Telecommunication: The world Telecommunication consists of two wards i.e. tele and communication. This is a Greek word which means “at a distance”. Hence telecommunication means that communication at distant end i.e. sending message at distant end which may be in the form of voice, data or pictorial.

There are three basic parts of modern Telecommunication System:1. Switching: The equipment is centrally located in which, the subscriber after having access can establish a call by dialing or in some cases by verbal order. The call charges and other metering equipment are also available in same place.2. Transmission: The equipment employed there in is for transmitting and receiving messages from distant ends.3. Access Network: It is a network which is actually installed for the connections of two parties which may be engaged in local, trunk or international calls. There are three types of access network:(i) Traditional Access Network: It is the old system and very much in practice. It is also known as out side plant. Cables and over head wires are constructed for connection of the subscribers in city area.(ii) Optical Fiber: It is the modern technique used for the interconnection of cities together for the transmission of speech channels, data channels, radio programs and T.V channels. It is also used for interconnection b/w local exchanges. Optical Fiber holds a great advantage over the two pair copper media because it can handle high speed signal over extended distances. There are two types of optical fiber.(a) Single Mode Optical Fiber: It can carry signal up to 3KM or 3000Meters without amplification.(b) Multi-Mode Optical Fiber: It can carry signal up to 2KM or 2000Meters without amplification.(III) Wireless Local Loop: Use of this equipment has eliminated the old copper wire and Local cables which are being used for the last so many years for the interconnection of subscriber premises. The atmosphere now being used as a media between the subscribers and the exchanges or we can say that it replaces the cable system & drop wire with radio system between the switch and end user’s home. The WLL provides an effective means to deploy telephone service in areas where geographical limitations make conventional cable installation difficult like rural areas, mountains, port areas and residential districts with fast growing service demands.Problems in Traditional Access Network:1- Lying of cables and erection of overhead wires is difficult in thickly populated areas.2- Cost of laying and erection is very high.3- Disturbance is caused to the public during laying process

4- Fault localization is not easy.5- Breakdown of communication occurs due to other agencies digging for lying of other services like sewerage, water supply, electricity, Sui-Gas etc.

Wireless Local Loop: Traditionally the provision of voice and data communications to the end user over the local loop or subscriber loop has been provided by wired systems. For residential subscribers twisted pair has been and continues to be the standard means of connection. For business and government subscribers twisted pair coaxial cable and optical fiber are in use.As subscribers have demanded greater capacity, particularly to support internet use, traditional twisted pair technology has become inadequate. Telecommunications providers have developed number technologies to meet the need including ISDN and a family of digital subscriber loop technologies known as xDSL. In addition cable operations have introduced two-way high-speed service using cable modem technology. Thus, wired technologies are responding to the need for reliable high speed access by residential, business, and government subscribers. However increasing interest is being shown in competing wireless technologies for subscriber access. These approaches are generally referred to as wireless local loop (WLL) or fixed wireless access. WLL alternatives are narrowband, which offer a replacement for existing telephony services, and broadband, which provide high-speed two-way voice and data service.Hence Wireless Local Loop (WLL) is a system that connects subscribers to the public switched Telephone Network (PSTN) using radio signals as a substitute for copper all or part of connection between the subscriber and the switch. As WLL is largely based on Mobile cellular Telephony’s architecture, it uses used Radio base stations connected to common public switching exchanges to reach via radio the subscriber’s residence or office fixed terminal, equipped with radio transceiver. The utilization of several radio base stations each of them covering an area, called Cell, guarantees the covering of the whole pre-determined region. Due to restricted mobility of the subscriber’s terminal, there is no need for handoff (mobility in between cells), the connection to a PSTN is generally availableThe Role of WLL:A WLL provider services one or more cells. Each cell. Each cell includes a base station antenna mounted on top of a tall building or tower. Individual subscribers have a fixed antenna mounted on a building or pole that has an unobstructed line of sight to the base station antenna. From the base station, there is a link, which may either be wired or wireless, to a switching center. The switching center typically a telephone company local office. This provides connections to the local and long distance telephone networks. An Internet service provider (ISP) may be collocated at the switch or connected to the switch by high-speed link.The figure below shows a two level hierarchy. More complex configurations have also been implemented in which a base station may serve a number of subordinate base station antennas, each of which supports a number of subscribers.

The WLL has a number of advantages over a wired approach to subscriber loop support:Cost: Wireless systems are less expensive than wired systems. Although the electronics of the wireless transmitter may be more expensive than those used for wired communications, with WLL

the cost of installing kilometers of cable, either underground or on poles is avoided, as well as the cost of maintaining the wired infrastructure.Installation Time: WLL system typically can be installed rapidly. The key stumbling blocks are obtaining permission to use a given frequency band and finding a suitable elevated site for the base station antennas. Once these hurdles are cleared, a WLL system can be installed in a small fraction of the time required for a new wired system.Selective Installation: Radio units are installed only for those subscribers who want the service at a given time. With a wired system, typically cable is laid out in anticipation of serving every subscriber in a local area.The other advantages are:-- Quality of wire less technology has improved the speech quality -- Cellular systems are too expensive with lesser signal quality than fixed broad band wireless which uses directional antennasWLL needs to be evaluated with respect to two alternatives:Wired scheme using existing installed cable: A large fraction of the earth’s inhabitants do not have a telephone line. For high speed applications, many subscribers with telephone lines do not have a line of sufficient quality or are too far from the central office to effectively use xDSL. Many of these same subscribers also do not have cable TV or their cable provider does not offer two way data service. Finally, because WLL has become cost competitive with wired schemes, new installation face a genuine choice between the wired and wireless approaches.Mobile cellular technology: Current cellular systems are too expensive and do not provide sufficient facilities to act as a realistic alternative to WLL. Even when 3G systems become available they are likely to be more expensive and less functional than broadband WLL alternatives. A major advantage of WLL over mobile cellular is that, because the subscriber unit is fixed, the subscriber can use a directional antenna pointed at the base station antenna, providing improved signal quality in both directions.Advantages of WLL:1- Cost of installation and Maintenance of WLL is lower than cable network2- Installation time is less.3- Selective installation for those who require connection at certain time.4- Quality of wire less technology has improved the speech quality 5- Cellular systems are too expensive with lesser signal quality than fixed broad band wireless which uses directional antennas Need for WLL:1- The Global for Telephone Network Access is driven by:

The pent-up demand of existing telecom services. The economic pressure to expand a region’s access to telecom. The impacts to deregulation.

2- The target of raising tele-density.3- There are many populations in remote rural Areas and rugged terrain.4- Cost of local loop is much less than traditional copper wire.5- Despite of much higher innovations in telecom sector, hundred of million of subscribers are waiting for dial tone.6- The motivation for local loop deployment thus can drive from:

The extension of existing service for new areas. The support of new or competitive network in both advance and underdeveloped

markets e.g. provision of direct local access. To by pass the existing local network.

WLL Primary Requirement:1- Fast and cost effective network deployment.2- Flexible – subscriber locations not known.3- Low cost.4- Integration in the existing network.5- Shortest possible time to market.6- Virtually un-limited capacity on demand.7- Support of value added services like ISDN.

8- Quality of service compatible with PSTN standards.9- Equal service access provided to urban and rural users.11- Key factor for success: competitive on price and service level.12- Customer’s equipment must be inexpensive and easy to use.Technical requirements of WLL Systems:1- Communications quality.2- Short construction period.3- Absence of interference with other wireless systems.4- High traffic volume.5- A successful WLL system must meet standards in:

Dropped calls and Fades (Fading: the net result of all the waves at the receiving and is zero)

Static and cross-talk. Privacy. Data rates. Voice quality. Fraud prevention capacities.

GSM An Introduction: Today GSM stand for Global System for Mobile Communication. It is the globally accepted standard for cellular communication.The idea of cell based mobile radio system (mobile phones) was born back in the early 1970 at Bell Laboratories in the USA. Although it was not until the early 1980 when the first commercial system was established in Scandinavia. Analogue cellular telephone systems experience a very rapid growth in Europe particularly in Scandinavia and the UK.At this early stage every country began developing its own system each with its own standard, which was incompatible with everyone else. This was an undesirable situation because it meant that equipment was limited to operate only one country’s network and the market for each mobile device was limited.In order to overcome these problems the Conference of European Post and Telecommunication (CEPT) formed the Group Special Mobile (GSM) in 1982 to develop a pan-European mobile cellular radio system. Only later did the acronym GSM come to stand for Global System for Mobile Communications.The standardized system had to meet the following criteria:

Spectrum efficiency (Limited radio frequencies) International roaming Low mobile base station costs Good subjective voice quality Compatibility with other systems / technology such as ISDN and PDAs Ability to support new services

Unlike the existing analogue cellular technology the GSM system was developed using digital technology. The decision to adopt digital technology was taken for a number of reasons.For example with the huge numbers of subscribers it was not going to be long before the analogue system would not be able to cope with the limited number of existing frequencies. Plans to expand the frequency range were proposed but these plans were rejected by al large number of countries because of the restricted spectrum.Also the restriction on frequencies combined with the development of new telecommunication technology such as ISDN presented many problems directly related to quality and compatibility. GSM intended to address these issues.From 1982 – 1985 discussions were held to decide between building an analogue or digital system. After multiple field tests, a digital system was adopted. The next task was to decide between narrowband or broadband solution. In May 1987 the narrowband time division multiple access (TDMA) solution was chosen.In 1989 the responsibility for the GSM specifications passed from CEPT to the European Telecommunication Standards Institute (ETSI). The aim of the GSM specifications was to describe the functionality and the interface for each component of the system and to provide

guidance on the design of the system. These specifications standardized the system guaranteeing the proper inter working of all the different elements of the GSM system. In 1990 phase I of the GSM specifications were published but the commercial use of GSM did not start until mid 1991. Initially only the larger cities and airports were covered. By the end of 1993 there were 36 GSM networks in 22 countries, with 25 additional countries having selected or considering GSM.Already GSM had gone far beyond its European target audience, South Africa Australia and many middle-east and far-east countries had chosen GSM. By the beginning of 1994 there were 1.3 million subscribers worldwide. In December 1995 there were over 10 million subscribers in Europe alone, and GSM systems existing in every continent around the world GSM earned its current meaning Global System for mobile communication.Today GSM has become the world’s most widely used mobile system used in over 700 networks in over 100 countries.At its 48th meeting on 16thOctober2002 the GSM Association (GSMA) announced that 95% of the world countries have now signed the GSM Memorandum of Understanding.Concept of Cellular System: The concept of cellular system is the use of a set of radio transceivers spread across a geographical area. These transceivers are the gateway for mobile devices to be connected to a communication network.GSM Architecture: The best way to create a manageable communication system is to divide it into various subgroups that are interconnected using standardized interfaces.A GSM system can be divided into three sub systems.

