Gsm Tutorial v1

8/8/2019 Gsm Tutorial v1

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This tutorial explains the basic components, technologies used, and operation of GlobalSystem for Mobile Communication - GSM - systems. You will discover why mobiletelephone service providers have upgraded from 1st generation analog systems to moreefficient and feature rich 2nd generation GSM systems. You will also discover how 2ndgeneration systems are gradually evolving into 3rd generation broadband multimediasystems.

This tutorial starts with the system components and basic services that the GSM systemcan provide. You will learn that the key types of GSM devices include single mode anddual mode mobile telephones, wireless PCMCIA cards, embedded radio modules, andexternal radio modems. You will then discover the different types of available servicessuch as multiple types of voice services, data services, group call, and messagingservices.

Explained are the physical and logical radio channel structures of the GSM system alongwith the basic frame and slot structures. Described are the fundamental capabilities andoperation of the GSM radio channel including channel coding, modulation types, speechcoding, RF power control, and mobile assisted handover. You will learn how each GSMradio 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 theneeds of voice and data communication are different, you will discover that the GSM


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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 theycommunicate with each other. You will learn how and why GSM is evolving into 3rdgeneration broadband systems including GPRS, EDGE, and WCDMA.

Mobile Communication expert Lawrence Harte has developed, designed, and managedmany types of communication products and services.

Mr. Harte is the publisher and editor of Mobile Video magazine and author of over 100books with over 50 on wireless communications. As a magazine editor, he hasinterviewed over 2100 companies in the mobile communication industry to discover thekey issues and solutions of the wireless industry.

Mr. Harte has worked for Ericsson/General Electric, Audiovox /Toshiba andWestinghouse and has consulted for hundreds of other companies. He is the inventor ofseveral patents on communication technology.

Mr. Harte holds many degrees and certificates including an Executive MBA from Wake


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Forest University (1995) and a BSET from the University of the State of New York, (1990).

Global System for Mobile Communication - A well defined radio system that is deployedin 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 formsthat 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 andbandwidth), 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 formatthat can be transmitted on GSM channels.

Radio Channels - The structure, frequencies, modulation types, and other RF


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characteristics of the GSM system.

Logical Channels - The logical control and data channels that share the physicaltransmission 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 servicesto devices and users.

GSM Evolution - How the system parts and services have evolved over time.

Mobile Communication System - Global system for mobile communication - GSM - is awide area wireless communications system that uses digital radio transmission toprovide voice, data, and multimedia communication services. A GSM system coordinatesthe communication between mobile telephones (mobile stations), base stations (cellsites), and switching systems.

Digital Media Formats - GSM is designed to transfer digital information. The initial versionof GSM transmitted digital media in circuit switched (continuous transmission) form and


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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 backto their digital form. The BSS is divided into base station - BS - parts that are located atthe cell site and base station controllers - BSC - that coordinate the distribution andreception of communication connections.

- Network and Switching System (NSS) - The NSS performs the interconnection betweenthe 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 dataswitches, databases, and administrative control services.

GSM services initially were focused on circuit switched voice service. The GSM systemevolved 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


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channel. The original design allowed for the use of a half rate voice service (lower qualityaudio) to increase 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 GSMsystem evolved to allow the combination of multiple circuit switched data connections toprovide high speed circuit switched data services - HSCSD.

GSM short messaging service - SMS is an electronic messaging service for extremelyshort text messages (140 characters). SMS evolved into executable messages that allowfor 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 servicewhich allowed for users to dynamically share packet data resources on one or more GSMchannels for services such as Internet browsing.


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GSM voice services provide continuous streams of digital audio to users. Each voiceuser shares time slots on a GSM radio channel.

Full Rate Voice - GSM full rate service allows 8 users to share each radio channel with avoice data rate of 13 kbps for each user.

Half Rate Voice - Half rate communication is a process where only half the normalchannel data rate (the full rate) is assigned to a user operating on a radiocommunications channel. Half rate GSM voice service allows up to 16 users to shareeach radio channel with a voice data rate of approximately 6.5 kbps for each user.

Enhanced Full Rate Voice - Enhanced full rate (EFR) is an improved form of digitalspeech compression used in GSM networks. The EFR rate speech coder uses the samedata transmission rate as the full rate speech coder. To use the EFR speech coder, boththe mobile device and the GSM system it is operating on must have EFR capability.

Voice Privacy - The GSM system has the capability to modify (encrypt) the digital audiousing a secret key that is shared by both the sender and receiver of the information(voice or data signal). Only users with the secret key can receive and decode theinformation. The key that is used by the GSM system constantly changes so even if thekey is compromised, it cannot be used again.

