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Introduction to GSM - an Overview of Global System for Mobile Communication
- 1. Introduction to GSM This slideshow describes the basic components, technologies used, and operation of Global System for Mobile Communication - GSM - systems. You will discover the evolution of GSM; 1st generation analog systems, 2nd generation GSM systems (digital voice), 3rd generation multimedia, and 4th generation wideband ultra broadband systems. ALTHOS Simplifying Knowledge (tm )
- 2. Introduction to GSM Lawrence Harte is a mobile communication expert who has developed, designed, and managed many types of communication products and services since 1985. Mr. Harte is the publisher and editor of Mobile Video magazine and author of over 112 books with over 53 on wireless communication. As a magazine editor, he has interviewed over 2100 companies in the mobile communication industry to discover the key issues and Lawrence Harte solutions of the wireless industry. LHarte@Althos.com Mr. Harte has worked for Ericsson/General Electric, Linkedin: Audiovox/Toshiba and Westinghouse and has consulted for hundreds of other companies. He is the inventor of www.linkedin.com/in/lharte/ several patents on communication technology. Mr. Harte Holds many degrees and certificates including an Executive MBA and a BSET. ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 2
- 3. GSM Topics Global System for Mobile Communication GSM Services GSM Products (Mobile Devices) GSM Radio Technology Digital Audio and Baseband Radio Channels Logical Channels GSM Network GSM System Operation GSM Evolution ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 3
- 4. What is GSM? Mobile Communication System Digital Media Formats Functional Sections - Mobile Stations (MS) - Base Station Subsystem (BSS) - Network and Switching System (NSS) ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 4
- 5. GSM Services Voice Services - Circuit Switched Data Services - Low to Medium Speed Short Messaging Services - Location Multicast Service - Group Call - Voice Broadcast Packet Data - Internet Browsing ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 5
- 6. GSM Devices Subscriber Identity Module (SIM) Mobile Telephones PCMCIA Air Cards Embedded Radio Modules External Radio Modems ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 6
- 7. Subscriber Identity Module(SIM) Holds User Identification Data Microprocessor Memory Software ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 7
- 8. GSM Radio Technology Frequency Reuse Time Division Multiple Access (TDMA) RF Power Control ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 8
- 9. Digital Audio andBaseband Analog to Digital Conversion Digital Speech Compression Channel Coding ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 9
- 10. Analog to DigitalConversion Voice Coding - 13 kbps Full Rate - 6.5 kbps Half Rate ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 10
- 11. Digital Speech Compression Analyze Voice Use Code Book to Identify Human Sounds ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 11
- 12. Channel Coding Error Protection - CRC - Block - Convolutional Signaling Commands - Fast - Slow Time Slot Allocation ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 12
- 13. Echo Cancellation Audio Analysis Time Shifting Echo Modeling - delay - amplitude Signal Subtraction ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 13
- 14. GSM Radio Channels Radio Channel Structure Time Slot Structure Frame Structure Dynamic Time Alignment A GSM radio channel is a 200 kHz communications channel that transfer digital information from a source to a destination. The GSM radio channel is divided into time slots and frames to allow multiple communication paths (logical channels) to share a single radio transmission channel. ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 14
- 15. Radio Channel Structure 200 kHz RF Channel Bandwidth Frequency Duplex Channels - Downlink - Uplink Time Offset ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 15
- 16. Time Slot Structure Normal Burst Random Access Burst (Shortened Burst) Frequency Correction Burst Synchronization Burst ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 16
- 17. Frame Structure GSM Frame - 4.615 msec Multiframes - Traffic - Control Superframes ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 17
- 18. Logical Channels Traffic Channels - User Data Control Channels - Signaling - Broadcast - System Information - Common Control - Paging and Access - Dedicated Control - On an Assigned Channel The GSM system divides up the RF communications channel into logical channels that serve different purposes. Traffic channels carry user data (voice and data). Control channel transport signaling messages. ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 18
- 19. Traffic Channels Full Rate (TCH/F) Half Rate (TCH/H) Eighth Rate (TCH/8) ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 19
- 20. Control Channels Beacon (Broadcast) Channels (BCH) Common Control Channels (CCCH) Dedicated Control Chanels ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 20
- 21. GSM Network Base Stations Repeaters Switching Centers Network Databases Wireless Network System Interconnection ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 21
- 22. Base Stations Radio Antenna Towers Radio Equipment - Transmitters - Combiners - Multicouplers - Receivers - Controllers Communication Links Power Supplies ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 22
