept behind the use of the various registers such as the HLR. In addition, the signaling between these functional entities uses Signaling System Number 7, an international standard already deployed in many countries and specified as the backbone signaling network for ISDN.  

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10. Bibliography

[1] Jan A. Audestad. Network aspects of the GSM system

[2] D. M. Balston. The pan-European system: GSM. In D. M. Balston and R.C.V. Macario, editors.

[3] David M. Balston. The pan-European cellular technology. In R.C.V. Macario, editor, Personal and Mobile Radio Systems. 

[4] David Cheeseman. The pan-European cellular mobile radio system. In R.C.V. Macario, editor, Personal and Mobile Radio Systems.

[5] C. Déchaux and R. Scheller. What are GSM and DCS.

[6] M. Feldmann and J. P. Rissen. GSM network systems and overall system integration.

[7] John M. Griffiths. ISDN Explained: Worldwide Network and Applications Technology. 

[8]  I. Harris. Data in the GSM cellular network.

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