This figure shows how the GSM system allows more than one simultaneous user perradio channel through the use of time multiplexing. This example shows GSM radiochannel can be divided to allow 8 or 16 users per channel. The top example shows thatone slot per frame is assigned to full rate users. The bottom example shows that one slotfor every other frame is assigned to half rate users.


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GSM Data services are the processes that can transfer digital information betweenwireless devices and the GSM system. GSM data services use a continuous connection -circuit switched data, or they may involve burst data transmission - packet switched data.

Circuit Switched Data - GSM circuit switched data is the reserving of a continuous path oftransmission resources from a sender to a receiver. GSM data may dedicate a variablenumber of transmission time slots per frame for circuit switched data connection. Forcircuit switched connections, the data transmission path is always available even if thereis no data to send.

Packet Switched Data - GSM packet switched data is the sending of data which is dividedinto small packets which can take different paths through a packet data network. TheGSM radio channel was modified in the GPRS system to allow the dynamic assignment oftime slots to enable packet data to be sent on the GSM radio channel.

Fax Services - Fax service is the transmission of facsimile (image) information betweenusers. Facsimile signals are normally sent using audio frequencies which cannot be sentthrough the GSM voice coder. To send fax signals through the GSM system, the GSMsystem must decode the fax signal and send it in digital form. When the fax data reachesits GSM exit point, the GSM system must reconvert the digital fax back into it originalanalog form.


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GSM Short Messaging Service - SMS - is a service that enables mobile phone subscriberswith the ability to send and receive text or data messages. GSM mobile devices can sendshort messages - 160 alphanumeric characters (7 bits each) or 140 data elements (8 bitseach).

Point-to-Point Messaging - Point to point messaging is the process of sending data, textor alphanumeric messages from one communication device to another communicationdevice.

Point-to-Multipoint Messaging - From one device to several other devices.

Cell Broadcast Messaging - From one device to all devices that are operating within aradio coverage area.

Executable Messages - An executable message is received by a subscriber identitymodule (SIM) card in a wireless system (such as a mobile phone system) that contains aprogram that instructs the SIM card to perform processing instructions.

Flash Messages - Flash messaging is the automatic displaying of SMS messages as soonas they are received (such as a weather alert or a news emergency).


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This diagram shows how an executable short message can be sent by a system operatorto add a new feature into a mobile phone. The executable short message is a programthat is stored in the SIM card and interacts with the operation of the mobile phone toallow the new feature to operate. The system simply sends the file as an executablemessage directly to the mobile phone identification. When the complete executable short

message has been received in the SIM card, it is stored in memory and this program cancomplete (run) instructions, allowing the new feature to operate.

Voice group call service - VGCS - is the process of transmitting a single voiceconversation on a channel or group of channels so it can be simultaneously received bya predefined group of service subscribers. VGS allows the simultaneous reception ofspeech conversation of a predefined group of mobile radios and/or a dispatch console.Each mobile radio that has group call capability is called a group call member.

Group call service is also known as push to talk - PTT service. PTT is a process ofinitiating transmission through the use of a push-to-talk button. VGCS operates in halfduplex (one-way at a time) communication mode. The push to talk process involves the


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talker pressing a talk button (usually part of a handheld microphone) that must bepushed before the user can transmit. If the system is available for PTT service (otherusers in the group are not talking), the talker will be alerted (possibly with anacknowledgement tone) and the talker can transmit their voice by holding the talk button.If the system is not available, the user will not be able to transmit/talk.

The list of members in a dispatch group, along with their identification, assignedpriorities, and capabilities is stored in a group call register - GCR.

This figure shows how voice group call service may operate in a GSM system. In thisdiagram, a single voice message is transmitted on GSM radio channels in a pre-definedgeographic area. Several mobile radios are operating within the radio coverage limits(group 5 in this example) of the cells broadcasting the group message. In this example, auser is communicating to a group. Each user in this group (including the dispatcher)listens and decodes the message for group 5. Other handsets in the area are not able toreceive and decode the group 5 message.

Voice broadcast service - VBS - is the process of transferring a single audio signal (suchas a news alert message) to be transmitted to all mobile devices that are operating within


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a radio geographic coverage area (one or more cell sites). VBS subscribers or devicesthat are capable of identifying and receiving the VBS communication signal receive theconversation or message.

This figure shows the basic operation of voice broadcast service. This example showshow an urgent news message (traffic alert) can be sent to all mobile devices that are

operating within the same radio coverage area.

GSM can provide location information using different types positioning systemsincluding network positioning (the system itself) or through the use of global positioningsystem GPS Information that is provided from mobile devices. GSM services includecommercial services, internal location services, emergency location services, and lawfulintercept services.