- 23. GSM Switching Systems MobileSwitching Center (MSC) - circuit switching Packet Nodes (SGSN) - packet data ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 23
- 24. Network Databases Home Location Register (HLR) - Registered with Operator Visitor Location Register (VLR) - Operating in System (including home subscribers) Equipment Identity Register (EIR) - Identifies Stolen Equipment Authentication Center (AuC) - Security Validation SMS Center (SMSC) - Message Reception and Routing Group Call Register (GCR) - Dispatch Type Services ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 24
- 25. GSM System Operation Mobile Telephone Transferring Calls Between Call Initialization Sites Updating Location Receiving a Call on a Mobile Waiting for Calls (Idle) Telephone Cell Reselection Conversation Mode System Access Connected Mode Mobile Call Origination Authentication Mobile Assisted Handoff ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 25
- 26. System Access Randomly Attempt Access Receive RF Channel and Time Slot Assignment ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 26
- 27. Mobile Call Origination Request Access - random attempts Assign Traffic Channel - frequency - time slot Setup Audio Connection Open Audio Path - hear ringing ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 27
- 28. Receiving a Call Send Page Alert Message to Last Known Mobile Location - start ringing Mobile Request Access Assign Traffic Channel Open Audio Path ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 28
- 29. Mobile Assisted Handoff(MAHO) Signal Quality Fall Below Allowable Level System Sends List of Channels for Mobile to Monitor Mobile Periodically Returns Measurements ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 29
- 30. GSM Evolution General Packet Radio Service (GPRS) Enhanced Data for Global Evolution (EDGE) Wideband Code Division Multiple Access (WCDMA) UMTS Long Term Evolution (UMTS LTE) ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 30
- 31. Wireless Resources Wireless Dictionary (17,000+ Definitions) – www.WirelessDictionary.com Intro to GSM Book (by Lawrence Harte) – http://www.althosbooks.com/intogsm.html - Also Available from Amazon and Barnes & Noble Mobile Video Magazine – www.mVideoMagazine.com Wireless Trade Shows - http://www.mvideomagazine.com/events.asp More Detailed GSM Tutorial (with notes): http://www.tvdictionary.com/tutorial/gsm-tutorial-title-slide.html ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 31
- 32. Summary GSM - Worldwide Mobile Wireless System Can Provide, Voice, Messaging, and Data Services Many Types of Mobile Devices - Voice, Data Only 200 kHz Digital RF Channel - 8 Slots per Frame Mix of Logical Channels - Traffic and Control GSM Network - Base Stations, Switches, and Databases Evolved into GPRS, EDGE, WCDMA, UMTS LTE GSM Continues to Evolve over Time ALTHOS Simplifying Knowledge (tm ) Introduction to GSM © Althos, 2009-2013 page 32
Editor's Notes
- #2 This tutorial explains the basic components, technologies used, and operation of Global System for Mobile Communication - GSM - systems. You will discover the evolution of GSM; 1 st generation analog systems, 2 nd generation GSM systems (digital voice), 3 rd generation multimedia, and 4 th generation wideband ultra broadband systems. This tutorial starts with the system components and basic services that the GSM system can provide. You will learn that the key types of GSM devices include single mode and dual mode mobile telephones, wireless PCMCIA cards, embedded radio modules, and external radio modems. You will then discover the different types of available services such as multiple types of voice services, data services, group call, and messaging services. Explained are the physical and logical radio channel structures of the GSM system along with the basic frame and slot structures. Described are the fundamental capabilities and operation of the GSM radio channel including channel coding , modulation types , speech coding , RF power control , and mobile assisted handover . You will learn how each GSM radio channel has 8 time slots per frame and that some of these are used for signaling (control channels) and others are used for user traffic (voice and data). Because the needs of voice and data communication are different, you will discover that the GSM system essentially separates circuit switched (primarily voice) and packet switched (primarily data) services. Described are key functional sections of a GSM network components and how they communicate with each other. You will learn how and why GSM is evolving into 3 rd generation broadband systems including GPRS, EDGE, and WCDMA.
- #3 01/23/13 Mobile Communication expert Lawrence Harte has developed, designed, and managed many types of communication products and services. Mr. Harte is the publisher and editor of Mobile Video magazine and author of over 100 books with over 50 on wireless communication. As a magazine editor, he has interviewed over 2100 companies in the mobile communication industry to discover the key issues and solutions of the wireless industry. Mr. Harte has worked for Ericsson/General Electric , Audiovox/Toshiba and Westinghouse and has consulted for hundreds of other companies. He is the inventor of several patents on communication technology. Mr. Harte holds many degrees and certificates including an Executive MBA from Wake Forest University (1995) and a BSET from the University of the State of New York, (1990).