1. The Mobile Station Sub System (MSS).2. The Base Station Subsystem (BSS).3. The Network and Signaling Subsystem (NSS).

1- Mobile Station Subsystem: The mobile station subsystem is the mobile phone equipment whether portable or within a vehicle which houses a radio transceiver and a Subscriber Identity Module other wise knows as a SIM smart card the size of a first class postage stamp. A SIM card can be moved between compatible mobile phone equipment since it is not fixed inside the equipment. It has memory (for data such as personal phone numbers and applications), a processor and the ability to interact with the user. Current SIM typically have 16 to 64 kb of memory which provides plenty of room for storing hundreds of personal phone numbers, text message and value added service. Each SIM card contains an International Mobile Subscriber Identity (IMEI) number and a security key which can be used by the GSM network to identify a unique user and provide authentication. It provides a secure infrastructure for the protection of signaling and user data which is especially useful for e and m commerce. The mobile equipment sometimes referred to as a terminal, contains an International Mobile Equipment Identity (IMEI) number which can be used by the network to identify the equipment. All this information is required by the GSM network to allow the mobile station access to services. These services are offered by a base station with which the mobile station communicates.

2- Base Station Subsystem: It can be divided into two parts(i) Base Transceiver Station (BTS)(ii) Base Station Controller (BSC)(i) Base Transceiver Station (BTS): The BTS handles radio communication of that particular base station with mobile station in a particular area called cell. To allow the good level of service to users the sizes of the cell depends on a number of factors such as

Power of BTS and mobile station. Geographical Features. Population Density

The tasks of BTS includes Channel Coding / Encoding Encryption / Decryption

It composed of transmitters, receivers, antennas and the interface to PCM facility. BTS contains on or more transceivers to provide the required call handling capacity. A cell site may be omni directional or split into typically three directional antenna.(ii) Base Station Controller (BSC): The BSC can manage more then one BTS and usually placed in the middle of the cell. The BSC is responsible for deciding the radio resources to the BTS such as channel setup, frequency, and power levels. The base station subsystem is connected to the network and switching subsystem by means of radio links or another network.The primary function of BCS is call maintenance. The mobile station normally send a report of their received signal strength to BSC every 480 ms with this information the BSC decides to initiate handover to other cell and change the BTS transmitter power etc.

3- Network and Switching Subsystem: This part of GSM network connects the base station subsystem to other network to allow users to communicate with other users by cal routing. The switching part of this subsystem is known as Mobile Switching Center MSC. It acts like a standard exchange in a fixed network and additionally provides all the function need to handle a mobile subscriber. The MSC also has a Gateway function for communication with other networks. It is called gateway MSC (GMSC). The network switching system is also responsible for providing the features like.

Mobility Authentication Security

To perform these functions it is necessary for the network to keep database containing user details so that decisions can be made of how service can be offered to the mobile users. There are four main data base which performs these functions.(i) The home Location Register (HLR)(ii) Visitor Location Register (VLR)(iii) Equipment Identity Register (EIR)(iv) Authentication CenterThese could be within a switching center or be linked remotely by means of some other communication channel.

(i) The Home Location Register (HLR): It is a data base used for management of mobile subscribers. It stores the International Mobile Subscriber Identity (IMEI), mobile station ISDN number and Current Visitor Location Register address. The main information stored their concerns the location of each mobile station in order to able to route calls to he mobile subscribers managed by each HLR. The HLR also maintains the services associated with each MS. One HLR can serve several MSC’s(ii) The Visitor Location Register (VLR): It contains the current location of the MS and 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. AVLR is connected to one MSC and is normally integrated into the MSC’s hardware.(iii) The Authentication Center (AUC): A protected data base that holds a copy of the secret key stored in each subscribers SIM card which is used for authentication and encryption over the radio channel. The AUC provides additional security against fraud. It is normally located close to each HLR with in a GSM network.(iv) The Equipment Identity Register (EIR): It is a data base that contains a list of all valid mobile station equipment with in the network where each mobile station is identified by its International Mobile Equipment Identity (IMEI) number. The EIR has three data bases:White List: for all known, good IMEI’sBlack list: for bad or stolen hand sets.Gray List: for hand sets / IMEI’s those are uncertain.

Operation and Maintenance Center: The OMC is a management system that oversees the GSM functional blocks. The OMC assists the network operator in maintaining satisfactory operation of the GSM network. Hardware redundancy and intelligent error detection mechanisms help prevent network down-time. The OMC is responsible for controlling and maintaining the MSC BSC and BTS. It can be incharge of an entire public Land Mobile Network (PLMN) or just some parts of the PLMN.Interface and Protocols: Providing voice and data transmission quality over the radio link is only part of the function of a cellular mobile network. AGSM mobile can seamlessly roam nationally and internationally, requiring standardized call routing and location updating functions in GSM network. A public communication system also needs solid security mechanisms to prevent misuse by third parties. Security functions such as authentication, encryption and the use of Temporary Mobile Subscriber Identities (TMSI’s) are an absolute must with in a GSM network; different protocols are needed to enable the flow of data and signaling between different GSM subsystems. Multiple Accesses: The ability of system to carry many signals at the same time is known as multiple accesses. Multiple Access allows the communication capacity of the system to be shared among a large number of subscribers (Mobile station or wireless phones), and to accommodate the different mixes of communication traffic that are transmitted by the different subscribers.Multiple Access Techniques:

1. Frequency Division Multiple Access (FDMA) (Narrow Band Technique)2. Time Division Multiple Access (TDMA) (Narrow Band Technique)3. Code Division Multiple Access (FDMA) (Wide Band Technique)

1- Frequency Division Multiple Accesses (FDMA): This technique involves subdividing a wide frequency band into narrower sub-bands which are assigned to various callers. This allows a single base station to serve many callers.

2- Time Division Multiple Access (TDMA): It is one of the several technologies used in wireless communication. TDMA provides each caller with time slots so that several calls can occupy one band width. Each caller is assigned a specific time slot and receives/ transmits signals with in the specified timeslot.

