GSM – Overview GSM is a globally accepted standard for digital cellular communications. What is GSM? If you are in Europe, Asia or Japan and using a mobile phone then most probably you must be using GSM technology in your mobile phone. GSM stands for Global System for Mobile Communication and is an open, digital cellular technology used for transmitting mobile voice and data services. The GSM emerged from the idea of cell-based mobile radio systems at Bell Laboratories in the early 1970s. The GSM is the name of a standardization group established in 1982 to create a common European mobile telephone standard.
The GSM standard is the most widely accepted standard and is implemented globally. The GSM is a circuit-switched system that divides each 200kHz channel into eight 25kHz time-slots. GSM operates in the 900MHz and 1. 8GHz bands in Europe and the 1. 9GHz and 850MHz bands in the US. The GSM is owning a market share of more than 70 percent of the world’s digital cellular subscribers. The GSM makes use of narrowband Time Division Multiple Access (TDMA) technique for transmitting signals. The GSM was developed using digital technology. It has an ability to carry 64 kbps to 120 Mbps of data rates.
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Presently GSM support more than one billion mobile subscribers in more than 210 countries throughout of the world. The GSM provides basic to advanced voice and data services including Roaming service. Roaming is the ability to use your GSM phone number in another GSM network. A GSM digitizes and compresses data, then sends it down through a channel with two other streams of user data, each in its own time slot. It operates at either the 900 MHz or 1,800 MHz frequency band. Why GSM? The GSM study group aimed to provide the followings through the GSM: ? Improved spectrum efficiency. ? International roaming. Low-cost mobile sets and base stations (BSs) ? High-quality speech ? Compatibility with Integrated Services Digital Network (ISDN) and other telephone company services. ? Support for new services. GSM Brief History Following table shows many of the important events in the rollout of the GSM system; other events were introduced, but had less significant impact on the overall systems. |Years |Events | |1982 |CEPT establishes a GSM group in order to develop the standards for a pan-European cellular mobile system. |1985 |A list of recommendations to be generated by the group is accepted. | |1986 |Field tests are performed to test the different radio techniques proposed for the air interface. | |1987 |Time Division Multiple Access (TDMA) is chosen as the access method (with Frequency Division Multiple Access | | |[FDMA]). The initial Memorandum of Understanding (MoU) is signed by telecommunication operators representing 12 | | |countries. | |1988 |GSM system is validated. |1989 |The responsibility of the GSM specifications is passed to the European Telecommunications Standards Institute | | |(ETSI). | |1990 |Phase 1 of the GSM specifications is delivered. | |1991 |Commercial launch of the GSM service occurs. The DCS1800 specifications are finalized. | |1992 |The addition of the countries that signed the GSM Memorandum of Understanding takes place. Coverage spreads to | | |larger cities and airports. | |1993 |Coverage of main roads’ GSM services starts outside Europe. |1994 |Data transmission capabilities launched. The number of networks rises to 69 in 43 countries by the end of 1994. | |1995 |Phase 2 of the GSM specifications occurs. Coverage is extended to rural areas. | |1996 |June: 133 network in 81 countries operational. | |1997 |July: 200 network in 109 countries operational, around 44 million subscribers worldwide. | |1999 |Wireless Application Protocol came into existence and 130 countries operational with 260 million subscribers | |2000 |General Packet Radio Service(GPRS) came into existence. |2001 |As of May 2001, over 550 million people were subscribers to mobile telecommunications | GSM – Architecture A GSM network consists of several functional entities whose functions and interfaces are defined. The GSM network can be divided into following broad parts. • The Mobile Station(MS) • The Base Station Subsystem (BSS) • The Network Switching Subsystem (NSS) • The Operation Support Subsystem(OSS) Following is the simple architecture diagram of GSM Network. [pic] The added components of the GSM architecture include the functions of the databases and messaging systems: ?
Home Location Register (HLR) ? Visitor Location Register (VLR) ? Equipment Identity Register (EIR) ? Authentication Center (AuC) ? SMS Serving Center (SMS SC) ? Gateway MSC (GMSC) ? Chargeback Center (CBC) ? Transcoder and Adaptation Unit (TRAU) Following is the diagram of GSM Netwrok alongwith added elements. [pic] The MS and the BSS communicate across the Um interface, also known as the air interface or radio link. The BSS communicates with the Network Service Switching center across the A interface. GSM Network Areas: In a GSM network, the following areas are defined: Cell: Cell is the basic service area: one BTS covers one cell. Each cell is given a Cell Global Identity (CGI), a number that uniquely identifies the cell. ? Location Area: A group of cells form a Location Area. This is the area that is paged when a subscriber gets an incoming call. Each Location Area is assigned a Location Area Identity (LAI). Each Location Area is served by one or more BSCs. ? MSC/VLR Service Area: The area covered by one MSC is called the MSC/VLR service area. ? PLMN: The area covered by one network operator is called PLMN. A PLMN can contain one or more MSCs.
GSM – The Mobile Station The MS consists of the physical equipment, such as the radio transceiver, display and digital signal processors, and the SIM card. It provides the air interface to the user in GSM networks. As such, other services are also provided, which include: ? Voice teleservices ? Data bearer services ? The features’ supplementary services [pic] The MS Functions: The MS also provides the receptor for SMS messages, enabling the user to toggle between the voice and data use. Moreover, the mobile facilitates access to voice. messaging systems.
The MS also provides access to the various data services available in a GSM network. These data services include: • X. 25 packet switching through a synchronous or asynchronous dialup connection to the PAD at speeds typically at 9. 6 Kbps. • General Packet Radio Services (GPRSs) using either an X. 25. or IP. based data transfer method at speeds up to 115 Kbps • High. speed, circuit. switched data at speeds up to 64 Kbps What is 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.