- #4 Global System for Mobile Communication - A well defined radio system that is deployed in almost all parts of the world. The GSM system continues to evolve into 3rd generation (multimedia) and 4th generation (broadband wireless) systems. GSM Services - The services available on GSM networks include voice, messaging, data, position location, and others. GSM Products (Mobile Devices) - Mobile devices convert radio signals into other forms that can be used. There are several types of GSM devices which include mobile telephones, data adapter cards, and embedded radio modules. GSM Radio - GSM radio system is the radio parameters (such as frequency and bandwidth), the structure of how data is transferred on the radio channel (frames, fields, and modulation). Digital Audio and Baseband - The processing of the information or media into a format that can be transmitted on GSM channels. Radio Channels - The structure, frequencies, modulation types, and other RF characteristics of the GSM system. Logical Channels - The logical control and data channels that share the physical transmission channel. GSM Network - The functional parts of the GSM system which include the Mobile Station (MS), Base Station, Mobile Switching Centre (MSC), and Service Nodes. GSM System Operation - How the GSM system coordinates access and provides services to devices and users. GSM Evolution - How the system parts and services have evolved over time.
- #5 Mobile Communication System - Global system for mobile communication - GSM - is a wide area wireless communications system that uses digital radio transmission to provide voice, data, and multimedia communication services. A GSM system coordinates the communication between mobile telephones (mobile stations), base stations (cell sites), and switching systems. Digital Media Formats - GSM is designed to transfer digital information. The initial version of GSM transmitted digital media in circuit switched (continuous transmission) form and later versions of GSM deliver data in packet data form. Functional Sections - GSM system is composed of three key sections: - Mobile Stations (MS) - A device that converts media to and from GSM radio signals. - Base Station Subsystem (BSS) - Assemblies that convert digital signals to radio signals that can be sent to mobile devices and receive radio signals that can be converted back to their digital form. The BSS is divided into base station - BS - parts that are located at the cell site and base station controllers - BSC - that coordinate the distribution and reception of communication connections. - Network and Switching System (NSS) - The NSS performs the interconnection between the base station parts and other networks such as the public switched telephone network - PSTN - and public Internet . The NSS is composed of circuit data and packet data switches, databases, and administrative control services.
- #6 GSM services initially was focused on circuit switched voice service. The GSM system evolved to offer additional types of data, messaging, multicast, and multimedia services. GSM voice service started as a full rate voice service that allowed 8 users per GSM radio channel. The original design allowed for the use of a half rate voice service (lower quality audio) to incrase the number of simultaneous GSM voice users to 16 per radio channel. GSM Data services started as low speed circuit switched data (9.6 kbps). The GSM system evolved to allow the combination of multiple circuit switched data connections to provide high speed circuit switched data services - HSCSD . GSM short messaging service - SMS is an electronic messaging service for extremely short text messages (140 characters). SMS evolved into executable messages that allow for advanced two-way messaging features. GSM Multicast - GSM has the capabilities of one to many type services such as group call ( dispatch type services) and voice broadcast (such as traffic alerts). GSM Packet Data - GPRS - The GSM system evolved into general packet radio service which allowed for users to dynamically share packet data resources on one or more GSM channels for services such as Internet browsing.
- #7 GSM devices range from mobile telephones to data cards that allow devices to connect and communicate with the GSM system.GSM mobile devices may include the capability to use GSM radio channels on different frequency bands. Mobile Telephones - Portable devices that can be used for voice communication. PCMCIA Air Cards - Cards that can slide into computers to provide data services. Embedded Radio Modules - Radio assemblies that can be built-in or installed in devices such as laptop computers, video cameras, or digital signage displays. External Radio Modems - Assemblies that can be connected to other devices through USB, Ethernet, or other connection types to provide data services.
- #8 A Subscriber Identity Module - SIM - is an intelligent card that holds service subscription information, identity, and personal information and is included with GSM end user devices. The SIM contains a microprocessor , memory and software to hold and process information that includes a phone number, billing identification information and a small amount of user specific data (such as feature preferences and short messages.) This information can be stored in the card rather than programmed into the phone itself. A SIM card can either be credit card-sized (ISO format) or the size of a postage-stamp (Plug-In format). SIM cards can be inserted into any SIM ready communication device. A SIM lock code ensures that a communication device will only work with one or a group of subscriber identity module (SIM) cards . The use of a SIM lock code by a service provider helps to ensure that a customer will only be able to use a communication device they provide at low cost with their SIM cards. If another SIM card is inserted to a communication device that is locked to a specific SIM card, the communication device will not operate. This figure shows a block diagram of a SIM card. This diagram shows that SIM cards have 8 electrical contacts. This allows for power to be applied to the electronic circuits inside the card and for data to be sent to and from the card. The card contains a microprocessor that is used to store and retrieve data. Identification information is stored in the cards protected memory that is not accessible by the customer. Additional memory is included to allow features or other information such as short messages to be stored on the card.
- #9 GSM Radio is the transmission of control and user information in digital format through a 200 kHz wide RF channel which usually operate on frequency bands around the world ranging from approximately 800 MHz to 3 GHz. Frequency Reuse - The ability to reuse radio channels that operate on the same frequency at different locations in a system area. This allows a limited number of radio channels to serve a large number of customers. Time Division Multiple Access - TDMA - Dividing up the radio channel into time slots that can be shared by 8 simultaneous users. RF Power Control - A system that dynamically commands mobile devices to increase or decrease their transmitter power level which can reduce interference (increase capacity) and save battery life.