Commercial Location Services - Value added services that are performed using locationdetermination equipment and services such as mapping and advertising.

Internal Location Services - Position discovery activities and data that are used fornetwork or service operation (find and page the subscriber).

Emergency Location Services - Discovery and transfer device location information to


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emergency facilities or services. Emergency LCS provides agencies with theidentification and location of a device that has dialed an emergency services number(such as 112 or 911).

Lawful Intercept Location Services - Providing of identification and location informationof a device to an authorized public safety agency.

This figure shows how mobile communication systems can use GPS signals to providelocation information. A mobile telephone has both mobile communication and GPSreception capability. When the user dials an emergency number, the GPS information canis sent to the public safety access point to allow emergency services to the location ofthe users mobile telephone.

GSM devices range from mobile telephones to data cards that allow devices to connectand communicate with the GSM system.GSM mobile devices may include the capabilityto 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.


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Embedded Radio Modules - Radio assemblies that can be built-in or installed in devicessuch as laptop computers, video cameras, or digital signage displays.

External Radio Modems - Assemblies that can be connected to other devices throughUSB, Ethernet, or other connection types to provide data services.

A Subscriber Identity Module - SIM - is an intelligent card that holds service subscriptioninformation, identity, and personal information and is included with GSM end userdevices.

The SIM contains a microprocessor, memory and software to hold and processinformation that includes a phone number, billing identification information and a smallamount of user specific data (such as feature preferences and short messages.) Thisinformation can be stored in the card rather than programmed into the phone itself. A SIMcard can either be credit card-sized (ISO format) or the size of a postage-stamp (Plug-Informat). 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 groupof subscriber identity module (SIM) cards. The use of a SIM lock code by a serviceprovider helps to ensure that a customer will only be able to use a communication device


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they provide at low cost with their SIM cards. If another SIM card is inserted to acommunication device that is locked to a specific SIM card, the communication devicewill not operate.

This figure shows a block diagram of a SIM card. This diagram shows that SIM cardshave 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 amicroprocessor that is used to store and retrieve data. Identification information is storedin the cards protected memory that is not accessible by the customer. Additional memoryis included to allow features or other information such as short messages to be stored onthe card.

GSM Radio is the transmission of control and user information in digital format through a200 kHz wide RF channel which usually operate on frequency bands around the worldranging from approximately 800 MHz to 3 GHz.

Frequency Reuse - The ability to reuse radio channels that operate on the samefrequency at different locations in a system area. This allows a limited number of radiochannels to serve a large number of customers.

Time Division Multiple Access - TDMA - Dividing up the radio channel into time slots that


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can be shared by 8 simultaneous users.

RF Power Control - A system that dynamically commands mobile devices to increase ordecrease their transmitter power level which can reduce interference (increase capacity)and save battery life.

Frequency reuse is the process of using the same radio frequencies on radio transmittersites within a geographic area, which are separated by sufficient distance to causeminimal interference with each other. Frequency reuse allows for a dramatic increase inthe number of customers that can be served (capacity) within a geographic area on alimited amount of radio spectrum (limited number of radio channels).

Frequency planning is the assignment (coordination) of radio channel frequencies inwireless systems that have multiple transmitters to minimize the amount of interferencecaused by transmitters that operate on the same frequency. Frequency planning is usedto help ensure that combined interference levels from nearby transmitters that areoperating on or near the same frequency do not exceed a certain interference (desiredsignal to interference) level compared to the desired signal.


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The ability to reuse frequencies depends on various factors, including the ability ofchannels to operate in with interference signal energy attenuation between thetransmitters. A frequency plan is the assignment of radio frequencies to radiotransmission sites (cell sites) that are located within a defined geographic area. Thefrequency plan may use ratios that are different dependent on the number of transmittingsites to the number of antennas (sectors) on each site. A common frequency reuse plan

for GSM is the ability to reuse a radio frequency on every 4th site that has three 120degree sectors each 12 total sectors. This plan is commonly called 4/12.

This figure shows how GSM can use frequency reuse to increase the system capacity.This diagram shows that a frequency in a GSM system can be reused at nearby cell sitesprovided the radio signal level from the interfering (unwanted) cell is 9 dB to 14 dB belowthe desired signal level

Time division multiple access - TDMA - is a process of sharing a single radio channel bydividing the channel into time slots that are shared between simultaneous users of theradio channel. When a mobile radio communicates with a TDMA system, it is assigned aspecific time position on the radio channel. By allowing several users to use differenttime positions (time slots) on a single radio channel, TDMA systems increase their abilityto serve multiple users with a limited number of radio channels.