You need to insert the SIM card into another GSM cellular phone to receive calls at that phone, make calls from that phone, or receive other subscribed services. GSM – The Base Station Subsystem (BSS) The BSS is composed of two parts: • The Base Transceiver Station (BTS) • The Base Station Controller (BSC) The BTS and the BSC communicate across the specified Abis interface, enabling operations between components that are made by different suppliers. The radio components of a BSS may consist of four to seven or nine cells. A BSS may have one or more base stations. The BSS uses the Abis interface between the BTS and the BSC.
A separate high-speed line (T1 or E1) is then connected from the BSS to the Mobile MSC. [pic] The Base Transceiver Station (BTS): The BTS houses the radio transceivers that define a cell and handles the radio link protocols with the MS. In a large urban area, a large number of BTSs may be deployed. [pic] The BTS corresponds to the transceivers and antennas used in each cell of the network. A BTS is usually placed in the center of a cell. Its transmitting power defines the size of a cell. Each BTS has between 1 and 16 transceivers, depending on the density of users in the cell.
Each BTS serves a single cell. It also includes the following functions: • Encoding, encrypting, multiplexing, modulating, and feeding the RF signals to the antenna. • Transcoding and rate adaptation • Time and frequency synchronizing • Voice through full- or half-rate services • Decoding, decrypting, and equalizing received signals • Random access detection • Timing advances • Uplink channel measurements The Base Station Controller (BSC): The BSC manages the radio resources for one or more BTSs. It handles radio channel setup, frequency hopping, and handovers.
The BSC is the connection between the mobile and the 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 (PSDN) or ISDN. It assigns and releases frequencies and time slots for the MS. The BSC also handles intercell handover. It controls the power transmission of the BSS and MS in its area. The function of the BSC is to allocate the necessary time slots between the BTS and the MSC. It is a switching device that handles the radio resources. Additional functions include: • Control of frequency hopping Performing traffic concentration to reduce the number of lines from the MSC • Providing an interface to the Operations and Maintenance Center for the BSS • Reallocation of frequencies among BTSs • Time and frequency synchronization • Power management • Time-delay measurements of received signals from the MS The Network Switching Subsystem (NSS) The Network switching system (NSS), the main part of which is the Mobile Switching Center (MSC), performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as authentication. pic] The switching system includes the following functional elements. Home Location Register (HLR) The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber’s service profile, location information, and activity status. When an individual buys a subscription in the form of SIM then all the information about this subscription is registered in the HLR of that operator. Mobile Services Switching Center (MSC) The central component of the Network Subsystem is the MSC.
The MSC performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others. Every MSC is identified by a unique ID. Visitor Location Register (VLR) The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC.
When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time. Authentication Center (AUC) 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. The AUC protects network operators from different types of fraud found in today’s cellular world. Equipment Identity Register (EIR)
The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where its International Mobile Equipment Identity (IMEI) identifies each MS. An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Operation Support Subsystem(OSS) The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS). Here are some of the OMC functions: • Administration and commercial operation (subscription, end terminals, charging and statistics). Security Management. • Network configuration, Operation and Performance Management. • Maintenance Tasks. The operation and Maintenance functions are based on the concepts of the Telecommunication Management Network (TMN) which is standardized in the ITU-T series M. 30. Following is the figure which shows how OMC system covers all the GSM elements. [pic] The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of OSS is to offer the customer cost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM network.
An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations. The GSM Specifications Specifications for different Personal Communication Services (PCS) systems vary among the different PCS networks. The GSM specification is listed below with important characteristics. Modulation: Modulation is a form of change process where we change the input information into a suitable format for the transmission medium. We also changed the information by demodulating the signal at the receiving end.
The GSM uses Gaussian Minimum Shift Keying (GMSK) modulation method. Access Methods: Because 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. GSM chose a combination of TDMA/FDMA as its method. 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 BS. 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. Transmission Rate: The total symbol rate for GSM at 1 bit per symbol in GMSK produces 270. 833 K symbols/second. The gross transmission rate of the time slot is 22. 8 Kbps. GSM is a digital system with an over-the-air bit rate of 270 kbps. Frequency Band: The uplink frequency range specified for GSM is 933 – 960 MHz (basic 900 MHz band only). The downlink frequency band 890 – 915 MHz (basic 900 MHz band only).
Channel Spacing: This indicates separation between adjacent carrier frequencies. In GSM, this is 200 kHz. Speech Coding: GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter, leaving behind a residual signal. Speech is encoded at 13 kbps. Duplex Distance: The duplex distance is 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies. A channel has two frequencies, 80 MHz apart. Misc: • Frame duration: 4. 615 mS Duplex Technique: Frequency Division Duplexing (FDD) access mode previously known as WCDMA. • Speech channels per RF channel: 8. GSM – Addresses & Identifiers GSM distinguishes explicitly between user and equipment and deals with them separately. Besides phone numbers and subscriber and equipment identifiers, several other identifiers have been defined; they are needed for the management of subscriber mobility and for addressing of all the remaining network elements. The most important addresses and identifiers are presented in the following: International Mobile Station Equipment Identity (IMEI):
The international mobile station equipment identity (IMEI) uniquely identifies a mobile station internationally. It is a kind of serial number. The IMEI is allocated by the equipment manufacturer and registered by the network operator and registered by the network operator who stores it in the EIR. By means of IMEI one recognizes obsolete, stolen or nonfunctional equipment. There are following parts of an IMEI: • Type Approval Code (TAC): 6 decimal places, centrally assigned. • Final Assembly Code (FAC): 6 decimal places, assigned by the manufacturer. • Serial Number (SNR): 6 decimal places, assigned by the manufacturer. Spare (SP): 1 decimal place. Thus, IMEI = TAC + FAC + SNR + SP. It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing. International Mobile Subscriber Identity ( IMSI): Each registered user is uniquely identified by its international mobile subscriber identity (IMSI). It is stored in the subscriber identity module (SIM) A mobile station can only be operated if a SIM with a valid IMSI is inserted into equipment with a valid IMEI. There are following parts of an IMSI: • Mobile Country Code (MCC): 3 decimal places, internationally standardized. Mobile Network Code (MNC): 2 decimal places, for unique identification of mobile network within the country. • Mobile Subscriber Identification Number (MSIN): Maximum 10 decimal places, identification number of the subscriber in the home mobile network. Mobile Subscriber ISDN Number ( MSISDN): The real telephone number of a mobile station is the mobile subscriber ISDN number (MSISDN). It is assigned to the subscriber (his or her SIM, respectively), such that a mobile station set can have several MSISDNs depending on the SIM. The MSISDN categories follow the international ISDN number plan and therefore have the following structure. Country Code (CC) : Up to 3 decimal places. • National Destination Code (NDC): Typically 2-3 decimal places. • Subscriber Number (SN): Maximum 10 decimal places. Mobile Station Roaming Number ( MSRN): The Mobile Station Roaming Number ( MSRN) is a temporary location dependent ISDN number. It is assigned by the locally responsible VLR to each mobile station in its area. Calls are also routed to the MS by using the MSRN. The MSRN has same structure as the MSISDN. • Country Code (CC) : of the visited network. • National Destination Code (NDC): of the visited network. • Subscriber Number (SN): in the current mobile network.