- #10 The baseband of GSM devices convert sound signals into digital audio, compresses the digital voice, and code the data signal into a format that can be transmitted. Analog to Digital Conversion - Sending the sound signal and digitizing it. Digital Speech Compression - Analyzing the digital sound signal and compressing it by approximately 10:1 (to 13 kbps). Channel Coding - Dividing the data into segments (for time slots), adding error protection bits, and merging in signaling control commands.
- #11 Analog to digital conversion - ADC - is a signal conversion process that periodically samples and converts a continuously varying signal - analog level - into digital values . The GSM system converts analog audio signals into digital form to be compressed and coded onto the radio channel. A typical analog to digital conversion process includes an initial filtering process to remove extremely high and low frequencies that could confuse the digital converter. This is followed by a periodic sampling section that measures the instantaneous level of the signals at fixed time intervals and converts the measured values (sampled voltages) into the equivalent digital number or pulses. This figure shows how an analog signal is converted to a digital signal. This diagram shows that an acoustic (sound) signal is converted to an audio electrical signal (continuously varying signal) by a microphone. This signal is sent through an audio band-pass filter that only allows frequency ranges within the desired audio band (removes unwanted noise and other non-audio frequency components). The audio signal is then sampled every 125 microseconds (8,000 times per second) and converted into 13 digital bits per sample (which is eventually compressed into 8 bits per sample). The digital bits represent the amplitude of the input analog signal.
- #12 Digital speech compression - speech coding - is a process of analyzing and compressing a digitized audio signal, transmitting that compressed digital signal to another point, and decoding the compressed signal to recreate the original (or approximate of the original) signal. The GSM digital speech compression process works by grouping the digital audio signals into 20 msec speech frames . These speech frames are analyzed and characterized (e.g. volume, pitch) by the speech coder. The speech coder removes redundancy in the digital signal (such as silence periods) and characterizes digital patterns that can be made by the human voice using code book tables. The code book table codes are transmitted instead of the original digitized audio signal . This results in the transmission of a 13 kbps compressed digital audio instead of the 64 kbps digitized audio signal. This figure shows the basic speech data compression process used for the GSM speech coder. This diagram shows that the analog voice signal is sampled 8,000 times each second and digitized into a 64 kbps digital signal. The digitized signal is grouped into 20 msec speech frames. The speech frames are analyzed and compressed into a new 13 kbps digital signal.
- #13 Channel coding is a process where one or more control and user data signals are combined with error protected or error correction information . After a sequence of digital data bits has been produced by a digital speech code or by other digital signal sources, these digital bits are processed to create a sequence of new bit patterns that are ready for transmission. This processing typically includes the addition of error detection and error protection bits, along with the rearrangement of bit order for transmission. Signaling Commands - Control messages that either replace (fast signaling) or sent along side (slow signaling) the user data. Time Slot Allocation - Which time slot(s) are assigned to the user.
- #14 ABC
- #15 A GSM radio channel is a 200 kHz communications channel that transfer digital information from a source to a destination. The GSM radio channel is divided into time slots and frames to allow multiple communication paths (logical channels) to share a single radio transmission channel. Radio Channel Bandwidth - ABC Modulation - ABC Duplex Channels - ABC Radio Channel Structure - ABC Time Slot Structure - ABC Frame Structure - ABC Dynamic Time Alignment - ABC
- #16 Radio channel structure is the division and coordination of a radio communication channel (wireless information transfer) into frames (groups) of data, time slots , and fields within the frames that hold specific types of information. The radio channel is divided into frames with 8 time slots per frame ( 0 through 7 ), and time slots are divided into field dependent on the purpose of the time slot. A forward (downlink) radio channel is paired with a reverse (uplink) radio channel to provide simultaneous two-way (duplex) voice communication. This pair of frequencies is known as absolute radio frequency channel number (ARFCN) or just plain channel. Between the downlink channel and uplink channel, the time slot numbers are offset by 3 slots . This allows the mobile telephone to transmit at different times than it receives. This allows the design of the mobile device to be simplified by replacing a frequency filter (duplexer) with a more efficient transmit/receive (T/R) switch. This figure shows that the GSM system uses a single type of radio channel. Each radio channel in the GSM system has a frequency bandwidth of 200 kHz and a data transmission rate of approximately 271 kbps. This example shows that each radio communication channel is divided into 8 time slots (0 through 7). This diagram shows that a simultaneous two-way voice communication session requires that at least one radio channel communicates from the base station to the mobile station (called the forward channel) and one channel communicates from the mobile station to the base station (called the reverse channel). This example also shows that some of the radio channel capacity is used to transfer voice (traffic) information and some of the radio channel capacity is used to transfer control messages.
