Introduction to GSM, page 1 Introduction to GSM 1. Development of GSM History of GSM Market situation GSM s future development Services offered by GSM GSM specifications 2. OSI reference model 3. RF interface
Introduction to GSM, page 2 1. Development of GSM History of GSM Communication any where, with anyone, at any time has been the aim of communications technology in recent years and still is. Without the progress in microelectronics, computer and software engineering, the creation of efficient algorithms and other achievements in many fields of communications technology this goal could not have been attained. Mobile communication has been around for quite some time now. Analog standards for mobile communication systems were developed and implemented as early as the end of the 70s. Subscribers were not well served by the plethora of different local standards. Fig. 1 gives an overview of the various analog mobile radio standards used in Europe in the mid 80s. NMT 450 NMT 900 TACS TACS NMT 450 NMT 900 NMT 450 C 450 NMT 450 NMT 900 NMT 450 TACS NMT 450 NMT 450 C 450 NMT 450 TACS Fig. 1: Analog mobile radio standards in Europe in the mid 80s With the rapid growth of the conventional analog cellular radio networks in Europe, it soon became clear that a system with a much greater capacity had to be planned. Another objective of such a system was to provide international compatibility to make the hodge-podge of different networks thrown together in the analog era obsolescent. In 1982, the Groupe Spécial Mobile (GSM) set up by CEPT started to develop a Pan- European standard for a digital cellular mobile radio network. After the foundation of the ETSI (European Telecommunications Standards Institute), GSM was reinterpreted to mean Global System for Mobile Communications which is now the official designation.
Introduction to GSM, page 3 The key event that lead to the current success of GSM was the signing of a Memorandum of Understanding by several European nations. It was agreed that just one standard should be introduced throughout Europe. Year Milestone 1982 Formation of the Groupe Spécial Mobile within CEPT 1987 Memorandum of Understanding signed 1989 GSM standardization work transferred to ETSI 1990 Phase 1 of specifications frozen 1991 The first GSM networks go into operation 1992 Most of the European networks start commercial voice services 1995 Phase 2 of the specifications is completed. New features: fax, data and SMS) 2000 More than 400 million subscribers in more than 130 countries and more than 350 network operators worldwide Fig. 2: History of GSM Market situation The boom in mobile communications has by far surpassed all the earliest prognoses and continues unabated. According to a market estimation (see also http://www.gsmworld.com), more than 400 million subscribers worldwide had made GSM calls by the end of 2000. GSM is being used in more than 130 countries where more than 350 network operators provide radio coverage. And this upward trend will continue. Penetration rates of more than 70% in certain countries mean that a further increase in subscriber numbers can be expected. Fig. 3 shows a 1999 study on expected subscriber numbers for the various mobile radio standards. The migration to other frequency bands also reflects the success of GSM. It was clear at a very early stage that further GSM networks had to be defined in other frequency bands. As early as the beginning of the 90s, DCS 1800 was specified. The name was later changed to GSM 1800 and this has remained the official designation. Since the GSM 900 and GSM 1800 frequency bands are not available in the United States, the Personal Communication System PCS 1900 (also known as GSM1900) was defined. The intention is now to move GSM into the 400 MHz band as well. The designations in this case are GSM 450 and GSM 480. The advantage of lower frequencies is a greater range. The GSM 400 network is, therefore, particularly attractive for thinly populated areas as the number of base stations can be reduced. The GSM-R system has, so far, not been mentioned. R stands for railway and GSM- R is a system that is used solely by railway companies across Europe on the basis of a common standard.
Introduction to GSM, page 4 Fig. 3: Predicted numbers of subscribers using the various mobile network systems that are available (mid 1999). GSM s future development Despite the enormous success of the GSM mobile radio standard, the GSM standard has weak points that hinder future applications. An essential objective is to increase the data transmission rate so that services like mobile Internet access, fast data transfer and mobile faxing can be implemented. The current GSM data rate of max. 14.4 kbit/s is too slow for these applications. Phase 2+ GSM specifications propose three further developments that aim at a higher data transmission rate and a more efficient use of available radio resources. The specified data rates are theoretical values, experience will show which data rates can really be achieved. HSCSD (high-speed circuit-switched data): HSCSD is based on multi-slot transmission,i.e. the bundling of timeslots. Not just one but several timeslots per frame are available to the subscriber. This, of course, increases the data transmission rate. Maximum transmission rates of 4 x 14.4 kbit/s = 57.6 kbit/s seem realistic at present. The main problem here is the circuit-switched assignment of physical channels, i.e. the network operator decides whether and how many timeslots are assigned to a subscriber. The consequence: Access may be denied to other subscribers.