Location Area Identity (LAI): Each LA of an PLMN has its own identifier. The Location Area Identifier (LAI) is also structured hierarchically and internationally unique as follows: • Country Code (CC) : 3 decimal places. • Mobile Network Code (MNC): 2 decimal places. • Location Area Code (LAC): maximum 5 decimal places or, maximum twice 8 bits coded in hexadecimal (LAC < FFFF). Temporary Mobile Subscriber Identity (TMSI): The VLR, which is responsible for the current location of a subscriber, can assign a temporary mobile subscriber identity (TMSI) which has only local significance in the area handled by the VLR.
It is stored on the network side only in the VLR and is not passed to the HLR. Together with the current location area, TMSI allows a subscriber to be identified uniquely and it can consist of upto 4×8 bits. Local Mobile Subscriber Identity (LMSI): The VLR can assign an additional searching key to each mobile station within its area to accelerate database access. This unique key is called the Local Mobile Subscriber Identity (LMSI). The LMSI is assigned when the mobile station registers with the VLR and is also sent to the HLR. An LIMSI consists of four octets ( 4 x 8 bits). Cell Identifier (CI):
Within an LA, the individual cells are uniquely identified with a cell identifier (CI), maximum 2 x 8 bits. Together with the global cell identity (LAI + CI) calls are thus also internationally defined in a unique way. Cellular Systems The Cellular Structure In a cellular system, the covering area of an operator is divided into cells. A cell corresponds to the covering area of one transmitter or a small collection of transmitters. The size of a cell is determined by the transmitter’s power. The concept of cellular systems is the use of low power transmitters in order to enable the efficient reuse of the frequencies.
In fact, if the transmitters used are very powerful, the frequencies cannot be reused for hundreds of kilometers as they are limited to the covering area of the transmitter. The frequency band allocated to a cellular mobile radio system is distributed over a group of cells and this distribution is repeated in all the covering area of an operator. The whole number of radio channels available can then be used in each group of cells that form the covering area of an operator. Frequencies used in a cell will be reused several cells away.
The distance between the cells using the same frequency must be sufficient to avoid interference. The frequency reuse will increase considerably the capacity in number of users. In order to work properly, a cellular system must verify the following two main conditions: • The power level of a transmitter within a single cell must be limited in order to reduce the interference with the transmitters of neighboring cells. The interference will not produce any damage to the system if a distance of about 2. 5 to 3 times the diameter of a cell is reserved between transmitters.
The receiver filters must also be very efficient. • Neighboring cells can not share the same channels. In order to reduce the interference, the frequencies must be reused only within a certain pattern. In order to exchange the information needed to maintain the communication links within the cellular network, several radio channels are reserved for the signaling information. Cluster The cells are grouped into clusters. The number of cells in a cluster must be determined so that the cluster can be repeated continuously within the covering area of an operator. The typical clusters contain 4, 7, 12 or 21 cells.
The number of cells in each cluster is very important. The smaller the number of cells per cluster is, the bigger the number of channels per cell will be. The capacity of each cell will be therefore increased. However a balance must be found in order to avoid the interference that could occur between neighboring clusters. This interference is produced by the small size of the clusters (the size of the cluster is defined by the number of cells per cluster). The total number of channels per cell depends on the number of available channels and the type of cluster used. Types of Cells
The density of population in a country is so varied that different types of cells are used: • Macro cells • Micro cells • Selective cells • Umbrella cells Macro cells The macro cells are large cells for remote and sparsely populated areas. Micro cells These cells are used for densely populated areas. By splitting the existing area into smaller cells, the number of channels available is increased as well as the capacity of the cells. The power level of the transmitters used in these cells is then decreased, reducing the possibility of interference between neighboring cells. Selective cells
It is not always useful to define a cell with a full coverage of 360 degrees. In some cases, cells with a particular shape and coverage are needed. These cells are called selective cells. A typical example of selective cell is the cell that may be located at the entrances of tunnels where coverage of 360 degrees is not needed. In this case, a selective cell with coverage of 120 degrees is used. Umbrella cells A freeway crossing very small cells produces an important number of handovers among the different small neighboring cells. In order to solve this problem, the concept of umbrella cells is introduced.