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The GSM system uses radio channel pairs. One RF channel for the downlink (from thesystem to the mobile) and one RF channel for the uplink (from the mobile to the system).The frequency separation of channel pairs of varies (45 MHz at 800 MHz to 90 MHz at1.9GHz) with the system. In general, the higher the frequency, the larger the frequencydifference between downlink and uplink frequencies.

GSM uses time division multiplexing (TDM) to share one modulated carrier frequencyradio waveform among 8 (full rate) to 16 (half rate) conversations. Therefore, documentsrelated to GSM are careful to distinguish between a radio carrier and a communicationchannel.

This figure shows how time division duplex (TDD) communications uses a signal channelor frequency to provide simultaneous two-way communications between devices bytime-sharing. When using TDD, one device transmits (device 1), the other device listens(device 2) for a short period time. After the transmission is complete, the devices reversetheir role so device 1 becomes a receiver and device 2 becomes a transmitter. Theprocess continually repeats itself so data appears to flow in both directions

simultaneously.

RF power control is a process of adjusting the power level of a mobile radio as it movescloser and further away from a base station. RF power control is typically accomplished


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by the sensing of the received signal strength level and the relaying of power controlmessages from a transmitter to the mobile device with commands that are used toincrease or decrease the mobile device's output power level. GSM RF power adjustmentsoccur in 2 dB steps.

The use of RF power control allows for the transmission of only the necessary RF signal

level to maintain a quality communication link. Some of the key benefits of RF powercontrol include reduced radio channel interference to other radio devices and increasedbatter life.

This figure shows how the radio signal power level output of a mobile telephone isadjusted by commands received from the base station to reduce the average transmittedpower from the mobile telephone. This lower power reduces interference to nearby cellsites. As the mobile telephone moves closer to the cell site, less power is required fromthe mobile telephone and it is commanded to reduce its transmitter output power level.The base station transmitter power level can also be reduced, although the base stationRF output power is not typically reduced. While the maximum output power varies for

different classes of mobile telephones, they typically have the same minimum powerlevel.

The baseband of GSM devices convert sound signals into digital audio, compresses the


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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 byapproximately 10:1 (to 13 kbps).

Channel Coding - Dividing the data into segments (for time slots), adding error protectionbits, and merging in signaling control commands.

Analog to digital conversion - ADC - is a signal conversion process that periodicallysamples and converts a continuously varying signal - analog level - into digital values.The GSM system converts analog audio signals into digital form to be compressed andcoded onto the radio channel.

A typical analog to digital conversion process includes an initial filtering process toremove extremely high and low frequencies that could confuse the digital converter. Thisis followed by a periodic sampling section that measures the instantaneous level of thesignals at fixed time intervals and converts the measured values (sampled voltages) intothe equivalent digital number or pulses.


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This figure shows how an analog signal is converted to a digital signal. This diagramshows that an acoustic (sound) signal is converted to an audio electrical signal(continuously varying signal) by a microphone. This signal is sent through an audioband-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 13digital bits per sample (which is eventually compressed into 8 bits per sample). Thedigital bits represent the amplitude of the input analog signal.

Digital speech compression - speech coding - is a process of analyzing and compressinga digitized audio signal, transmitting that compressed digital signal to another point, anddecoding the compressed signal to recreate the original (or approximate of the original)signal.

The GSM digital speech compression process works by grouping the digital audiosignals into 20 msec speech frames. These speech frames are analyzed andcharacterized (e.g. volume, pitch) by the speech coder. The speech coder removesredundancy in the digital signal (such as silence periods) and characterizes digital


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patterns that can be made by the human voice using code book tables. The code booktable codes are transmitted instead of the original digitized audio signal. This results inthe transmission of a 13 kbps compressed digital audio instead of the 64 kbps digitizedaudio 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 eachsecond and digitized into a 64 kbps digital signal. The digitized signal is grouped into 20msec speech frames. The speech frames are analyzed and compressed into a new 13kbps digital signal.

Channel coding is a process where one or more control and user data signals arecombined with error protected or error correction information. After a sequence of digitaldata 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 readyfor transmission. This processing typically includes the addition of error detection anderror protection bits, along with the rearrangement of bit order for transmission.

Signaling Commands - Control messages that either replace (fast signaling) or sent alongside (slow signaling) the user data.


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Time Slot Allocation - Which time slot(s) are assigned to the user

A GSM radio channel is a 200 kHz communications channel that transfer digitalinformation from a source to a destination. The GSM radio channel is divided into timeslots and frames to allow multiple communication paths (logical channels) to share asingle radio transmission channel.

Radio Channel Bandwidth The GSM system uses a single type of RF channel which is200 kHz wide.