- #17 Time slot structure is the division of a time slot period into different fields (information parts). Slot structure fields include a preamble for synchronization, control header, user data , signaling data , and error detection . A single time slot transmission is called a radio burst . Four types of radio bursts are defined in the GSM system; normal burst, shortened burst, frequency correction burst, and synchronization burst. The time period for a GSM time slot is 577 microseconds. Time slots include ramp up and ramp down periods to minimize rapid changes in radio transmitter power. The ramp up and ramp down time is used to reduce unwanted radio emissions that occur from rapidly changing signals. Normal Burst - A normal burst is used for normal communication between the mobile device and the base station. Each normal burst can transfer 114 bits of user information data (after error protection is removed). Random Access Burst (Shortened Burst) is a short 88 bit transmission burst that is used to request access to the GSM system. Mobile devices use a shortened burst when transmitting an access request to the GSM system to avoid the possibility of burst overlap with transmission bursts in adjacent time slots. Frequency Correction Burst - is a time slot of information that contains a 142 bit pattern of all “0” values. The reception and decoding of the frequency correction burst allows the mobile device to adjust (frequency correct) its timing so it can better receive and demodulate the radio channel. Synchronization Burst - is a transmission burst that contains system timing information. It contains a 78 bit code to identify the hyperframe counter. The synchronization burst follows the frequency correction burst.
- #18 Frame structure is the division of defined length of digital information into different fields (information parts). A GSM frame is 4.615 msec and it is composed of 8 time slots (numbered 0 through 7). During voice communication, one user is typically assigned to each time slot within a frame. The GSM system also combines frames to form Multiframes. Multiframes are frames that are grouped or linked together to perform specific functions . Multiframes on the GSM system use established schedules for specific purposes, such as coordinating with frequency hopping patterns. Multiframes used in the GSM system include the 26 traffic multiframe, 51 control multiframe, superframe, and hyperframe. Traffic Multiframe Structures - The 26 traffic multiframe structure is used to send information on the traffic channel. The 26 traffic multiframe structure is used to combine user data (traffic), slow control signaling (SACCH), and idle time period. The idle time period allows a mobile device to perform other necessary operations such as monitoring the radio signal strength level of a beacon channel from other cells. The time interval of a 26 frame traffic multiframe is 6 blocks of speech coder data (120 msec). Control Multiframe Structures - The 51 control multiframe structure is used to send information on the control channel. The 51 frame control multiframe is sub divided into logical channels that include the frequency correction burst, the synchronization burst, the broadcast channel (BCCH), the paging and access grant channel (PAGCH), and the stand-alone dedicated control channel (SDCCH). The PAGCH is logically sub divided into PCH and AGCH. Superframe - A superframe is a multiframe sequence that combines the period of a 51 multiframe with 26 multiframes (6.12 seconds). The use of the superframe time period allows all mobile devices to scan all the different time frame types at least once. Hyperframe - A hyperframe is a multiframe sequence that is composed of 2048 superframes, and is the largest time interval in the GSM system (3 hours, 28 minutes, 53 seconds). Every time slot during a hyperframe has a sequential number (represented by an 11 bit counter) that is composed of a frame number and a time slot number. This counter allows the hyperframe to synchronize frequency hopping sequence, encryption processes for voice privacy of subscribers' conversations. This figure shows the different types of GSM frame and multiframe structures. This diagram shows that a single GSM frame is composed of 8 time slots. When a radio channel is used to provide a control channel, time slot 0 and the other time slots are used for traffic channels. 51 frames are grouped together to form control multiframes (for the control channel). 26 frames are grouped together to form traffic multiframes (for the traffic channels). Superframes are the composition of 26 control multiframes or 51 traffic multiframes to provide a common time period of 6.12 seconds. 2,048 superframes are grouped together to form a hyperframe. A hyperframe has the longest time period in the GSM system of 3 hours, 28 minutes, and 53 seconds.
- #19 The GSM system divides up the RF communications channel into logical channels that serve different purposes . Traffic channels carry user data (voice and data). Control channel transport signaling messages. Traffic Channels - ABC Control Channels - Transfer command messages. Broadcast Channels - Continuously transmit information to all users in the radio coverage area. This can be system identification information (such as the name of the system). Common Control Channel Signaling - Transfers command messages to users that are sharing the channel (such as sending paging messages to mobile devices that are listening to the paging channel). Dedicated Control Channel Signaling - Sending command messages to specific devices (such as sending a power level adjustment message to a mobile device).
- #20 Traffic channels - TCH - are the combination of voice and data signals (time slot assignments) that exit within a communication channel. There are three basic types of traffic channels in the GSM system; full rate, half rate and eighth rate. Variants of these channels also exist. A full rate traffic channel - TCH/F - dedicates one slot per frame for a communication channel between a user and the cellular system. A half rate traffic channel - TCH/H dedicates one slot per every two frames for a communication channel between a user and the cellular system. The eighth rate traffic channel - TCH/8 - dedicates one time slot for every eight frames . The TCH/8 channel is only used for call setup and/or short message service, to provide limited data transmission rates.