An umbrella cell covers several microcells. The power level inside an umbrella cell is increased comparing to the power levels used in the microcells that form the umbrella cell. When the speed of the mobile is too high, the mobile is handed off to the umbrella cell. The mobile will then stay longer in the same cell (in this case the umbrella cell). This will reduce the number of handovers and the work of the network. GSM-Transmission From Source Information to Radio Waves [pic] Figure: Different Operations that have to be performed in order to pass from the speech source to radio waves and vice versa.
Speech Coding The transmission of speech is at the moment, the most important service of a mobile cellular system. The GSM speech codec, which will transform the analog signal (voice) into a digital representation, has to meet the following criteria: • A good speech quality, at least as good as the one obtained with previous cellular systems. • To reduce the redundancy in the sounds of the voice. This reduction is essential due to the limited capacity of transmission of a radio channel. • The speech codec must not be very complex because complexity is equivalent to high costs.
The final choice for the GSM speech code is a code named RPE-LTP (Regular Pulse Excitation Long-Term Prediction). This code uses the information from previous samples this information does not change very quickly) in order to predict the current sample. The speech signal is divided into blocks of 20 milliseconds. These blocks are then passed to the speech code, which has a rate of 13 kbps, in order to obtain blocks of 260 bits. Channel Coding Channel coding adds redundancy bits to the original information in order to detect and correct, if possible, errors occurred during the transmission.
Channel Coding for the GSM Data TCH channels The channel coding is performed using two codes: a block code and a convolution code. The block code corresponds to the block code defined in the GSM Recommendations 05. 03. The block code receives an input block of 240 bits and adds four zero tail bits at the end of the input block. The output of the block code is consequently a block of 244 bits. A convolution code adds redundancy bits in order to protect the information. A convolution encoder contains memory. This property differentiates a convolution code from a block code.
A convolution code can be defined by three variables: n, k and K. The value n corresponds to the number of bits at the output of the encoder, k to the number of bits at the input of the block and K to the memory of the encoder. The ratio, R, of the code is defined as follows: R = k/n. Let’s consider a convolution code with the following values: k is equal to 1, n to 2 and K to 5. This convolution code uses then a rate of R = 1/2 and a delay of K = 5, which means that it will add a redundant bit for each input bit. The convolution code uses 5 consecutive bits in order to compute the redundancy bit.
As the convolution code is a 1/2-rate convolution code, a block of 488 bits is generated. These 488 bits are punctured in order to produce a block of 456 bits. Thirty two bits, obtained as follows, are not transmitted: C (11 + 15 j) for j = 0, 1… 31 The block of 456 bits produced by the convolution code is then passed to the interleaver. Channel Coding for the GSM Speech Channels Before applying the channel coding, the 260 bits of a GSM speech frame are divided in three different classes according to their function and importance. The most important class is the class IA containing 50 bits.
Next in importance is the class IB, which contains 132 bits. The least important is the class II, which contains the remaining 78 bits. The different classes are coded differently. First of all, the class IA bits are block-coded. Three parity bits, used for error detection, are added to the 50 class IA bits. The resultant 53 bits are added to the class IB bits. Four zero bits are added to this block of 185 bits (50+3+132). A convolutional code, with r = 1/2 and K = 5, is then applied, obtaining an output block of 378 bits. Channel Coding for the GSM Control Channels
In GSM the signaling information is just contained in 184 bits. Forty parity bits, obtained using a fire code, and four zero bits are added to the 184 bits before applying the convolutional code (r = 1/2 and K = 5). The output of the convolutional code is then a block of 456 bits, which does not need to be punctured. Interleaving Interleaving rearranges a group of bits in a particular way. It is used in combination with FEC codes in order to improve the performance of the error correction mechanisms. The interleaving decreases the possibility of losing whole bursts during the transmission, by dispersing the errors.
Being the errors less concentrated, it is then easier to correct them. Interleaving for the GSM Control Channels A burst in GSM transmits two blocks of 57 data bits each. Therefore the 456 bits corresponding to the output of the channel coder fit into four bursts (4*114 = 456). The 456 bits are divided into eight blocks of 57 bits. The first block of 57 bits contains the bit numbers (0, 8, 16 … 448), the second one the bit numbers (1, 9, 17 … 449), etc. The last block of 57 bits will then contain the bit numbers (7, 15 … 455). The first four blocks of 57 bits are placed in the even-numbered bits of four bursts.
The other four blocks of 57 bits are placed in the odd-numbered bits of the same four bursts. Therefore the interleaving depth of the GSM interleaving for control channels is four and a new data block starts every four bursts. The interleaver for control channels is called a block rectangular interleaver. Interleaving for the GSM Speech Channels The block of 456 bits, obtained after the channel coding, is then divided in eight blocks of 57 bits in the same way as it is explained in the previous paragraph. But these eight blocks of 57 bits are distributed differently.
The first four blocks of 57 bits are placed in the even-numbered bits of four consecutive bursts. The other four blocks of 57 bits are placed in the odd-numbered bits of the next four bursts. The interleaving depth of the GSM interleaving for speech channels is then eight. A new data block also starts every four bursts. The interleaver for speech channels is called a block diagonal interleaver Interleaving for the GSM Data TCH Channels A particular interleaving scheme, with an interleaving depth equal to 22, is applied to the block of 456 bits obtained after the channel coding.
The block is divided into 16 blocks of 24 bits each, 2 blocks of 18 bits each, 2 blocks of 12 bits each and 2 blocks of 6 bits each. It is spread over 22 bursts in the following way: • the first and the twenty-second bursts carry one block of 6 bits each • the second and the twenty-first bursts carry one block of 12 bits each • the third and the twentieth bursts carry one block of 18 bits each • from the fourth to the nineteenth burst, a block of 24 bits is placed in each burst A burst will then carry information from five or six consecutive data blocks.