Modulation The first version of GSM used a robust but inefficient GSMK modulationtype.

Duplex Channels The GSM system uses a pair of RF channels for communication. Onechannel is used from the system to the mobile (the downlink) and the other channel isfrom the mobile to the system (the uplink).

Radio Channel Structure The GSM radio channel is divided into frames and time slots.

Time Slot Structure The structure of the GSM time slots can vary based on the mode ofoperation.


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Frame Structure There are 8 time slots per GSM frame.

Dynamic Time Alignment The GSM system can dynamically adjust the relative transmittime of mobile devices to compensate for the distance the device is located from thebase station.

Radio channel structure is the division and coordination of a radio communicationchannel (wireless information transfer) into frames (groups) of data, time slots, and fieldswithin the frames that hold specific types of information.

The radio channel is divided into frames with 8 time slots per frame (0 through 7), andtime 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 providesimultaneous two-way (duplex) voice communication. This pair of frequencies is knownas absolute radio frequency channel number (ARFCN) or just plain channel.

Between the downlink channel and uplink channel, the time slot numbers are offset by 3slots. This allows the mobile telephone to transmit at different times than it receives. Thisallows the design of the mobile device to be simplified by replacing a frequency filter


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(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 radiochannel in the GSM system has a frequency bandwidth of 200 kHz and a datatransmission rate of approximately 271 kbps. This example shows that each radiocommunication 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 oneradio channel communicates from the base station to the mobile station (called theforward channel) and one channel communicates from the mobile station to the basestation (called the reverse channel). This example also shows that some of the radiochannel capacity is used to transfer voice (traffic) information and some of the radiochannel capacity is used to transfer control messages.

Time slot structure is the division of a time slot period into different fields (informationparts). Slot structure fields include a preamble for synchronization, control header, userdata, signaling data, and error detection.

A single time slot transmission is called a radio burst. Four types of radio bursts are


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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 andramp down periods to minimize rapid changes in radio transmitter power. The ramp upand 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 mobiledevice and the base station. Each normal burst can transfer 114 bits of user informationdata (after error protection is removed).

Random Access Burst (Shortened Burst) is a short 88 bit transmission burst that is usedto request access to the GSM system. Mobile devices use a shortened burst whentransmitting an access request to the GSM system to avoid the possibility of burstoverlap with transmission bursts in adjacent time slots.

Frequency Correction Burst - is a time slot of information that contains a 142 bit patternof all 0 values. The reception and decoding of the frequency correction burst allows themobile device to adjust (frequency correct) its timing so it can better receive anddemodulate the radio channel.

Synchronization Burst - is a transmission burst that contains system timing information.It contains a 78 bit code to identify the hyper frame counter. The synchronization burstfollows the frequency correction burst.


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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, on ne use r is typically assigned toeach time slot within a frame. The GSM system also combines frames to formMultiframes.

Multiframes are frames that are grouped or linked together to perform specific functions.Multiframes on the GSM system use established schedules for specific purposes, suchas coordinating with frequency hopping patterns. Multiframes used in the GSM systeminclude the 26 traffic multiframe, 51 control multiframe, superframe, and hyperframe.

Traffic Multiframe Structures - The 26 traffic multiframe structure is used to sendinformation on the traffic channel. The 26 traffic multiframe structure is used to combineuser data (traffic), slow control signaling (SACCH), and idle time period. The idle timeperiod allows a mobile device to perform other necessary operations such as monitoringthe radio signal strength level of a beacon channel from other cells. The time interval of a26 frame traffic multiframe is 6 blocks of speech coder data (120 msec).

Control Multiframe Structures - The 51 control multiframe structure is used to sendinformation on the control channel. The 51 frame control multiframe is sub divided into


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logical channels that include the frequency correction burst, the synchronization burst,the broadcast channel (BCCH), the paging and access grant channel (PAGCH), and thestand-alone dedicated control channel (SDCCH). The PAGCH is logically sub divided intoPCH 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 periodallows 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 2048superframes, and is the largest time interval in the GSM system (3 hours, 28 minutes, 53seconds). Every time slot during a hyperframe has a sequential number (represented byan 11 bit counter) that is composed of a frame number and a time slot number. Thiscounter allows the hyperframe to synchronize frequency hopping sequence, encryptionprocesses 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 radiochannel is used to provide a control channel, time slot 0 and the other time slots are usedfor traffic channels. 51 frames are grouped together to form control multiframes (for thecontrol channel). 26 frames are grouped together to form traffic multiframes (for thetraffic channels). Superframes are the composition of 26 control multiframes or 51 trafficmultiframes to provide a common time period of 6.12 seconds. 2,048 superframes aregrouped together to form a hyperframe. A hyperframe has the longest time period in theGSM system of 3 hours, 28 minutes, and 53 seconds.