3- Code Division Multiple Access (CDMA): CDMA is a direct sequence spread spectrum system where the entire band width of the system i.e 1.25 MHz is mode available to each user. This technique eliminates the frequency reuse problem in cellular systems.Unlike TDMA and FDMA system where user signals never overlap in either the time or the frequency domains respectively, a CDMA system allows transmission at the same time while using the same frequency. The mechanism separating the users in CDMA system consist of assigning a unique code that modulates the signal from each user. The number of unique codes in a CDMA link is equal to the number of active users. The code modulating the users signal is also called a separating code, spreading sequence or chip sequence.

Comparison between Narrow band and Wide band Access TechniquesFDMA / TDMA CDMA

Hard Limit on total number of channels (simultaneous user)

No hard limit. Soft limit applies

Narrow band technique, low data rate limited by channel bandwidth

Wide band technique, high data rates possible

Channel arrangement in near vicinity is a problem due too co-channel interference

No problem of co-channel interference

Sophisticated tight filtering required at receiver No requirement of tight filteringTolerance to interfering signal is low High tolerance Capacity improvement is possible by adding hard ware / controlling interference

Capacity improvement possible by reducing interference only. No hard ware requiredBy using voice activity detection

1 The Air Interface (Um Interface): The International Telecommunication Union (ITU) which manages international allocation of radio spectrum has allocated the following bands.GSM 900 Uplink : 890-915 MHz (Mobile Station to Base Station)Downlink: 935-960 MHz (Base Station to Mobile Station)GSM1800Uplink : 1710-1785 MHzDownlink: 1805-1880 MHzGSM 1900Uplink: 1850-1910 MHzDownlink: 1930-1990 MHz

The air interface for GSM is known as Um interface. Since radio spectrum is a limited resource shared by all users, a method was devised to divide the band width among as many users as possible. The method chosen by GSM is a combination of time division multiple access (TDMA) and frequency division multiple access (FDMA)The FDMA part involves the division by frequency of maximum of 25 MHz allocated 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 TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts approximately 0.577 ms. Eight burst period are grouped into a TDMA frame (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.

Bandwidth = 25 MHzChannel = 200 KHzSpeech Channels / Frame = 8Each slot has 156.25 bits (0.577 ms) 6 Trail bits 8.25 Guard bits 26 synchronization bits 2x58=116 Data BitsTransmission rate = 270.833 kbpsTime duration of slot = 0.577 msTime duration of one TDMA frame = 4.615 msOne TDMA multi-frame = 26 TDMA frames Time duration of one TDMA multi-frame = 120 ms

Logical Channels on the Air Interface: Several logical channels are mapped on to the physical channels. The organization of logical channel depends on the application and the direction of the information flow (uplink/downlink or bidirectional). The logical channel can be either a traffic channel (TCH), which carries user data, or a signaling channel.

Traffic Channel on the Air Interface: A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined using a 26 frame multi-frame or group of 26 TDMA frames. The length of a 26 frame multi-frame is 120ms.Hence length of multi-frame is defined as:

Length of one TDMA multi-frame / Total no of frames in one multi-frame * Total no of time sots in one TDMA frame=120 / 26 * 8 = 0.577 ms

Out of 26 frames, 24 are used for traffic. One is used for Slow Associated Control Channel (SACCH) and one is currently unused. TCH for the uplink & down link are separated in time by 3 burst periods so that the mobile station does not have to transmit and receive simultaneously, thereby simplifying the electronic circuitry.In addition to these full rate TCHs (TCH/F, 22.8 kbit/s), half rate TCHs (TCH/H, 11.4 kbit/s) are also defined. Half rate TCHs double the capacity of a system effectively by making it possible to transmit two calls in a single channel. If a TCH/F is used for data communications, the usable data rate drops to 9.6 kbit/s (in TCH/H: max. 4.8 kbit/s). Eight-rate TCHs are also specified, and are used for signaling. In the GSM recommendations, they are called Stand-Alone Dedicated Control Channels (SDCCH).Signaling Channels on the Air Interface: The signaling channels on the air interface are used for call establishment, paging, call maintenance, synchronization, etc. There are three groups of signaling channels(i)The Broadcast Channels (BCH): Carry only downlink information and are mainly responsible for synchronization and frequency correction. This is the only channel type enabling point-to-multipoint communication in which short messages are simultaneously transmitted to several mobiles. These are:-- The Broadcast Control Channel: This channel gives the mobile station information of the base station identity and the carrier frequency available-- The Frequency Correction Channel (FCCH): The mobile station must be corrected if necessary to ensure it is using the correct frequency with the network-- The Synchronization channel (SCH): This is used to correct the timing of the frames and burst periods(ii) The Common Control Channels (CCCH): The group of uplink and downlink channels between the MS and BTS. These channels are used to carry information from the network to mobile stations and provide access to the network. This includes:-- The Paging Channel (PCH): Downlink only. This channel notifies a mobile station of an incoming call-- The Random Access Channel (RACH): Uplink only. Used by mobile station when it is to request access to the GSM network-- The Access Grant Channel (AGCH): Downlink only. The base station uses this channel to inform the mobile station which dedicated channel it should use for communication after requesting access on the RACH channel(iii) The Dedicated Control Channels (DCCH): They are responsible for roaming, handovers, encryption, etc. The DCCHs includes:

-- The Standalone Dedicated Control Channel (SDCCH): Used to swap signaling information between the uplink and downlink channels-- The Slow Associated Control Channel (SACCH): It is used to perform maintenance and control of the channels-- The Fast Associated Control Channel (FACCH): It is similar to SDCCH but used in parallel to operation of the TCHAlmost all the signaling channel uses the normal burst format except the following channels.RACH (Which uses random access burst)FCCH (Which uses frequency correction burst)SCH (Which uses synchronization burst)Burst Format: A time slot is a 577 us or 0.577ms time interval i.e 156.25 bits duration and its physical contents are known as a burst. Five different types of bursts exist in the system. They are distinguished by different TDMA frame divisions.