Introduction to GSM, page 5 GPRS (general packet radio service): GPRS, too, is based on multi-slot transmission and timeslot bundling, but it uses packet switching instead of circuit-switching. This means GPRS is compatible with other packet-switched networks like TCP/IP from the Internet and mobile radio. Transmission only takes place when data are present and transmission resources are assigned dynamically by the network. GPRS chooses one of four coding schemes depending on transmission quality. Theoretically, GPRS can provide a maximum data rate of about 170 kbit/s. EDGE (enhanced data rate for GSM evolution): This latest step in the evolution of the GSM standard uses a different modulation method. Several data symbols are transmitted with one modulation symbol so that data rates of up to 384 kbit/s can be achieved. A disadvantage for the network operator is that a better S/N ratio is required. EDGE will be of interest for all network operators who could not obtain an UMTS license but would still like to be market players. The next mobile radio generation called UMTS (universal mobile telecommunication system) is now in the standardization phase, the ultimate objective being a mobile radio standard that applies all over the world. Fig. 4 shows the change from the 2nd generation mobile radio standard through the "2.5 th " to the 3rd generation. The path that was actually taken differed from network operator to network operator. GSM TDMA/FDMA 9,6 (14,4) kbit/s IS-136 (D-AMPS) TDMA 19,2 kbit/s IS-95 (cdmaone) CDMA 8,0 (13,0) kbit/s PDC TDMA/FDMA 9,6 kbit/s HSCSD 56 kbit/s GPRS 45-160 kbit/s EDGE 384 kbit/s IS-136+ 43,2 kbit/s IS-136 HS 384 kbit/s 1 Mbit/s IS-95 B 64 kbit/s UMTS W-CDMA/TD-CDMA 144; 384 kbit/s 2 Mbit/s UWC-136 TDMA 144; 384 kbit/s 2 Mbit/s cdma 2000 CDMA n * 64 kbit/s 2 Mbit/s IMT-2000 Fig. 4: Evolution of the various global mobile radio standards
Introduction to GSM, page 6 Services offered by GSM Teleservices Supplementary services Bearer services Fig. 5: GSM services There are three categories of services provided by GSM networks: Teleservices Bearer services (data transmission) Supplementary services The term teleservice refers to services provided on a user-terminal to user-terminal basis. The most important teleservice is straight voice communication, fax transmission also belongs to this category. Another example is the short-message service (a form of alphanumeric paging) in which a message received by the mobile can be read directly from the display. With bearer services, the end user provides his own terminal equipment and the responsibility of the network operator ends at the end-user transfer point. Many forms of data transmission at rates between 300 and 14400 bit/s fall into this category. GSM does not provide any special error control for transparent data links - the user must provide this for himself. With non-transparent data services, a GSM protocol provides error control but at the same time reduces the maximum data rate to 14.4 kbit/s. Supplementary services were developed along the lines of planned ISDN services, but vary greatly from country to country. Among the first services implemented in mobile radio networks are - call forwarding - advice of call charge - call restriction - conference facilities
Introduction to GSM, page 7 GSM Specifications The GSM Specifications fill some 5000 pages. They contain a great number of technical recommendations for the mobile radio network and the main headings are as follows: 00 Preamble 01 General Vocabulary, Abbreviations 02 Service Aspects 03 Network 04 MS-BS Interface and Protocols 05 Physical Layer on Radio Path 06 Audio Aspects 07 Terminal Adapters for Mobiles 08 BTS/BSC Interface (A bis ) and BSC/MSC Interface (A) 09 Network Interworking 10 Service Interworking 11 Equipment Specifications and Type-approval Specifications 12 Network Management, Operations and Maintenance Aspects Fig. 6: Main sections of the GSM Specifications 2. The OSI Reference Model A system of this complexity requires a great deal of planning and organization at both the definition and the implementation phase. A framework for structuring data communications networks in general has been developed by the International Standards Organization ISO in the form of the open system interconnection (OSI) model. The OSI model provides for a number of horizontal layers, each layer communicating exclusively, and according to well-defined rules, with the layers immediately above and below it. Communication is, therefore, vertical except for the lowest, or physical layer, where information passes from one system to another. The GSM specifications follow the stipulations for the bottom three layers of the OSI model.
Introduction to GSM, page 8 OSI model Application in GSM 7 6 Application Presentation Tasks of user 5 4 Session Transport Tasks of fixed network 3 Network Call control Mobility management Radio resources management 2 Data Link Block building and concatenation of messages Acknowledgement mode Tasks of GSM network 1 Physical Error protection coding Channel coding Modulation Fig. 7: OSI reference model in GSM At the lowest layer (layer 1), the physical characteristics of the transmission medium are specified. In the context of GSM radio links, this definition not only includes frequencies, modulation types, etc, but also the structure of the bursts and frames because a time-division multiplex technique is used. Since this layer is responsible for the correct transmission of single bits, it requires some form of error control coding. The second GSM layer (layer 2), referred to as the data link layer, consists of an intelligent entity responsible for the secure communication of data messages between the radio stations. To this end, the transmit side structures the messages from the higher layer to match the physical constraints of the layer 1 medium and requests, in many situations, a confirmation (acknowledgement) from the receiving side. At the receive side of layer 2, messages are reconstructed from the received frames and the acknowledgements formulated and sent back.
