GIÁO TRÌNH GSM

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Hard to fathom, but it really wasn't all that long ago that even a plain old telephone was a luxury item. But, as we all know, technology's only constant is change. In this day and age, many folks need to be accessible everywhere, whether they're at work or play, in the office or at home. To meet this demand, the GSM standard (Global System for Mobile Communications) for mobile telephony was introduced in the mid- 1980s. Today, GSM is the most popular mobile radio standard in the world. A boom is underway, such that many GSM users find life without their phone practically inconceivable....

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Pocket Guide for Fundamentals and GSM Testing Publisher: Wandel & Goltermann GmbH & Co Elektronische Meûtechnik P O. Box 12 62 . D-72795 Eningen u.A. Germany e-mail: solutions@wg.com http://www.wg.com Author: Marc Kahabka
CONTENTS 1 ªMobilityº ± The magic word . . . . . . . . . . . . .. .. .. . . . . . . . 3 2 GSM overview . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . 5 3 GSM system architecture . . . . . . . . . . . . . . . .. .. .. . . . . . . . 7 4 Interfaces and protocols . . . . . . . . . . . . . . . .. .. .. . . . . . . . 11 5 The air interface Um . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . 13 5.1 Logical channels on the air interface . . . . .. .. .. . . . . . . . 15 5.2 Traffic channels on the air interface . . . . . .. .. .. . . . . . . . 17 5.3 Signaling channels on the air interface . . .. .. .. . . . . . . . 18 5.4 Burst formats . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . 20 5.5 Protocols on the air interface . . . . . . . . . .. .. .. . . . . . . . 22 6 The Abis interface . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . 24 6.1 The TRAU frame . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . 26 6.2 Protocols on the Abis interface . . . . . . . . .. .. .. . . . . . . . 28 7 The A interface . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . 30 7.1 Protocols on the A interface . . . . . . . . . . .. .. .. . . . . . . . 30 8 MSC-based interfaces . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . 32 8.1 MSC protocols . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . 33 9 Call setup . . . . . . . . . . . . . . . . . .. . . . . . . . .. .. .. . . . . . . . 35 10 Test and measurement problems in GSM . . . .. .. .. . . . . . . . 37 11 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . 46 12 GSM glossary . . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . 47 13 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . 51 1
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1 ªMobilityª ± Hard to fathom, but it really wasn't all that long ago that even a plain The magic word old telephone was a luxury item. But, as we all know, technology's only constant is change. In this day and age, many folks need to be access- ible everywhere, whether they're at work or play, in the office or at home. To meet this demand, the GSM standard (Global System for Mo- bile Communications) for mobile telephony was introduced in the mid- 1980s. Today, GSM is the most popular mobile radio standard in the world. A boom is underway, such that many GSM users find life without their phone practically inconceivable. Nowadays, when we speak of GSM, we usually mean ªoriginalº GSM ± also known as GSM900 since 900 MHz was the original frequency band. To provide additional capacity and enable higher subscriber den- sities, two other systems were added later: GSM1800 (also DCS1800) and GSM1900 (also PCS 900). Compared to GSM 900, GSM1800 and GSM1900 differ primarily in the air interface. Besides using another fre- quency band, they use a microcellular structure (i.e. a smaller coverage region for each radio cell). This makes it possible to reuse frequencies at closer distances, enabling an increase in subscriber density. The dis- advantage is the higher attenuation of the air interface due to the higher frequency. The rest of this booklet will mainly focus on GSM900. Where now? A few years ago, Michael Jackson sang ª. . . just call my name and I'll be thereº. While this might seem inconceivable now, it might become reality sooner than we think, given the rapid pace of technological evolution. Faced with a whirlwind of speculation, ETSI 3
(the telecom standardization authority in Europe) decided to base the air interface of the planned universal mobile telecommunications sys- tem (UMTS) on a mix of WCDMA and TD/CDMA technologies. The in- frastructure of the existing GSM networks will most likely be used. This booklet is intended to provide communications engineers & techni- cians with basic information about the GSM system ± a starting point for further study of any given area. A word of warning: Look further if you need complete GSM system specifications. Research sources are listed in the appendix. Also: This booklet assumes you, the reader, have a basic understanding of telecommunications technology. Enjoy! Marc Kahabka 4
2 GSM overview Fig. 1: The Mobile Evolution Before GSM networks there were public mobile radio networks (cellu- lar). They normally used analog technologies, which varied from country to country and from manufacturer to another. These analog networks 5