(i) The Normal Burst (NB): Used to carry information on traffic and control channels, except RACH. It contains 116 encrypted bits.

(ii) The Frequency Correction Burst (FB): Used for the frequency synchronization of the mobile. The contents of this burst are used to calculate an un-modulated, sinusoidal oscillation, onto which the synthesizer of the mobiles is clocked

(iii) The Synchronization Burst (SB): Used for time synchronization of the mobile. It contains a long training sequence and carries the information of a TDMA frame number.

(iv) The Access Burst (AB): Used for random access and characterized by a longer guard period (256 us) to allow for burst transmission from a mobile that doses not know the correct timing advance at the first access to a network (or after handover).

(v) The Dummy Burst (DB): Transmitted as a filter in un used time slots of the carrier, does not carry any information but has the same format as a normal burst (NB).

The Abis Interface: It lies between the base station sub system (BSS) and represents the dividing line between the BSC function and the BTS.The BSC and BTS can be connected using leased lines, radio links or metropolitan area networks (MANs). Basically two channel types exist between the BSC and BTC.Traffic Channels (TCH): It can be configured in 8, 16, 64 kbits/s formats and transport user data.Signaling Channel: It can be configured is 16, 32, 56 & 64 kbits/s formats and are used for signaling purpose between BTS & BSCThe A Interface: It lies between the BCS & MSC.Other MSC based Interfaces: All of the interfaces around the MSC use SS7 based protocols. The B, C, D, F and G interfaces are referred to as MAP interfaces. These connect either the MSC to registers or registers to other registers. The E interface supports the MAP protocol and call set up protocols (TUP). This interface connects one MSC to another MSC with in the same network or to another network; MSC. They are designed as follows:

B Interface between MSC & VLR (use MAP/TCAP protocol)C Interface between MSC & HLR (use MAP/TCAP protocol)D Interface between HLR & VLR (use MAP/TCAP protocol)E Interface between two MSC’s (use MAP/TCAP + ISUP/TUP protocol)F Interface between MSC & EIR (use MAP/TCAP protocol)G Interface between VLR’s (use MAP / TCAP protocols)Fixed Network Interfaces:Via TUP protocol: Between MSC and Analog / Digital NetworksVia ISUP protocol: Between MSC and Analog / Digital Networks (provides more features than TUP)Via NAP protocol: Between MSC and IN

The SCCP protocol provides connectionless message transport to and from the GSM network databases for TCAP and MAP messaging. Here, two connection types are also distinguished:Circuit related call control: Related to ISUP and TUPNon circuit related call control: The mobile application part (MAP) protocol is used here, allowing implementation of functions such as location updating / roaming, SMS delivery, handover, authentication and incoming call routing information. The MAP protocol uses the transaction capability application part (TCAP) protocol to transfer real time information (between MSCs, HLRs and VLRs)MSC ProtocolsMAP (Mobile Application Part): GSM Recommendation used to control queries to the different databases in the mobile radio network (HLR, VLR and EIR). MAP responsibilities include access and location management (e.g. where is the called subscriber currently?),MSC-MSC handover, security functions, O&M, SMS and supplementary services.TCAP (Transaction Capabilities Application Part): Provided universal calls and functions for handling requests to distributed application processes.ISUP (ISDN user part): Controls inter-working (e.g. call setup / take-down) between PLMNs and other networks, and provides the same basic functionalities as TUP.INAP (Intelligent Network Application Part): Implements intelligent supplementary services (e.g. free call, time dependent routing functions is a central service center).TUP (Telephone User Part): Implements inter-working between PLMNs and other networks. TUP is normally used to provide international connections and is slowly being replaces by ISUP.Cellular Structure: As already mentioned a particular area controlled by a BSC is called a cell. The size of a cell depends on the radio transmitter power of that cell. The frequency-reuse principle forms the basis for cellular radio communication. Nearby cells must use different radio frequencies for communication channels with mobile stations to prevent interference. Only when the cells are sufficiently far apart can the same set of frequencies be used again. A cluster is the term given to the number of adjacent cells that use different sets of radio frequencies for communication channels until they need to be repeated. On the diagram below each cell contains a number that indicates the set of frequencies that are being used in that cell. Notice no adjacent cells use the same set of frequencies.

Transmission of a radio signal at a certain power level results in a coverage area composed of the region where the signal power remains significant. By lowering the power level, the coverage area can be reduced and vice versa.In a populated area where there is likely to be a higher number of mobile stations it may be necessary to have smaller cells so that there is enough different radio frequencies for channels to give a good level of service to users. Each cell also uses some of its channels for signaling information to mobile stations, so not all channels can be used for user communication. The