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Dynamic time alignment is a technique that allows a radio system base station to receivetransmitted signals from mobile radios in an exact time slot, even though not all mobiletelephones are the same distance from the base station. Time alignment keeps differentmobile radio transmit bursts from colliding or overlapping. Dynamic time alignment isnecessary because subscribers are moving, and their radio waves' arrival time at thebase station depends on their changing distance from the base station. The greater thedistance, the more delay in the signal's arrival time.

This figure shows how the relative transmitter timing in a mobile radio (relative to thereceived signal) is dynamically adjusted to account for the combined receive andtransmit delays as the mobile radio is located at different distances from the base stationantenna. In this example, the mobile telephone uses a received burst to determine whenits burst transmission should start.

As the mobile radio moves away from the tower, the transmission time increases and thiscauses the transmitted bursts to slip outside its time slot when it is received at the basestation (possibly causing overlap to transmissions from other radios.) When the basestation receiver detects the change in slot period reception, it sends commands to themobile telephone to advance its relative transmission time as it moves away from thebase station, and to retard its relative transmission time as it moves closer.


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The GSM system divides up the RF communications channel into logical channels thatserve different purposes. Traffic channels carry user data (voice and data). Controlchannel transport signaling messages.

Traffic Channels - ABC

Control Channels - Transfer command messages.

Broadcast Channels - Continuously transmit information to all users in the radiocoverage area. This can be system identification information (such as the name of thesystem).

Common Control Channel Signaling - Transfers command messages to users that aresharing the channel (such as sending paging messages to mobile devices that arelistening 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).


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Traffic channels - TCH - are the combination of voice and data signals (time slotassignments) that exit within a communication channel.

There are three basic types of traffic channels in the GSM system; full rate, half rate andeighth rate. Variants of these channels also exist.

A full rate traffic channel - TCH/F - dedicates one slot per frame for a communicationchannel between a user and the cellular system.

A half rate traffic channel - TCH/H dedicates one slot per every two frames for acommunication 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 providelimited data transmission rates.


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Control channels are communication channels used in a system (such as a radio controlchannel), which are dedicated to the sending and/or receiving of command messagesbetween devices (such as a base station and a mobile radio). On the GSM system, thecontrol channel sends messages that include paging (alerting), access control (channelassignment) and system broadcast information (access parameters and systemidentification).

Beacon (Broadcast) Channels (BCH) - are used to transfer system information such astiming references and synchronization information. The broadcast channel providessystem information, system configuration information (such a paging channel sleepgroups), and lists of neighboring radio channels to all mobile devices operating within itsradio coverage area.

Common Control Channels (CCCH) - are communication channels that are used tocoordinate 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 communication channels that transfer signalingmessages to specific devices.


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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 controlchannels include the frequency correction channel, synchronization channel, broadcastchannel, paging channel, and access grant channel and the reverse logical controlchannel includes an access request channel. The traffic channel carries user data in bothdirections. This example shows that while on the traffic channel, fast control channel

messages (FACCH) and slow control channel messages (SACCH) can be sent.

Traffic channel signaling is the sending of command messages on a channel that isassigned to a specific device - signaling on a dedicated channel.

Fast Channel Signaling - FACCH - Sending control messages in place of data (cantransfer the messages faster). FACCH messages could be used for power controlcommands which should be reached to quickly.

Slow Channel Signaling - SACCH - Sending control messages along with user data.SACCH signaling channel transfers a single control message in approximately 1/2second. Messages. SACCH signaling can be used to send continuous channel qualityreports to assist in the handover decision.


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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 andnetworks.

Base Stations - are radio systems that can transmit and receive signals that are used andshared by mobile devices.

Switching Centers - provide the circuit and packet data connections between GSMnetwork components.

Network Databases - contain the lists of subscribers, devices, and services that are usedby the GSM system.

Wireless Network System Interconnection - are the devices that adapt the signaling andmedia that is sent between the GSM network and other networks such as the publicswitched telephone network - PSTN and the Internet.


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Base stations - BS - may be stand alone transmission systems that are part of a cell siteand are composed of an antenna system (typically a radio tower), building, and basestation 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 transceiversection is divided into transmitter and receiver assemblies. The transmitter sectionconverts a voice signal to RF for transmission to wireless telephones and the receiversection converts RF from the mobile device to signals that are routed to the MSC orpacket switching network. The controller section commands insertion and extraction ofsignaling information.

Radio Antenna Towers - Wireless base station antenna heights can vary from a few feetto more than three hundred feet. Radio towers raise the height of antennas to providegreater area coverage. There may be several different antenna systems mounted on thesame radio tower.