The data blocks are said to be interleaved diagonally. A new data block starts every four bursts. Burst Assembling The burst assembling procedure is in charge of grouping the bits into bursts. Section 5. 2. 3 presents the different bursts structures and describes in detail the structure of the normal burst. Ciphering Ciphering is used to protect signaling and user data. First of all, a ciphering key is computed using the algorithm A8 stored on the SIM card, the subscriber key and a random number delivered by the network (this random number is the same as the one used for the authentication procedure).
Secondly, a 114 bit sequence is produced using the ciphering key, an algorithm called A5 and the burst numbers. This bit sequence is then XORED with the two 57 bit blocks of data included in a normal burst. In order to decipher correctly, the receiver has to use the same algorithm A5 for the deciphering procedure. Modulation The modulation chosen for the GSM system is the Gaussian Minimum Shift Keying (GMSK). The GMSK modulation has been chosen as a compromise between spectrum efficiency, complexity and low spurious radiations (that reduce the possibilities of adjacent channel interference).
The GMSK modulation has a rate of 270 kbps and a BT product equal to 0. 3. [pic]Figure: GMSK Modulation Scheme Transmission Problems Path Loss Path loss (or path attenuation) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system. This term is commonly used in wireless communications and signal propagation. Path loss may be due to many effects, such as free-space loss, refraction, diffraction, reflection, aperture-medium coupling loss, and absorption.
Path loss is also influenced by terrain contours, environment (urban or rural, vegetation and foliage), propagation medium (dry or moist air), the distance between the transmitter and the receiver, and the height and location of antennas. Shadowing In wireless communications, fading is deviation or the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modeled as a random process. A fading channel is a communication channel that experiences fading.
In wireless systems, fading may either be due to multipath propagation, referred to as multipath induced fading, or due to shadowing from obstacles affecting the wave propagation, sometimes referred to as shadow fading. Rayleigh Fading Rayleigh fading is caused by multipath reception. The mobile antenna receives a large number, say N, reflected and scattered waves. Because of wave cancellation effects, the instantaneous received power seen by a moving antenna becomes a random variable, dependent on the location of the antenna.
Below is the figure showing a sample of a Rayleigh fading signal. Signal amplitude (in dB) versus time for an antenna moving at constant velocity. Notice the deep fades that occur occasionally. Although fading is a random process, deep fades have a tendency to occur approximately every half a wavelength of motion. | | | | [pic]Figure: Typical signal in a channel with Rayleigh Fading Time Dispersion Time Dispersion is another problem relating to multiple paths to the Rx antenna of either an MS or BTS.
However, in contrast to Rayleigh Fading, the reflected signal comes from an object far away from the Rx antenna Time Dispersion causes Inter Symbol Interference (ISI) where consecutive symbols (bits) interfere with each other making it difficult for the receiver to determine which symbol is the correct one. Time Alignment Each MS on a call is allocated a time slot on a TDMA frame. This is an amount of time during which the MS transmits information to the BTS. The information must also arrive at the BTS within the time slot.
The Time Alignment problem occurs when part of the information transmitted by an MS does not arrive within the allocated time slot. Instead that may arrive during the next time slot, and may interfere with information from anther MS using that other time slot. GSM – Operations The operation of the GSM system can be understood by studying the sequence of events that takes place when a call is initiated from the Mobile Station. Call from Mobile Phone to PSTN: When a mobile subscriber makes a call to a PSTN telephone subscriber, the following sequence of events takes place: 1.
The MSC/VLR receives the message of a call request. 2. The MSC/VLR checks if the mobile station is authorized to access the network. If so, the mobile station is activated. If the mobile station is not authorized, service will be denied. 3. MSC/VLR analyzes the number and initiates a call setup with the PSTN. 4. MSC/VLR asks the corresponding BSC to allocate a traffic channel (a radio channel and a time slot). 5. The BSC allocates the traffic channel and passes the information to the mobile station. 6. The called party answers the call and the conversation takes place. 7.
The mobile station keeps on taking measurements of the radio channels in the present cell and neighboring cells and passes the information to the BSC. The BSC decides if handover is required, if so, a new traffic channel is allocated to the mobile station and the handover is performed. If handover is not required, the mobile station continues to transmit in the same frequency. Call from PSTN to Mobile Phone: When a PSTN subscriber calls a mobile station, the sequence of events is as follows: 1. The Gateway MSC receives the call and queries the HLR for the information needed to route the call to the serving MSC/VLR. . The GMSC routes the call to the MSC/VLR. 3. The MSC checks the VLR for the location area of the MS. 4. The MSC contacts the MS via the BSC through a broadcast message, that is, through a paging request. 5. The MS responds to the page request. 6. The BSC allocates a traffic channel and sends a message to the MS to tune to the channel. The MS generates a ringing signal and, after the subscriber answers, the speech connection is established. 7. Handover, if required, takes place, as discussed in the earlier case. The MS codes the speech at 13 Kbps for transmission over the radio channel in the given time slot.
The BSC converts (or transcodes) the speech to 64 Kbps and sends it over a land link or radio link to the MSC. The MSC then forwards the speech data to the PSTN. In the reverse direction, the speech is received at 64 Kbps rate at the BSC and the BSC does the transcoding to 13 Kbps for radio transmission. In its original form, GSM supports 9. 6 Kbps data, which can be transmitted in one TDMA time slot. Over the last few years, many enhancements were done to the GSM standards (GSM Phase 2 and GSM Phase 2+) to provide higher data rates for data applications. GSM – Protocol Stack
The layered model of the GSM architecture integrates and links the peer-to-peer communications between two different systems. The underlying layers satisfy the services of the upper-layer protocols. Notifications are passed from layer to layer to ensure that the information has been properly formatted, transmitted, and received. The GMS protocol stacks diagram is shown below: [pic] MS Protocols: The signaling protocol in GSM is structured into three general layers, depending on the interface. • Layer 1: The physical layer, which uses the channel structures over the air interface. • Layer 2: The data-link layer.