GSM network has given rise to four main types of cells. These are Macro Cells, Micro Cells, Selective Cells and Umbrella Cells.Macro Cells: This type of cell is mainly used where there is a very low population density so the demand for channels from the network at any one time is low, but the area of coverage is large. The transmitter power may need to be high to allow for the large coverage area.Micro Cells: For high population densities the cell structure is small which increases the number of channels available over the area. The power level of the transmitters may need to be lowered to prevent cells from interfering with each other.Umbrella Cells: This type of cell, as the name suggests contains other cells inside of it. When a mobile station is moving on a train or a motorway for example, the cell used by the mobile may change often creating many handovers from each cell to the next. To reduce these handovers when the GSM network recognizes a mobile station moving between cells it often hands the mobile station to the Umbrella Cell, which is a high power cell within an area of other cells. By doing this the overheads of communication required for the handover are substantially lowered.Selective Cells: Due to the physical nature of some areas, a cell often does not need to cover a full 360o. In case such as these the BTSs are situated so that they give the desired area of coverage. An example of where coverage may only be required in one direction is at the entrance and exits of tunnels.System Features: This section provides a brief description of the GSM network features:Roaming: The roaming feature allows a user to make and receive calls in any GSM network and to use the same user specific services worldwide. This requires a roaming agreement between the individual operators. With worldwide roaming the MS is accessible under the same phone number every where.Handover: In a cellular network the radio and fixed voice connections are not permanently allocated for the duration of a call. Handover or handoff means switching on ongoing call to a different channel of cell.There are four different types of handovers in GSM which involve transferring a connection between:

Channels (timeslots) in the same cell (intra BTS handover) Cells under the control of the same BSC (inter BTS handover) Cells under the control of different BSCs but belonging to the same MSC (inter BSC

handover) Cells under the control of different MSCs (inter MSC handover)

The first two types of handover involve only one base station controller (BSC). To save signaling bandwidth, they are managed by the BSC without involving the MSC, except to notify it upon completion of the handover. The last two types of handover 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 function with the exception of subsequent inter-BSC handovers under the control of the new MSC, called the relay MSC.Handover can be initiated by either the BSC or the MSC. During its idle timeslots, the mobile scans the broadcast control channel of up to 16 neighboring cells, and forms 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 decision on when to initiate a handover is a function of the following parameters.

Receive quality Receive level

Successful handover in GSM can take place at propagation speeds up to 250 km/h.Multi-path Equalization: At the 900 MHz range radio waves bounce off everything- buildings, hills, cars, airplanes etc. Many reflected signals each with a different phase, can reach an antenna (also known as multi-path propagation). Equalization is used to extract the desired signal form the unwanted reflections. It works by finding out how a known transmitted signal is modified by multi-path 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 very time slot burst.Frequency Hopping: The mobile station has to be frequency-agile meaning it can move between different frequencies in order to transmit and receive data, etc. A normal handset is able to switch frequencies 217 times per second. GSM makes use of this frequency agility to implement slow frequency hopping, where the mobile and the BTS transmit each TDMA frame on a different carrier frequency. The frequency hopping algorithm is broadcast on the broadcast control channel. Since multipath fading is dependent on the carrier frequency, slow frequency hoping helps alleviate the problem. In addition co-channel interference is in effect randomized. The broadcast and common control channels are not subject to frequency hopping and are always transmitted on the same frequency.Discontinuous Transmission (DTX): To reduce the MS’s power consumption and minimize interference on the air interface, user signal transmission is interrupted during pauses in speech. “Comfort noise” is artificially generated by the MS to avoid disruption due to an abrupt interruption in speech.Discontinuous Reception (DRX): Another method used to conserve power at the mobile station is discontinuous reception. The paging channels, 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.Power Control: Several classes of mobile station are defined in the GSM specifications according to their peak transmitter power. To minimize co-channel interference and to conserve power, both the mobile and the base transceiver station operate at the lowest power level that will maintain in steps of 2 dBm from the peak power for the class down to a minimum of 13 dBm (20 milliwatts for MS).The mobile station and BTS continually measure the signal strength or signal quality (based on the bit error ratio) and pass the information to the base station controller, which ultimately decides if and when the power level should be changed.Mobility Management: The GSM network keeps track of which mobile telephones are powered on and active in the network. To provide as efficient call delivery as possible the network keeps track of the last known location of the MS in the VLR and HLR. Radio sites connected to the MSC are divided into groups called location areas. When a call is designation for an MS the network looks for the MS in the last known location area.Authentication: Authentication normally takes place when the MS is turned on with each incoming call and outgoing call. A verification that the security code, stored in the AuC matches the security code stored in SIM card of the MS completes this process.The user must key in a PIN code on the handset in order to activate the hardware before this automatic procedure can start.

The GSM network protocol stack can be fitted into three different layers. The first layer can be said to be the physical layer, the second to be the data link layer and the third to be the application layer. The third layer compromises of radio resource management, mobility management and call management. The next section will describe further the services offered through these layers.GSM Services: Today GSM offers many different services to its subscribers. These were not all introduced when GSM came into existence, but have been introduced through time in a regular fashion under the GSM Memorandum of Understanding (MoU).From the beginning the planners of GSM wanted ISDN compatibility, but because of radio transmission limitations, in terms of bandwidth and cost, it was not possible to provide the standard ISDN B-Channel bit rate of 64 kbps.There are two types of basic services offered through GSM: telephony (also referred to as tele-services) and data (also referred to as bearer services).Telephony Services: Telephony services are mainly voice services that provide subscribers with the complete capability (including necessary terminal equipment) to communicate with other

subscribers. Data services provide the capacity necessary to transmit appropriate data signals

between two access points creating an interface to the network. In addition to normal telephony