Introduction to GSM, page 9 The third GSM layer (layer 3), also referred to as the network layer, is responsible for the management of all calls and associated activities of the radio network. These tasks are further subdivided into sublayers designated: - Call control management (CC) - Mobility management (MM) - Radio resource management (RR) Voice Voice User data Signalling OSI Layer 3 Network functions Signalling User data Frame building Acknowledgement Request OSI Layer 2 Data transmission Frame restoration Acknowledgement Channel coding Error protection Spreading Error correction Despreading Equalization RF modulation Transmitter OSI Layer 1 Physical layer RF demodulation Receiver Fig. 8: Block diagram of GSM mobile station As can be seen from the greatly simplified functional block diagram of the GSM transmitter and receiver, this segregation of functions does not only provide a useful basis for apportioning the design effort, it also ensures that the measurement interfaces within the system are clearly defined.
Introduction to GSM, page 10 3. RF Interface Certain frequency bands could be reserved worldwide for the GSM system despite the rival mobile radio systems. All signatories to the Memorandum of Understanding for GSM have agreed to implement their GSM systems in these frequency bands. The following frequency bands are now used in Europe: GSM 900 (D network) Frequency range P band G1 band Uplink (mobile sending) Downlink GSM 1800 (E network) 890 to 915 MHz 880 to 890 MHz 1710 to 1785 MHz 935 to 960 MHz 925 to 935 MHz 1805 to 1880 MHz (BS sending) Duplex spacing 45 MHz 95 MHz Spectrum 2 x 35 MHz 2 x 75 MHz Number of channels 124 49 374 Channel numbers 1 to 124 975 to 1023 512 to 885 Channel spacing 200 khz Modulation GMSK with B x T = 0.3 Data transmission rate 270.833 kbit/s Bit period 3.69 µs Fig. 9: Frequency assignment in D and E networks The relatively wide channel spacing compared to conventional radio services which often have to make do with bandwidths of 20 to 25 khz is striking. However, to ensure that the spectrum is used efficiently, the useful duration of a GSM channel is divided up into 0.577 ms timeslots. This means that the transmission channel is not permanently available to one specific user (TDMA system: time division multiple access). Eight synchronized users share a TDMA frame so that each user can send or receive a 0.577 ms data packet every 4.62 ms. This is shown in Fig. 10. Each data packet contains 2 x 57 bits = 114 bits which are, subjected to comprehensive error control. The nominal data rate is therefore: 114 bits / 4.62 ms 24.7 kbit / s Normally, 13.0 kbit/s of the nominal data rate are available for voice transmission (full-rate codec), the rest is for error protection.
Introduction to GSM, page 11 TDMA-frame = 4,62 msec 0 1 2 3 4 5 6 7 Time slot = 0,577 msec Information 57 Bit Training sequence and flag-bits Information 57 Bit Guard period Fig. 10: Time organization of a GSM frequency channel A timeslot contains three tail bits (not shown in the drawing above), an information packet (57 bits of raw data), a training sequence (including flag bits for special signalling tasks) mainly for channel equalization, another information packet (57 bits of raw data) and a guard interval including tail bits to ensure that a burst does not spill over from one timeslot into the next during assignment. To allow a mobile to transmit and receive in a quasi duplex mode, the transmit and receive windows are offset in time: TDMA-frames (time slot 0... 7) 4,62 msec each Down link TX RX TX RX TX RX (transm. frequency 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Base station) Uplink (rcv. frequency mobile station) 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 RX TX RX TX RX TX Timeslot numbering offset 3 timeslots between mobile and base station Fig. 11: Transmission and reception with time offset For the sake of clarity, time slot numbering on the MS side is shifted by three slots with respect to that of the BS side. Strictly speaking, this intermittent transmission and reception corresponds to halfduplex operation. However, because of voice data compression, AF data transmission seems to be continuous. As far as hardware requirements are concerned, this has the advantage that only one synthesizer is required for the
Introduction to GSM, page 12 receiver and transmitter in the mobile station. In "transmission gaps", the synthesizer may even be used to measure the field strength of neighbouring base stations. This field strength measurement is used to decide whether communication should be handed over from one base station to another. Other GSM systems are available besides the described GSM networks. They are either still in the implementation phase, intended for closed subscriber groups or have had to be shifted to other frequency bands because of the current channel assignment. Comparison of main specifications: Downlink Uplink GSM 900 (P band) 935 to 960 MHz 890 to 915 MHz GSM 900 (extended) 925 to 960 MHz 880 to 915 MHz GSM 1800 1805 to 1880 MHz 1710 to 1785 MHz GSM 900 Rail 921 to 925 MHz 876 to 880 MHz GSM 450 460.4 to 467.6 MHz 450.4 to 457.6 MHz GSM 480 488.8 to 496 MHz 478.8 to 486 MHz GSM 1900 1930 to 1990 MHz 1850 to 1910 MHz Fig. 12: Comparison of the various GSM systems GSM 900 Rail is used by European railway companies as a train radio telephony system and cannot therefore be regarded as a public voice system. GSM 450 and GSM 480 are mainly intended for rural networks where a greater coverage is required (max. 70 km compared to the approx. 35 km for all other GSM networks). GSM 1900 is used in the United States.