Across the Um interface, the data-link layer is a modified version of the Link access protocol for the D channel (LAP-D) protocol used in ISDN, called Link access protocol on the Dm channel (LAP-Dm). Across the A interface, the Message Transfer Part (MTP), Layer 2 of SS7 is used. • Layer 3: The third layer of the GSM signaling protocol is divided into three sublayers: o Radio Resource management (RR) o Mobility Management (MM) and o Connection Management (CM). The MS to BTS Protocols: The RR layer oversees the establishment of a link, both radio and fixed, between the MS and the MSC.
The main functional components involved are the MS, the BSS, and 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. The MM layer is built on top of the RR layer and handles the functions that arise from the mobility of the subscriber, as well as the authentication and security aspects. Location management is concerned with the procedures that enable the system to know the current location of a powered-on MS so that incoming call routing can be completed.
The CM layer is responsible for CC, supplementary service management, and Short Message Service (SMS) management. Each of these may be considered as a separate sublayer within the CM layer. Other functions of the CC sublayer include call establishment, selection of the type of service (including alternating between services during a call), and call release. BSC Protocols: After the information is passed from the BTS to the BSC, a different set of interfaces is used. The Abis interface is used between the BTS and BSC.
At this level, the radio resources at the lower portion of Layer 3 are changed from the RR to the Base Transceiver Station Management (BTSM). The BTS management layer is a relay function at the BTS to the BSC. The RR protocols are responsible for the allocation and reallocation of traffic channels between the MS and the BTS. These services include controlling the initial access to the system, paging for MT calls, the handover of calls between cell sites, power control, and call termination. The RR protocols provide the procedures for the use, allocation, reallocation, and release of the GSM channels.
The BSC still has some radio resource management in place for the frequency coordination, frequency allocation, and the management of the overall network layer for the Layer 2 interfaces. From the BSC, the relay is using SS7 protocols so the MTP 1-3 is used as the underlying architecture, and the BSS mobile application part or the direct application part is used to communicate from the BSC to the MSC. MSC Protocols: At the MSC, the information is mapped across the A interface to the MTP Layers 1 through 3 from the BSC. Here the equivalent set of radio resources is called the BSS MAP.
The BSS MAP/DTAP and the MM and CM are at the upper layers of Layer 3 protocols. This completes the relay process. Through the control-signaling network, the MSCs interact to locate and connect to users throughout the network. Location registers are included in the MSC databases to assist in the role of determining how and whether connections are to be made to roaming users. Each user of a GSM MS is assigned a HLR that is used to contain the user’s location and subscribed services. A separate register, the VLR, is used to track the location of a user.
As the users roam out of the area covered by the HLR, the MS notifies a new VLR of its whereabouts. The VLR in turn uses the control network (which happens to be based on SS7) to signal the HLR of the MS’s new location. Through this information, MT calls can be routed to the user by the location information contained in the user’s HLR. AXE 10 System Survey Introduction AXE is a multi-application, open-ended digital switching product for public telecommunications networks. It has real-time processing capacity and can handle high volumes of traffic.
AXE is based on a model in which all functionality (switching, subscriber and network access, operation and maintenance, traffic control, charging control) is handled by each node in the network. AXE as a multi-application platform When AXE was introduced into the market it supported only the major telecommunications application, PSTN. Since then AXE has been continuously developed in response to the demands of modern telecommunications. AXE supports a wide range of applications in addition to PSTN: ISDN PLMN Business Communications
Overlaying these networks are Intelligent Networks (IN) and signaling networks, which AXE also supports. AXE provides functionality at different levels in these networks. AXE in Ericsson’s GSM Systems Ericsson’s GSM systems are based on AXE. This means that the features and services built into AXE can be provided as standard within CME 20/CMS 40. It also means that Ericsson’s GSM systems will benefit from the future development of AXE. The AXE-based nodes in Ericsson’s GSM systems are: • MSC/VLR • GMSC • HLR • ILR • SCP • BSC AXE System Architecture The key to the success of AXE is its unique flexibility and modularity.
Modularity allows AXE to readily adapt to the changing requirements of networks and of end-users. This modularity means ease of handling which leads to reduced costs and the flexibility to adapt to the changing world of telecommunications. Modularity is implemented in a number of ways in AXE. These are described below: Functional Modularity AXE is designed in such a way that nodes with different functions can be generated from the same system. For example, an AXE can act as an MSC/VLR or as a HLR. This can be achieved due to software and hardware modularity. Software Modularity
AXE consists of a set of independent building blocks (known as function blocks), each performing a specific function and communicating with each other by means of defined signals and interfaces. Software modularity means that function blocks can be added, deleted or modified without requiring changes or redesign of other parts of the system. Hardware Modularity The physical packaging of AXE offers a high degree of flexibility and is based on industry-standard specifications. The packaging system contributes to ease of handling during design, manufacturing, installation and operation and maintenance.
The basic building blocks of the packaging system are the plug-in units and the containers for these units, sub racks. Plug-in units can be replaced or removed without disturbing other equipment. Technological Modularity AXE is an open-ended switching platform. This allows new technologies and functions to be added, enabling the continuous development of AXE. For example, AXE was not originally conceived for mobile applications, but when mobile was being developed, AXE proved to be the most suitable platform. AXE Structure There are currently two basic types of structure for AXE: Non-Application Modularity based AXE systems (AXE 105) • Application modularity based AXE systems (AXE 106) Non-Application Modularity based AXE Systems An example of an AXE node which is implemented without using Application Modularity is the BSC. System Level 1 System Level 1 is the AXE 105 system itself and is a combination of System Level 2 systems. System Level 2 At System Level 2, AXE 105 is divided into two: • APT: this is the switching and telecommunications applications part of AXE • APZ: this is the control or operating system part of AXE Subsystem Level
Every AXE is a combination of APT and APZ subsystems. Similar functions (e. g. charging functions) are grouped together in one subsystem. Set of Parts If required, a set of parts can be used between the subsystem level and function block level. This groups a set of function blocks which perform tasks relating to a similar function Function Block Level The tasks allocated to a certain subsystem are further divided into individual function blocks. Each function block constitutes a well defined unit with its own data and with standardized signal interworking.