A typical cell site antenna system has multiple antennas. One antenna is used fortransmittingand two are used for reception for each radio coverage sector.


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Radio Equipment - in the base station contains audio processing, modulation, and RFpower amplifier assemblies. In the GSM system, the transmitter power level for thecontrol channel is usually fixed to define the cell boundaries (e.g. a control channel). Thepower level of dedicated (individual) channels may dynamically change to the lowestlevel possible that allows quality communication with the wireless telephone.

Communication Links - are dedicated connections (such as E1 or T1 lines) that transfercontrol and media signals from the base station to GSM network components.

A switching center coordinates all communication channels and processes. There aretwo types of switches used in the GSM system; a mobile switching center (MSC) and apacket switching system.

Mobile Switching Center - MSC - processes requests for service connections from mobiledevices and land line callers, and routes calls between the base stations and the publicswitched telephone network (PSTN). The MSC receives the dialed digits, creates andinterprets call processing tones, and routes the call paths.

Serving General Packet Radio Service Support Node - SGSN - is a packet switching nodethat coordinates the operation of packet data devices that are operating within its service


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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 SGSNregisters and maintains a list of active packet data radios in its network and coordinatesthe packet transfer between the mobile radios.

GSM network databases contain lists of subscribers, devices, services, and otherIdentifiable items that may be used by the GSM system. Some of the key GSM networkdatabases include a master subscriber database (home location register), temporaryactive user subscriber database (visitor location register), unauthorized or suspect userdatabase (equipment identity register), billing database, and authorization and validationcenter (authentication).

Home Location Register - HLR - is a subscriber database containing each customersinternational mobile subscriber identity - IMSI and international mobile equipmentidentifier - IMEI to uniquely identify each customer. The HLR holds each customers userprofile, which includes the selected long distance carrier, calling restrictions, service feecharge rates, and other selected network options.

Visitor Location Register - VLR - contains a subset of a subscribers HLR information foruse while a mobile telephone is active on a particular MSC. The VLR holds both visitingand home customers information. The user's required HLR information is temporarily


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stored in the VLR memory and then erased, either when the wireless telephone registerswith another MSC or in another system, or after a specified period of inactivity.

Equipment Identity Register - EIR - is a database that contains the identity oftelecommunications devices (such as wireless telephones) and the status of thesedevices 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 usagepatterns that may indicate fraudulent use.

Authentication Center - AuC - The authentication center (AuC) stores and processesinformation that is required to validate the identity ("authenticate") of a wirelesstelephone 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 andthe 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 thecurrent location of group members.

The GSM system performs many tasks to setup, manage, and end communication


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sessions.

Mobile Telephone Initialization - is the process of obtaining information that isbroadcasted from the mobile system that identifies how the mobile device should accessthe 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 bemonitored.

System Access - is the process of coordinating how to get the attention of the systemand 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 connectionshould be transferred to another cell site and coordinating the handover process.

Mobile Assisted Handoff - is the process of transferring information (such as RF signalquality) from the mobile device to the system which can be used to assist in the transferprocess.

Receiving a Call on a Mobile Telephone - is the process of alerting a mobile device thatan 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 userduring a call.

Connected Mode - is the process of maintaining the availability of a device that istransferring bursts of data (packet data) on the network.

Authentication - is the validation of the identifying of the device through the use ofsecurity processes and secret information.


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System access control is the process of gaining the attention of the system, obtainingauthorization to use system services, and the initial assignment to the communicationchannel to setup a communication session.

Access control and initial assignment occur when a mobile device responds to a paging(incoming connection request), desires to setup a call, or makes any attempt. Access tothe GSM system is a random occurrence referred to as slotted ALOHA protocol (notusually preplanned.) To avoid access collisions between mobile devices, a seizurecontention avoidance process is used. Before a mobile device attempts access to thesystem, it first waits until the channel is available (not busy serving other users). Themobile device then begins transmitting an access request message on the randomaccess channel (RACH) at a power level assigned by the broadcast channel.

If the system acknowledges the mobile devices request for service, the mobile devicewill send additional information to the system that allows it to setup a dedicatedcommunications channel on which conversation or data transmission can begin. Thededicated communication channel may be a traffic channel (for user voice and data) oran interim signaling channel. The stand alone dedicated control channel (SDCCH) allowsthe mobile device to setup (e.g. authenticate) while it is waiting for a traffic channelassignment.


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Mobile call origination is the process of initiating a communication session by a mobiledevice. Mobile origination typically occurs when a user dials a telephone number andpresses the SEND button.