- #21 Control channels are communication channels used in a system (such as a radio control channel), which are dedicated to the sending and/or receiving of command messages between devices (such as a base station and a mobile radio). On the GSM system, the control channel sends messages that include paging (alerting), access control (channel assignment) and system broadcast information (access parameters and system identification). Beacon (Broadcast) Channels (BCH) - are used to transfer system information such as timing references and synchronization information. The broadcast channel provides system information, system configuration information (such a paging channel sleep groups), and lists of neighboring radio channels to all mobile devices operating within its radio coverage area. Common Control Channels (CCCH) - are communication channels that are used to coordinate the control of mobile devices operating within its cell radio coverage area. GSM control channels include the random access channel (RACH), paging channel (PCH), and access grant channel (AGCH). Dedicated Control Channels - are communicatio channels that transfer signaling messages to specific devices. channels used in the GSM system. This diagram shows that the TDMA physical channel is divided into a control channel (time slot 0) and a traffic channel (time slot 4 in this example). The forward logical control channels include the frequency correction channel, synchronization channel, broadcast channel, paging channel, and access grant channel and the reverse logical control channel includes an access request channel. The traffic channel carries user data in both directions. This example shows that while on the traffic channel, fast control channel messages (FACCH) and slow control channel messages (SACCH) can be sent.
- #22 GSM networks consist of cell site radio towers - base stations, communication links, switching center(s), and network databases which can be linked to other systems and networks. Base Stations - are radio systems that can transmit and receive signals that are used and shared by mobile devices. Switching Centers - provide the circuit and packet data connections between GSM network components. Network Databases - contain the lists of subscribers, devices, and services that are used by the GSM system. Wireless Network System Interconnection - are the devices that adapt the signaling and media that is sent between the GSM network and other networks such as the public switched telephone network - PSTN and the Internet.
- #23 Base stations - BS - may be stand alone transmission systems that are part of a cell site and are composed of an antenna system (typically a radio tower), building, and base station radio equipment. Base station radio equipment consists of RF equipment (transceivers and antenna interface equipment), controllers, and power supplies. Base station radios are coordinated by the GSM system's BSC. The radio transceiver section is divided into transmitter and receiver assemblies. The transmitter section converts a voice signal to RF for transmission to wireless telephones and the receiver section converts RF from the mobile device to signals tha tare routed to the MSC or packet switching network. The controller section commands insertion and extraction of signaling information. Radio Antenna Towers - Wireless base station antenna heights can vary from a few feet to more than three hundred feet. Radio towers raise the height of antennas to provide greater area coverage. There may be several different antenna systems mounted on the same radio tower. A typical cell site antenna system has multiple antennas. One antenna is used for transmitting and two are used for reception for each radio coverage sector. Radio Equipment - in the base station contains audio processing, modulation, and RF power amplifier assemblies. In the GSM system, the transmitter power level for the control channel is usually fixed to define the cell boundaries (e.g. a control channel). The power level of dedicated (individual) channels may dynamically change to the lowest level possible that allows quality communication with the wireless telephone. Communication Links - are dedicated connections (such as E1 or T1 lines) that transfer control and media signals from the base station to GSM network components.
- #24 A switching center coordinates all communication channels and processes. There are two types of switches used in the GSM system; a mobile switching center (MSC) and a packet switching system. Mobile Switching Center - MSC - processes requests for service connections from mobile devices and land line callers, and routes calls between the base stations and the public switched telephone network (PSTN). The MSC receives the dialed digits, creates and interprets call processing tones, and routes the call paths. Serving General Packet Radio Service Support Node - SGSN - is a packet switching node that coordinates the operation of packet data devices that are operating within its service coverage range. The SGSN operates in a process similar to that of a MSC and a VLR, except the SGSN performs packet switching instead of circuit switching. The SGSN registers and maintains a list of active packet data radios in its network and coordinates the packet transfer between the mobile radios.