Function Unit Level Every function block consists of function units. There are 3 types of function units: • A hardware unit • A regional software unit which deals with routine work such as the scanning of hardware devices • A central software unit which is responsible for the more complex analysis functions required, e. g. call set-up A function block may consist of all three together or central software only. Control System Architecture Another important factor behind the flexibility of AXE is the control system architecture.
AXE is a Stored Program Control (SPC) exchange. That is, software programs stored in the AXE computer control the operation of the AXE switching equipment. This is a two level system with both central and distributed control. This approach offers reliability and call handling efficiency. The control system, APZ, is a two-level system with centralized and distributed logic. The central processing level consists of a duplicated Central Processor (CP) working in parallel synchronous mode. At the distributed level there are a number of Regional Processors (RP) working in pairs.
The control system, APZ, is a two-level system with centralized and distributed logic. The central processing level consists of a duplicated Central Processor (CP) working in parallel synchronous mode. At the distributed level there are a number of Regional Processors (RP) working in pairs. The RPs control hardware units called Extension Modules (EM). One RP-pair can control up to 16 EMs. The number of EMs connected to one RP-pair depends on the complexity of the tasks to be performed. The more RP capacity required, the fewer EMs can be connected. Two RPs in a pair share the work load of controlling the EMs.
The Input/Output (I/O) system provides connections with I/O devices such as terminals, printers, alarm displays, data links, flexible disks, hard disks and magnetic tapes. The I/O system performs all input/output functions and processes various maintenance, administrative, performance and call-related data. Introduction to Physical and Logical Channels Each timeslot on a TDMA frame is called a Physical Channel. Therefore, there are 8 physical channels per carrier frequency in GSM. Physical Channels can be used to transmit speech, data or signaling information.
MHz 890915 TDMA Frame n TDMA Frame n+1 TDMA Frame n+2 TDMA Frame n+x Physical Channel 5 TDMA Frame n n+1 n+2 n+x TCH TCH FACCH TCH Figure: The TDMA Channel Concept A Physical Channel may carry different messages, depending on the information that is to be sent. These messages are called Logical Channels. For example, on one of the physical channels used for traffic, the traffic itself is transmitted using a Traffic Channel (TCH) message, while a handover instruction is transmitted using a Fast Associated Control Channel (FACCH) message.
Logical Channels Many types of logical channels exist each designed to carry a different message to or from a Mobile Station (MS). All information to and from an MS must be formatted correctly, so that the receiving device can understand the meaning of different bits in the message. For example, as seen previously, in the burst used to carry traffic, some bits represent the speech or data itself, while others are used as a training sequence. Broadcast Channels (BCH’s) | |Logical Channel |Direction |BTS |MS | |Frequency Correction Channel |Downlink |Transmits a Carrier |Identifies BCCH carrier by | |(FCCH) |Point to Multipoint |Frequency |the carrier frequency and | | | | |synchronizes with the | | | | |frequency | |Synchronization Channel |Downlink |Transmits information about|Synchronizes with the frame | |(SCH) |Point to Multipoint |the TDMA Frame Structure in|structure within a particular| | | |a cell(e. g. Frame number) |cell, and ensures that the | | | |and the BTS identity -BSIC |chosen BTS is a GSM BTS-BSIC | | | | |can only be decoded by an MS | | | | |if the BTS belongs to a
GSM | | | | |network | |Broadcast Control Channel |Downlink |Broadcasts some general |Receives LAI and will signal | |(BCCH) |Point to Multipoint |cell information such as |to the network as part of the| | | |Location Area Identity |Location Updating procedure | | | |(LAI), maximum output power|if the LAI is different to | | | |allowed in the cell and |the one already stored on its| | | |identity of BCCH carriers |SIM.
MS sets its output power| | | |for neighboring cells |level based on the | | | | |information received on the | | | | |BCCH carriers on which it | | | | |will perform measurements to | | | | |assist in efficient handover. | Dedicated Control Channels (DCCH’s) | |Logical Channel |Direction |BTS |MS | |Stand Alone Dedicated Control |Uplink and Downlink |The BTS switches to the |The MS switches to the | |Channel (SDCCH) |Point to Point |assigned SDCCH. The call |assigned SDCCH. Call set-up | | | |set-up procedure is |is performed. The MS receives| | | |performed in idle mode. The |TCH assignment information. | | |BSC assigns a TCH |(carrier and time slot) | | | |(SDCCH is also used to | | | | |transmit text messages SMS) | | |Cell Broadcast Channel |Downlink |Uses this logical channel to|MS receives cell broadcast | |(CBCH) |Point to Multipoint |transmit short message |messages. | | | |service cell broadcast. | |Slow Associated Control Channel |Uplink and Downlink |Instructs the MS the |Sends averaged measurements | |(SACCH) |Point to Point |transmitting power to use |on its own BTS (signal | | | |and gives instructions on |strength and quality) and | | | |Timing Advance (TA) |neighboring BTS’s (signal | | | | |strength). The MS continues | | | | |to use SACCH for this purpose| | | | |during a call. | |Fast Associated Control Channel |Uplink and Downlink |Transmits handover |Transmits necessary handover | |(FACCH) |Point to Point |information |information in access burst. | Common Control Channels (CCCH’s) | |Logical Channel |Direction |BTS |MS | |Paging Channel (PCH) |Downlink |Transmits a paging message |At certain time intervals the| | |Point to Point |to indicate an incoming |MS listens to the PCH. If it | | | |call or short message. The |identifies its own mobile | | | |paging message contains the|subscriber identity number on| | | |identity number of the |the PCH, it will respond | | | |mobile subscriber that the | | | | |network wishes to contact. | |Random Access Channel |Uplink |Receives request from MS |Answers paging message on the| |(RACH) |Point to Point |for a signaling channel( to|RACH by requesting a | | | |be used for call set-up) |signaling channel. | |Access Grant Channel |Downlink |Assigns a signaling channel|Receives signaling channel | |(AGCH) |Point to Point |(SDCCH) to the MS. |assignment (SDCCH) | Traffic Channels (TCH)
Once call set-up procedures have been on the control physical channel, the MS tunes to a Traffic Physical Channel. It uses the Traffic Channel (TCH) logical channel. There are two types of TCH. • Full Rate TCH: Transmits full rate speech (13 kbps). A full rate TCH occupies one physical channel. • Half Rate TCH: Transmits half rate speech (6. 5 kbps). Two half rate TCH’s can share one physical channel thus doubling the capacity. GSM – User Services GSM has much more to offer than voice telephony. Additional services allow you greater flexibility in where and when you use your phone. You should contact your local GSM network operator for information on the specific services available to you.