did not comply with any uniform standard. There was no way to use a single mobile phone from one country to another. The speech quality in most networks was not satisfactory. GSM became popular very quickly because it provided improved speech quality and, through a uniform international standard, made it possible to use a single telephone number and mobile unit around the world. The European Telecommunications Standardization Institute (ETSI) adopted the GSM standard in 1991, and GSM is now used in 135 countries. The benefits of GSM include: ± Support for international roaming ± Distinction between user and device identification ± Excellent speech quality ± Wide range of services ± Interworking (e.g. with ISDN, DECT) ± Extensive security features GSM also stands out from other technologies with its wide range of services1: ± Telephony ± Asynchronous and synchronous data services (2.4/4.8/9.6 kbit/s) ± Access to packet data network (X.25) ± Telematic services (SMS, fax, videotext, etc.) ± Many value-added features (call forwarding, caller ID, voice mailbox) ± E-mail and Internet connections 1 Available services vary from operator to operator 6
3 GSM system architecture Fig. 2 The best way to create a manageable communications system is to divide it into various subgroups that are interconnected using standardized interfaces. A GSM network can be divided into three groups (see Fig. 2): The mobile station (MS), the base station subsystem (BSS) and the network subsystem. 7
They are characterized as follows: The mobile station A mobile station may be referred to as a ªhandsetº, a ªmobileº, a ªport- (MS) able terminalº or ªmobile equipmentº ME). It also includes a subscriber identity module (SIM) that is normally removable and comes in two sizes. Each SIM card has a unique identification number called IMSI (international mobile subscriber identity). In addition, each MS is as- signed a unique hardware identification called IMEI (international mobile equipment identity). In some of the newer applications (data communications in particular), an MS can also be a terminal that acts as a GSM interface, e.g. for a laptop computer. In this new application the MS does not look like a normal GSM telephone. The seemingly low price of a mobile phone can give the (false) impres- sion that the product is not of high quality. Besides providing a trans- ceiver (TRX) for transmission and reception of voice and data, the mobile also performs a number of very demanding tasks such as authentication, handover, encoding and channel encoding. The base station The base station subsystem (BSS) is made up of the base station subsystem (BSS) controller (BSC) and the base transceiver station (BTS). The base transceiver station (BTS): GSM uses a series of radio trans- mitters called BTSs to connect the mobiles to a cellular network. Their tasks include channel coding/decoding and encryption/decryption. A BTS is comprised of radio transmitters and receivers, antennas, the in- terface to the PCM facility, etc. The BTS may contain one or more 8
transceivers to provide the required call handling capacity. A cell site may be omnidirectional or split into typically three directional cells. . The base station controller (BSC): A group of BTSs are connected to a particular BSC which manages the radio resources for them. Today's new and intelligent BTSs have taken over many tasks that were previously handled by the BSCs. The primary function of the BSC is call maintenance. The mobile sta- tions normally send a report of their received signal strength to the BSC every 480 ms. With this information the BSC decides to initiate handovers to other cells, change the BTS transmitter power, etc. . The mobile switching center (MSC): Acts like a standard exchange The network subsystem in a fixed network and additionally provides all the functionality needed to handle a mobile subscriber. The main functions are regis- tration, authentication, location updating, handovers and call routing to a roaming subscriber. The signaling between functional entities (registers) in the network subsystem uses Signaling System 7 (SS7). If the MSC also has a gateway function for communicating with other networks, it is called Gateway MSC (GMSC). . The home location register (HLR): A database used for management of mobile subscribers. It stores the international mobile subscriber identity (IMSI), mobile station ISDN number (MSISDN) and current visitor location register (VLR) address. The main information stored there concerns the location of each mobile station in order to be able to route calls to the mo- bile subscribers managed by each HLR. The HLR also maintains the ser- vices associated with each MS. One HLR can serve several MSCs. 9
. The visitor location register (VLR): Contains the current location of the MS and selected administrative information from the HLR, neces- sary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. A VLR is connected to one MSC and is normally integrated into the MSC's hardware. . The authentication center (AuC): A protected database that holds a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and encryption over the radio channel. The AuC provides additional security against fraud. It is normally located close to each HLR within a GSM network. . The equipment identity register (EIR): The EIR is a database that contains a list of all valid mobile station equipment within the net- work, where each mobile station is identified by its international mo- bile equipment identity (IMEI). The EIR has three databases: ± White list: for all known, good IMEIs ± Black list: for bad or stolen handsets ± Grey list: for handsets/IMEIs that are uncertain Operation and The OMC is a management system that oversees the GSM functional Maintenance Center blocks. The OMC assists the network operator in maintaining satisfac- (OMC) tory operation of the GSM network. Hardware redundancy and intelli- gent error detection mechanisms help prevent network down-time. The OMC is responsible for controlling and maintaining the MSC, BSC and BTS. It can be in charge of an entire public land mobile network (PLMN) or just some parts of the PLMN. 10