- #25 GSM network databases contain lists of subscribers , devices , services , and other Identifiable items that may be used by the GSM system. Some of the key GSM network databases include a master subscriber database (home location register), temporary active user subscriber database (visitor location register), unauthorized or suspect user database (equipment identity register), billing database, and authorization and validation center (authentication). Home Location Register - HLR - is a subscriber database containing each customer’s international mobile subscriber identity - IMSI and international mobile equipment identifier - IMEI to uniquely identify each customer. The HLR holds each customer’s user profile, which includes the selected long distance carrier, calling restrictions, service fee charge rates, and other selected network options. Visitor Location Register - VLR - contains a subset of a subscriber’s HLR information for use while a mobile telephone is active on a particular MSC. The VLR holds both visiting and home customers’ information. The user's required HLR information is temporarily stored in the VLR memory and then erased, either when the wireless telephone registers with another MSC or in another system, or after a specified period of inactivity. Equipment Identity Register - EIR - is a database that contains the identity of telecommunications devices (such as wireless telephones) and the status of these devices in the network (such as authorized or not-authorized). The EIR is primarily used to identify wireless telephones that may have been stolen or have questionable usage patterns that may indicate fraudulent use. Authentication Center - AuC - The authentication center (AuC) stores and processes information that is required to validate the identity ("authenticate") of a wireless telephone before service is provided. SMS Center - SMSC - receives, stores, delivers, and confirms receipt of short messages. Group Call Register - GCR - is a network database that holds a list of group members and the attributes that allow the set-up and processing of calls to and from group members. The GCR holds the membership lists, account features, priority authorization and the current location of group members.
- #26 The GSM system performs many tasks to setup, manage, and end communication sessions. Mobile Telephone Initialization - is the process of obtaining information that is broadcased from the mobile system that identifies how the mobile device should access the system. Updating Location - is the process of sending a registration message to the system. Waiting for Calls (Idle) - is the process of waiting for call alerts or other messages. Cell Reselection - is the process of determining when a different cell site should be monitored. System Access - is the process of coordinating how to get the attention of the system and receive channel assignments. Mobile Call Origination - is the process of requesting access to the GSM system after a user has initiated a call. Transferring Calls Between Call Sites - is the process of identifying when a connection should be transferred to another cell site and coordinating the handover process. Mobile Assisted Handoff - is the process of transferring information (such as RF signal quality) from the mobile device to the system which can be used to assist in the transfer process. Receiving a Call on a Mobile Telephone - is the process of alerting a mobile device that an incoming call is waiting to be answered and connecting the audio path if the mobile user answers the call. Conversation Mode - is the process of maintaining an audio connection with a user during a call. Connected Mode - is the process of maintaining the availability of a device that is transferring bursts of data (packet data) on the network. Authentication - is the validation of the identify of the device through the use of security processes and secret information.
- #27 System access control is the process of gaining the attention of the system , obtaining authorization to use system services, and the initial assignment to the communication channel to setup a communication session. Access control and initial assignment occur when a mobile device responds to a page (incoming connection request), desires to setup a call , or makes any attempt. Access to the GSM system is a random occurrence referred to as slotted ALOHA protocol (not usually preplanned.) To avoid access “collisions” between mobile devices, a seizure contention avoidance process is used. Before a mobile device attempts access to the system, it first waits until the channel is available (not busy serving other users). The mobile device then begins transmitting an access request message on the random access channel (RACH) at a power level assigned by the broadcast channel. If the system acknowledges the mobile device’s request for service, the mobile device will send additional information to the system that allows it to setup a dedicated communications channel on which conversation or data transmission can begin. The dedicated communication channel may be a traffic channel (for user voice and data) or an interim signaling channel. The stand alone dedicated control channel (SDCCH) allows the mobile device to setup (e.g. authenticate) while it is waiting for a traffic channel assignment.
- #28 Mobile call origination is the process of initiating a communication session by a mobile device . Mobile origination typically occurs when a user dials a telephone number and presses the SEND button . When initiating a call, a mobile telephone attempts to gain service from the GSM system by transmitting a system access request and indicating the access request is a call origination type. The access type is indicated by a 3-bit code that is contained in the access request message. This figure shows a functional diagram of how a mobile telephone initiates a call to a GSM network. In step 1, the mobile telephone sends a system access request message indicating it desires to initiate a call. When the system acknowledges the request, the mobile telephone is assigned to a traffic channel (step 2). The dialed digits are then sent to the system and the call is routed to the destination telephone (step 3). If the called person answers, the GSM system will open an audio path between the GSM user and the destination telephone (step 4).
- #29 Mobile call reception is the process of the GSM system sending paging messages on a paging control channel to alert mobile devices that they are receiving a call. Mobile devices listen for paging messages with their identification code (IMSI number or TMSI) on a paging channel. To receive a call , the mobile device synchronizes to the system and continuously monitors the paging channel. When the mobile device receives its identification number on the paging channel, it will attempt to access the mobile system indicating its access request is in response to a paging message . The system may then validate the identity of the mobile device and assign it to a traffic channel. The mobile device then alerts the user of an incoming call (ringing the mobile device) and, if the user answers the call (pressing SEND), the mobile device alerts the system that the call has been answered, and the cellular system can connect the audio path between the mobile device and the caller. This figure shows the basic process for receiving calls on a GSM system. In step 1, the mobile telephone receives a page message in its paging group. The mobile telephone sends access request messages to the system indicating the access is in response to a page message (step 2). The system assigns the mobile telephone to a traffic channel (step 3). The audio paths are then opened between the caller and the mobile telephone (step 4).