But there are three basic types of services offered through GSM which you can ask for: • Telephony (also referred to as teleservices) Services • Data (also referred to as bearer services) Services. • Supplementary Services Teleservices or Telephony Services: A Teleservice utilises the capabilities of a Bearer Service to transport data, defining which capabilities are required and how they should be set up. Voice Calls: The most basic Teleservice supported by GSM is telephony. This includes Full-rate speech at 13 Kbps and emergency calls, where the nearest emergency? service provider is notified by dialing three digits. A very basic example of emergency service is 911 service available in USA. Videotext and Facsmile:
Another group of teleservices includes Videotext access, Teletex transmission, Facsimile alternate speech and facsimile Group 3, Automatic facsimile Group 3 etc. Short Text Messages: SMS (Short Messaging Service) service is a text messaging which allow you to send and receive text messages on your GSM Mobile phone. Services available from many of the world’s GSM networks today – in addition to simple user generated text message services – include news, sport, financial, language and location based services, as well as many early examples of mobile commerce such as stocks and share prices, mobile banking facilities and leisure booking services.
Bearer Services or Data Services Using your GSM phone to receive and send data is the essential building block leading to widespread mobile Internet access and mobile data transfer. GSM currently has a data transfer rate of 9. 6k. New developments that will push up data transfer rates for GSM users are HSCSD (high speed circuit switched data) and GPRS (general packet radio service) are now available. Supplementary Services Supplementary services are provided on top of teleservices or bearer services, and include features such as caller identification, call forwarding, call waiting, multi? party conversations, and barring of outgoing (international) calls, among others.
A brief description of supplementary services is given here: • Multiparty Service or conferencing: The multiparty service allows a mobile subscriber to establish a multiparty conversation. that is, a simultaneous conversation between three or more subscribers to setup a conference call. This service is only applicable to normal telephony. • Call Waiting: This service allows a mobile subscriber to be notified of an incoming call during a conversation. The subscriber can answer, reject, or ignore the incoming call. Call waiting is applicable to all GSM telecommunications services using a circuit-switched connection. • Call Hold: This service allows a subscriber to put an incoming call on hold and then resume this call. The call hold service is only applicable to normal telephony. Call Forwarding: The Call Forwarding Supplementary Service is used to divert calls from the original recipient to another number, and is normally set up by the subscriber himself. It can be used by the subscriber to divert calls from the Mobile Station when the subscriber is not available, and so to ensure that calls are not lost. A typical scenario would be a salesperson turns off his mobile phone during a meeting with customers, but does not with to lose potential sales leads while he is unavailable. • Call Barring: The concept of barring certain types of calls might seem to be a supplementary disservice rather than service. However, there are times when the subscriber is not the actual user of the Mobile Station, and as a consequence may wish to limit its functionality, so as to limit the charges incurred.
Alternatively, if the subscriber and user are one and the same, the Call Barring may be useful to stop calls being routed to international destinations when they are routed. The reason for this is because it is expected that the roaming subscriber will pay the charges incurred for international re-routing of calls. So, GSM devised some flexible services that enable the subscriber to conditionally bar calls. • Number Identification: There are following supplementary services related to number identification: o Calling Line Identification Presentation: This service deals with the presentation of the calling party’s telephone number. The concept is for this number to be presented, at the start of the phone ringing, so that the called person can determine who is ringing prior to answering.
The person subscribing to the service receives the telephone number of the calling party. o Calling Line Identification Restriction: A person not wishing their number to be presented to others subscribes to this service. In the normal course of event, the restriction service overrides the presentation service. o Connected Line Identification Presentation: This service is provided to give the calling party the telephone number of the person to whom they are connected. This may seem strange since the person making the call should know the number they dialled, but there are situations (such as forwardings) where the number connected is not the number dialled.
The person subscribing to the service is the calling party. o Connected Line Identification Restriction: There are times when the person called does not wish to have their number presented and so they would subscribe to this person. Normally, this overrides the presentation service. o Malicious Call Identification: The malicious call identification service was provided to combat the spread of obscene or annoying calls. The victim should subscribe to this service, and then they could cause known malicious calls to be identified in the GSM network, using a simple command. This identified number could then be passed to the appropriate authority for action.
The definition for this service is not stable. • Advice of Charge (AoC): This service was designed to give the subscriber an indication of the cost of the services as they are used. Furthermore, those Service Providers who wish to offer rental services to subscribers without their own Subscriber Identity Module (SIM) can also utilize this service in a slightly different form. AoC for data calls is provided on the basis of time measurements. • Closed User Groups (CUGs): This service is provided on GSM to enable gr