and emergency calling, the following subscriber services are supported by GSM:Dual Tone Multi-Frequency (DTMF): DTMF is a tone signaling scheme often used for control purposes via the telephone network i.e. remote control of answering machines.Facsimile Group III: GSM supports CCITT Group 3 facsimile. As standard fax machines are designed to be connected to a telephone using analogue signals, a special fax converter connected to the exchange is used in the GSM system. This enables a GSM – connected fax to communicate with any analogue fax in the network.Short Message Service (SMS): A convenient facility of the GSM Network. A message consisting of a maximum of 160 Latin alphanumeric characters, or 70 Chinese or Arabic characters can be sent to or from a mobile station. SMS is essentially similar to paging, with the advantage that if the subscriber’s mobile unit is powered off or has left the coverage area, the message is stored and offered back to the subscriber when the mobile is turned on or has re-entered the coverage area of the network. It also features confirmation of delivery, and message can be sent and received simultaneously with GSM voice, data and fax calls. This ensures that the message will be received.Cell broadcast: A variation of the short message service is the cell broadcast facility. Whereas SMS messages are sent point-to-point, Cell Broadcast (SMS-CB) messages are sent point-to-area. A message of a maximum of 93 characters can be broadcast to all mobile subscribers in a certain geographic area. Typical applications include traffic congestion warnings and weather reports.Voice mail: This service is an answering machine within the network which is controlled by the subscriber. Calls can be forwarded to the subscriber’s voice mail, from which they can be retrieved by the subscriber via a personal security number.Fax mail: With this service the subscribers can receive fax message at any fax machine. The messages are stored in a service centre from which they can be retrieved by the subscriber via a personal security number to the desired fax number.Supplementary Services: GSM supports a comprehensive set of supplementary services that can complement and support both telephony and data services. Supplementary services are defined by GSM and are characterized as revenue-generating features. A partial listing of supplementary services follows.Call Forwarding: This is a network based feature that can be activated by the MS. CF allows calls to be sent to other number under conditions defined by the user. These conditions can be either unconditional or dependent on certain criteria (no answer, busy, not reachable).Call Barring: There are different types of ‘call barring’ services:-- Barring of All outgoing Calls (BAOC).-- Barring of Outgoing International Calls (BOIC).

-- Barring of Outgoing International calls except those directed toward the home PLMN Country (BOIC-exHC).-- Barring of all Incoming Calls (BAIC).-- Barring of incoming calls when roaming.Call Hold: Puts an active call on hold to be resumed at a later time.Call waiting: CW is a network based feature that must also be supported by the GSM telephone. With CW GSM users with a call in progress will receive an audible beep to alert them that there is an incoming call for the MS. The incoming call cab be accepted sent to voice mail or rejected. If the incoming call is rejected the caller will receive a busy signal. Once the call is accepted the original call is put on hold to allow a connection to the new incoming call.Advice of Charge: Provides the user with online account information based on time measurements.Multiparty Service: Possibility of establishing a simultaneous conversation between three and six subscribers.Closed User Group: They are a group of subscribers who are capable of only calling themselves and certain numbers. Calling Line ID: Calling Line ID must be supported by the GSM network and the telephone. The GSM telephone displays the originating telephone number of incoming calls. This feature requires the caller’s network to deliver the calling line ID (telephone number) to the GSM network.Calling Line Identification Restriction: It enables the calling user to restrict the presentation of their ISDN.Connected Line Identification Presentation: It supplies the calling user with the directory number he gets if his call is forwarded.Operator determined barring: Restriction of different of services and call types by the operator.Bearer Services: GSM users can send and receive 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. Due to the fact that the GSM is a digital network it does not require a modem to reach other users on it or any other GSM network, but does require a audio modem inside the GSM network to inter-work with analogue POTS network.Some of the bearer services are listed below:Asynchronous and Synchronous data: Both these types of data can be transported to or from an ISDN terminal with data rates between 300 and 9600 bps. Data can use either the transparent service, which has a fixed delay but no guarantee of data integrity, or a non-transparent service, which guarantees data integrity through an Automatic Repeat Request (ARO) mechanism, but with a variable delay.Alternate speech and data: It allows a user to exchange voice and data modes during a call, and then resume the conversation as desired at data rates that range from 300 to 9600 bps.Asynchronous PAD: (packet switched, packet assembler/dissembler) access, this is for users who lack the necessary equipment to directly form packets, so they can access packet assemblers/dissemblers at asynchronous data rates that range from 300 to 9600 bps.Synchronous dedicated packet data access: Users who do have the necessary equipment can gain access to a packet network at synchronous data rates that range from 2400 to 9600 bps.

Call Setup:The following example describes a cal from a fixed network subscriber to a mobile subscriber in a GSM network.

1. The incoming call is passed from the fixed network to the gateway MSC (GMSC).2. Then based on the IMSI numbers of the called party, its HLR is determined.3. The HLR checks for the existence of the called number. Then the relevant VLR is

requested to provide a mobile station roaming number.4. This is transmitted back to the GMSC5. Then the connection is switched through to the responsible MSC6. Now the VLR is queried for the location range and reach ability status of the mobile

subscriber.7. If the MS is marked reachable a radio call is enabled.8. And executed in all radio zones assigned to the VLR9. When the mobile subscriber telephone responds to the page request from the current

radio cell10. All necessary security procedures are executed.11. If this is successful the VLR indicated to the MSC.12. That the call can be completed.