4 Interfaces and protocols Fig. 3: OSI Layer structure in GSM Note: Numbers in parentheses indicate the relevant ETSI-GSM Recommendations. Providing voice or data transmission quality over the radio link is only part of the function of a cellular mobile network. A GSM mobile can seamlessly roam nationally and internationally, requiring standardized call routing and location updating functions in GSM networks. A public communications system also needs solid security mechanisms to pre- vent misuse by third parties. Security functions such as authentication, encryption and the use of Temporary Mobile Subscriber Identities (TMSIs) are an absolute must. 11
Within a GSM network, different protocols are needed to enable the flow of data and signaling between different GSM subsystems. Figure 3 shows the interfaces that link the different GSM subsystems and the protocols used to communicate on each interface. GSM protocols are basically divided into three layers: . Layer 1: Physical layer ± Enables physical transmission (TDMA, FDMA, etc.) ± Assessment of channel quality ± Except on the air interface (GSM Rec. 04.04), PCM 30 or ISDN links are used (GSM Rec. 08.54 on Abis interface and 08.04 on A to F interfaces). . Layer 2: Data link layer ± Multiplexing of one or more layer 2 connections on control/signaling channels ± Error detection (based on HDLC) ± Flow control ± Transmission quality assurance ± Routing . Layer 3: Network layer ± Connection management (air interface) ± Management of location data ± Subscriber identification ± Management of added services (SMS, call forwarding, conference calls, etc.) 12
5 The air interface Um Fig. 4: GSM Air Interface, TDMA frame The International Telecommunication Union (ITU), which manages inter- national allocation of radio spectrum (among many other functions), has allocated the following bands: GSM900: Uplink: 890±915 MHz (= mobile station to base station) Downlink: 935±960 MHz (= base station to mobile station). 13
GSM1800 (previously: DCS-1800): Uplink: 1710±1785 MHz Downlink: 1805±1880 MHz GSM1900 (previously: PCS-1900): Uplink: 1850±1910 MHz Downlink: 1930±1990 MHz The air interface for GSM is known as the Um interface. Since radio spectrum is a limited resource shared by all users, a method was devised to divide the bandwidth among as many users as possible. The method chosen by GSM is a combination of time- and frequency-division multiple access (TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum) 25 MHz allocated bandwidth into 124 carrier frequencies spaced 200 kHz apart. One or more carrier frequencies are assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts approx. 0.577 ms. Eight burst periods are grouped into a TDMA frame (approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame. 14
5.1 Logical channels on the air inter- face Fig. 5: GSM Air Interface, logical channels 15
Several logical channels are mapped onto the physical channels. The organization of logical channels depends on the application and the direction of information flow (uplink/downlink or bidirectional). A logical channel can be either a traffic channel (TCH), which carries user data, or a signaling channel (see following chapters). Fig. 6 16
5.2 Traffic channels A traffic channel (TCH) is used to carry speech and data traffic. Traffic on the air inter- channels are defined using a 26-frame multiframe, or group of 26 TDMA face frames. The length of a 26-frame multiframe is 120 ms, which is how the length of a burst period is defined (120 ms divided by 26 frames divided by 8 burst periods per frame). Out of the 26 frames, 24 are used for traffic, 1 is used for the slow associated control channel (SACCH) and 1 is currently unused (see Fig. 5). TCHs for the uplink and downlink are separated in time by 3 burst periods, so that the mobile station does not have to transmit and receive simultaneously, thereby simplifying the electronic circuitry. This method permits complex an- tenna duplex filters to be avoided and thus helps to cut power con- sumption. In addition to these full-rate TCHs (TCH/F, 22.8 kbit/s), half-rate TCHs (TCH/H, 11.4 kbit/s) are also defined. Half-rate TCHs double the capa- city of a system effectively by making it possible to transmit two calls in a single channel. If a TCH/F is used for data communications, the usable data rate drops to 9.6 kbit/s (in TCH/H: max. 4.8 kbit/s) due to the enhanced security algorithms. Eighth-rate TCHs are also specified, and are used for signaling. In the GSM Recommendations, they are called stand-alone dedicated control channels (SDCCH). 17
5.3 Signaling The signaling channels on the air interface are used for call establish- channels on the ment, paging, call maintenance, synchronization, etc. There are 3 groups air interface of signaling channels: . The broadcast channels (BCH): Carry only downlink information and are responsible mainly for synchronization and frequency correc- tion. This is the only channel type enabling point-to-multipoint com- munications in which short messages are simultaneously transmitted to several mobiles. The BCHs include the following channels: ± The broadcast control channel (BCCH): General information, cell- specific; e.g. local area code (LAC), network operator, access parameters, list of neighboring cells, etc. The MS receives signals via the BCCH from many BTSs within the same network and/or different networks. ± The frequency correction channel (FCCH): Downlink only; correc- tion of MS frequencies; transmission of frequency standard to MS; it is also used for synchronization of an acquisition by providing the boundaries between timeslots and the position of the first time- slot of a TDMA frame. ± The synchronization channel (SCH): Downlink only; frame syn- chronization (TDMA frame number) and identification of base station. The valid reception of one SCH burst will provide the MS with all the information needed to synchronize with a BTS. 18