When initiating a call, a mobile telephone attempts to gain service from the GSM systemby transmitting a system access request and indicating the access request is a callorigination type. The access type is indicated by a 3-bit code that is contained in theaccess request message.

This figure shows a functional diagram of how a mobile telephone initiates a call to aGSM network. In step 1, the mobile telephone sends a system access request messageindicating it desires to initiate a call. When the system acknowledges the request, themobile telephone is assigned to a traffic channel (step 2). The dialed digits are then sentto the system and the call is routed to the destination telephone (step 3). If the calledperson answers, the GSM system will open an audio path between the GSM user and thedestination telephone (step 4).


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Mobile call reception is the process of the GSM system sending paging messages on apaging control channel to alert mobile devices that they are receiving a call. Mobiledevices 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 continuouslymonitors the paging channel. When the mobile device receives its identification numberon the paging channel, it will attempt to access the mobile system indicating its accessrequest is in response to a paging message. The system may then validate the identity ofthe mobile device and assign it to a traffic channel. The mobile device then alerts theuser 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 thecaller.

This figure shows the basic process for receiving calls on a GSM system. In step 1, themobile telephone receives a page message in its paging group. The mobile telephonesends access request messages to the system indicating the access is in response to apage 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).


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Mobile assisted handover - MAHO - is a process that is used to allow a mobile phone toassist the base station in the decision to transfer the call (handoff/handoff) to anotherbase station. The mobile radio assists by providing RF signal quality information thattypically 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 oneslot, and has 6 idle slots available in each frame. During the idle time periods, the mobiletelephone can tune to other radio channel frequencies and measure their signalstrengths.

This figure illustrates the basic mobile assisted handoff process. The mobile telephoneinitially receives a list of nearby radio channels to monitor. During the idle mobiletelephone periods (between transmission and reception bursts), the mobile telephonemonitors other radio channels for signal strength. The mobile telephone can report thesemeasurements, along with its own received signal strength and channel quality (bit errorrate) back to the base station. The base station can use this information, along with otherinformation, to determine if a new radio channel should be assigned, and which channelto assign the mobile telephone to.


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Authentication is a process of exchanging information between a communications deviceand the mobile network which allows the carrier or network operator to confirm the trueidentity of the user (or device). This validation of the authenticity of the user or deviceallows a service provider to deny service to users that cannot be identified.

The authentication algorithm used in the GSM system is contained in the subscriberidentity module - SIM - card. The GSM authentication process can use different versionsof authentication.

The GSM authentication process starts with the transmission of a random number(RAND) from the base station. This random number is used, along with other informationincluding the secret data value (Ki), to calculate a signed response (SRES). The secretnumber Ki is stored in both the mobile telephone and GSM system and it is nottransmitted over the radio link. When the GSM system performs the authenticationprocess, it compares the SRES it calculates to the SRES returned by the mobiletelephone. If both SRESs match, the GSM system allows call processing to continue. Thecodes generated in the authentication step may be used for voice privacy (encryption)mode.

This figure shows the basic GSM authentication process. As part of a typicalauthentication process, a random number that changes periodically (RAND) is sent from


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the base station. This number is regularly received and temporarily stored by the mobileradio. The random number is then processed with the shared secret data (Ki) that hasbeen previously stored in the SIM card, along with additional subscriber information, tocreate an authentication response (SRES). The authentication response is sent back tothe system to validate the mobile radio. The system processes the same information tocreate its own authentication response. If both authentication responses match, service

may be provided. This process avoids sending any secret information over the radiocommunication channel.

The GSM system evolved in phases starting from basic voice services to medium speedpacket data services. Other systems have been developed that use or build on the GSMradio and network structure including GPRS, EDGE, WCDMA, and UMTS LTE.

General Packet Radio Service - GPRS - Packet data on a 200 kHz GSM channel. Addedpacket switching nodes.

Enhanced Data for Global Evolution (EDGE) - Higher speed data on a 200 kHz wide GSMchannel through the use of new modulation types.

Wideband Code Division Multiple Access (WCDMA) - A new 5 MHz wide radio channel


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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 GSMnetwork.

GSM was initially designed for digital voice communication. The GSM system hasevolved to provide messaging, data, and multimedia services.

GSM - Worldwide Mobile Wireless System - In almost every country. Large volumeproduction 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, anddevices with embedded radio devices.

200 kHz Digital RF Channel - 8 Slots per Frame - A single type of channel that can beshared by multiple users.


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Mix of Logical Channels - Traffic and Control - map multiple communication functions totime slots.

GSM Network - Base Stations, Switches, and Databases - Base stations are the accesspoints. 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.

Documents

Gsm Tutorial v1