The Future of GSM: GSM is growing and evolving and already offers an expanded and feature-rich ‘family’ of voice and data enabling services. The next step in its evolution is the introduction of General Packet Radio Service (GPRS). This service promises to offer ‘always-on’, higher capacity, Internet-based content and packet-based data services. This enables services such as colour Internet browsing, e-mail on the move, powerful visual communications, multimedia messages and location based services.Further enhancements in data capability over the core GSM network will be provided with the introduction of Enhanced Data rates for GSM Evolution (Edge). This will achieve the delivery of advanced mobile services such as the downloading of video and music clips, full multimedia messaging, high-speed colour Internet access and e-mail.These and other new services will be known as third generation services (3GSM). They will be delivered on an evolved core GSM network and are the future of mobile communication. Fundamentals of PCMSampling Theorem: The sampling theorem is used to determine the minimum rate at which and analog signal can be sampled without information is being lost when the original signal is

received. The sampling frequency must be more than twice the highest frequency contained in the analog signal

fA>2fsAnalog to Digital ConventionSampling: A sampling frequency of 8000Hz has been specified internationally for the frequency band (300 Hz to 3400 Hz) used in telephone systems i.e. voice signal is sampled 8000 times per second. The interval between two consecutive samples from the same voice signal (sampling interval) is calculated as:

TA = 1/fA = 1/8000 = 125usWhere TA= Sampling Interval

fA= Sampling Frequency

The figure below shows how the voice signal is fed via a low pass filter to an electronic switch. The low pass filter limits the frequency band to be transmitted i.e. it suppresses frequencies higher than half of the sampling frequency. The electronic switch driven at the sampling frequency of 8000Hz takes samples from the telephone signal once every 125 us. A Pulse Amplitude Modulated signal (PAM signal) is thus obtained at the output of the electronic switch.

Quantizing: The pulse amplitude modulated signal (PAM signal) still represents the voice signal in analog form. The samples can however be transmitted and further processed much more easily in digital form. The first stage in the conversion of a digital signal in case of PAM signal is Quantizing the whole range of possible amplitude values is divided into quantizing intervals and the quantizing intervals is determined for each sample. The quantizing principle is shown in the figure below:In order to simplify the explanation only 16 equal quantizing intervals are indicated. The quantizing intervals are numbered +1 to +8 in the positive range and -1 to -8 in the negative range of the voice signal. The several different analog values fall with in the same quantizing intervals.

Quantizing distortion decreases as the number of quantizing intervals are increased. If the quantizing intervals are made sufficiently small the distortion will be minimum and the noise imperceptible.Types of QuantizationUniform Quantization: Quantization in which all the quantizing intervals are of equal size. In this type equal and large quantizing intervals are used over the whole amplitude range, relatively large discrepancies will occur in case of small signal amplitude.Non Uniform Quantization: Quantization in which the quantizing interval are not of equal size. Small quantizing intervals are usually allocated to small signal values (samples) and large quantizing intervals to large signal values to make the quantizing distortion ratio nearly independent of the signal level.Encoding: The PCM signal to be transmitted is obtained by encoding the quantizing intervals. The electronic encoder allocates on 8-bit PCM word to each individual sample. In PCM transmission system an 8-digit binary code is used for 128 positive and 128 negative quantizing intervals (128+128=256)Multiplexing: The 8 bit PCM words of a number of voice signals can be transmitted consecutively in repeated cycles. A PCM word of one voice signal is followed by the PCM words of all other telephone signals arranged in consecutive order. This creates a PCM time division multiple signal. The process involved in multiplexing are carried out fully electronically.

Digital to Analog ConversionDe-multiplexing: On the receive side the individual PCM signal are recovered from the time division multiplex signal i.e. 8 bit PCM words are distributed to the appropriate outputs. The de-multiplexing process is also controlled fully electronically.Decoding: On the receive side a signal amplitude Vout is allocated to every 8 bit PCM word. It corresponds to the midpoint of particular quantizing interval. In this way the PCM words are decoded in the order in which they are received and converted to a PAM signal. Finally the PAM signal is fed to a low pass filter, which reproduces the original analog telephone signal.

SummaryTransmit Side:

1. Band limiting of a voice signal by means of a low pass filter.2. Sampling voice signal. The resulting samples form a PAM signal.3. Quantizing samples i.e. the quantizing interval is determined for each sample.4. Encoding sample i.e. a binary PCM word is allocated to the determined quantizing

interval. The voice signal which consists of 8-bits PCM word is known as a PCM signal.5. Multiplexing PCM signals i.e. the PCM word in a voice signal are interleaved with PCM

words from other voice signals to form a PCM time division multiplex signal.Receive Side:

1. De-multiplexing PCM time division multiplex signal i.e. the PCM word of the voice signals are distributed to the individual lines.

2. Decoding PCM words in the PCM signal i.e. a signal amplitude is allocated to each PCM word. The signal amplitude is equal to the mid point value of the particular quantizing interval. A PAM signal is produced again.

3. Reproducing the original analog voice signal from the PAM signal with the aid of a low pass filter.

The sequence in which the individual steps are carried out depends on the system used. It may differ from the sequence discussed above the samples from several voice signal could for example be composed into a PAM time division multiplex signal and then quantized and encoded into one common unit.Synchronization of Received and Transmit Section: PCM transmission systems terminate at both ends in a digital multiplex unit. Each multiplex unit contains a transmit section and a receive section. The transmit section from the 8-bit PCM words to be transmitted, and the receive section convert the received PCM words back to analog signal. In either speech direction the receive section must recover the analog signals using the same timings signal as its associated transmit section. Thus the information received from the transmit section by the receive section contains not only the PCM signals but also the timing signal used to form them. In order to carry out these functions the transmit section is provided with a timing signal generator and the receive section with a timing signal detector which extracts the timing signal from the received PCM signal. The received section is thus synchronized i.e. it operates in step with the transmit section of the same speech direction.PCM Characteristics: Sampling Frequency=8 kHz or 8000HzNo of samples per voice signal=8000 samplesPulse frame period= 1/fA=1/8000=125usNo of bits in a PCM word=8bitsBit rate of a voice channel=8000x8 bit=64 kbits/sNo of channel time slots per pulse frame=32No of bits per pulse frame=8x32=256 bitsPeriod for an 8-bit channel time slot=125usx8/256=3.9usBit rate of time division multiplex signal=8000x256=2048kbit/s=2Mbits/s.