- #30 Mobile assisted handover - MAHO - is a process that is used to allow a mobile phone to assist the base station in the decision to transfer the call (handoff/handoff) to another base station. The mobile radio assists by providing RF signal quality information that typically includes the received signal strength indication - RSSI - and bit error rate - BER - of its own and other candidate channels. MAHO is an official term of the GSM system. During GSM communication, the mobile device transmits on one slot, receives on one slot, and has 6 idle slots available in each frame. During the idle time periods, the mobile telephone can tune to other radio channel frequencies and measure their signal strengths. This figure illustrates the basic mobile assisted handoff process. The mobile telephone initially receives a list of nearby radio channels to monitor. During the idle mobile telephone periods (between transmission and reception bursts), the mobile telephone monitors other radio channels for signal strength. The mobile telephone can report these measurements, along with its own received signal strength and channel quality (bit error rate) back to the base station. The base station can use this information, along with other information, to determine if a new radio channel should be assigned, and which channel to assign the mobile telephone to.
- #31 The GSM system evolved in phases starting from basic voice services to medium speed packet data services. Other systems have been developed that use or build on the GSM radio and network structure including GPRS, EDGE, WCDMA, and UMTS LTE. General Packet Radio Service - GPRS - Packet data on a 200 kHz GSM channel. Added packet switching nodes. Enhanced Data for Global Evolution (EDGE) - Higher speed data on a 200 kHz wide GSM channel through the use of new modulation types. Wideband Code Division Multiple Access (WCDMA) - A new 5 MHz wide radio channel that can be used with GSM network elements. UMTS Long Term Evolution (UMTS LTE) - Another new variable bandwidth (up to 20 MHz) radio channel that uses an all IP switching system. It can transfer to and from the GSM network.
- #32 Internet television service providers (ITVSPs) help customers find Internet television channels and manage connections between media sources. While it is possible in some cases for end users to directly connect to a media source by using an IP address or even a web link, Internet television provides may simplify the programming guide choices. Internet television service providers may also provide connections to subscription controlled television sources. For this role, the Internet TV service provider makes a business relationship with the media source. The IP television service provider may pay the media provider from funds it collects from their end user's. Content Sources - Content sources for Internet TV systems can include a mix of traditional television content sources and new media sources. While some Internet TV providers provide traditional content sources such as television programming form other countries, Internet TV service providers commonly provide specialty content that is not available through traditional broadcast networks. Examples include specialty sports channels (such as fly fishing), hobby channels (such as whiskey making), and others. If the consumer wants access to the content, they may only able to get it through Internet TV. Internet TV Headend - Internet TV headends are the network components that are used to mange how subscribers are connected to media sources. In some Internet TV systems , the headend manages and process media (convert formats if necessary) for distribution to subscribers and other Internet TV systems simply manage how viewing devices connect to media sources (direct connection). Internet TV Distribution - Internet TV distribution is the processes that are used to manage TV connections through the Internet . To minimize the aggregation of many channels onto a single high speed connection, Internet TV distribution systems may use mirror sites or multicast distribution systems to copy and redistribute programs multiple viewers who are simultaneously watching the same program. Internet TV Viewing Devices - Internet TV viewing devices can receive and convert media such as video, audio or images into a form that can be viewed by people. Internet TV viewing devices include multimedia computers and Internet TV set top box (STB) adapter boxes. Internet TV STBs are similar to IP STBs except they have the capability to receive TV programs over unmanaged broadband Internet connections. This figure shows that Internet television service providers (ITVSPs) are primarily made of computers that are connected to the Internet and software to operate call processing and other services. In this diagram, a computer keeps track of which customers are active (registration) and what features and services are authorized. When television channel requests are processed, the ITVSP sends messages to gateways via the Internet allowing television channels to be connected to a selected media gateway source (such as television channels). These media gateways transfer their billing details to a clearinghouse so the ITVSP can pay for the gateway's usage. The ITVSP then can use this billing information to charge the customer for channels viewed.
- #33 GSM was initially designed for digital voice communication. The GSM system has evolved to provide messaging, data, and multimedia services. GSM - Worldwide Mobile Wireless System - In almost every country. Large volume production of equipment has resulted in very low cost mobile and system equipment.. Can Provide, Voice, Messaging, and Data Services - Carriers can offer a mix of voice, data, and multimedia services. Many Types of Mobile Devices - Voice, Data Only -Mix of telephones, data adapters, and devices with embedded radio devices. 200 kHz Digital RF Channel - 8 Slots per Frame - A single type of channel that can be shared by multiple users. Mix of Logical Channels - Traffic and Control - map multiple communication functions to time slots. GSM Network - Base Stations, Switches, and Databases - Base stations are the access points. Switches and routers connect the data. Databases are lists of subscribers, devices, and services. Evolved into GPRS, EDGE, WCDMA, UMTS LTE - All these systems started from GSM.