<p>GSM(Global System for Mobile Communication)<br/> I he success of mobile systems across the world is a sign that com—munication is moving towards a more personalized，convenient system. People who have to use a mobile phone on business soon begin to realize that the ability to phone any time，any place in one’S personal life rapidly becomes a necessity，not a convenience．<br/> The speed and rapidity with which the personal communications revo-lution takes place is，unlike fixed transmission systems,highly dependent on technology and communication standards．<br/> For mobile the three key elements to achieving service take—up are the cost，the size and the weight of the phone，and the cost and quality of the link3．If any of these are wrong，especially the fi rst two，then market g rowth is liable to be severely restricted．<br/> The fixed telephone service is global and the interconnection varies from coaxial cable to optical fib re and satellite．The national standards are different，but with common interfaces and interface conversion，intercon-nection can take place．For mobile the problem is far more complex，with the need to roam creating a need for complex networks and systems-f hus in mobile the question Of standa rds is far more c rucial to success than fixed systems4．In addition，there is also the vexed question Of spectrum alloca-tion in the mobile area．<br/> Mobile systems originally operated in analog mode in the 450 MHz band，moving later to 900 MHz with digital GSM and then to 1 800 MHz with personal co~nmunication systems．The history of mobility can split into gen-erations．The fi rst generation systems were the advanced mobile phone systems(AMPS)in the U．S．，total access communication system(TACS) in most of Eu rope and Nordic mobile telephone system(NMT)；which were all analogue systems．The second generation is vary much dominated by the standard first set out in Eu rope by the g roup special mobile(GSM) committee，which was designed as a global mobile communication system．<br/> The GSM system is based on a cellular communications principle which was first proposed as a concept in the 1940s by Bell System engineers in the U．S．．The idea came out of the need to increase network capacity and got round the fact that broadcast mobile networks，operating in densely populated areas，could be jammed by a very small number of simultaneous calls5．The power of the cellula r system was that it allowed frequency reuse．<br/> The cellular concept is defined by two features，frequency reuse and cell splitting．Frequency reuse comes into play by using radio channels on the same frequency in coverage areas that are far enough apart not to cause co-channel interference．This allows handIing of simultaneous calls<br/>that exceed the theoretical spectral capacity．Cell splitting is necessary when the traffic demand on a cel J has reached the maximum and the ceII is then divided into a micro-cellular system．The shape of celI in a cellular system is always depicted as a hexagon and the cluster size can be seven，<br/>nine or twelve．<br/> The GSM system requires a nutuber of functions to be created for a fully operational mobile system．<br/> The cell coverage area is controlled by a base station which is itself made up of two elements．The fitst element is the transmission system which communicates out to the moblle and also receives information from it to set up and maintain calls when actually in operation．The base station transceiver(BST)is controlled by the base station controller(BSC)，which communicates with the mobile switching center(MSC)一the essentiaI link to the Iocal public switched telephone network(PSTN)，and to the sub-scriber data which is stored in registers within the system．The subsc riber registers allow the GSM system to check a subscriber who requests the use<br/>of the network，allow access and then set up the charging function，etc．<br/> The GSM system was allocated part of the 900 MHz band at fhe 1978 World Administratlon Conference(WAC)，the actual bands being 890 to 915 MHz for the uplink transmission and 935 to 960 MHz for ihe down Jink．The access method is time division multiple access(TDMA)．</p><p> The GSM system operates in a burst transmission mode with 124 radk channels in the 900 MHz band，and these bursts can carry different types of information．The fi rst type of information is speech，which js coded a 6. 5 kbit／s or 13 kbit／s．The second type is data，which can be sent a<br/>3．6 kbit／s，6 kbit／s or 12．6 kbit／s．These two forms of transmission are the useful parts of the transmission，but have to be supported by overhead information which is sent in controI channels(CCH)．<br/> The use of digital radio transmission and the advanced handover algo-rIthms between radio cells in GSM network allows for significantly bette frequency usage than in analogue cellular systems,thus in creasing the number of subscribers thal can be served⒎．Since。GSM provides commor standard．cellular subscribers wiII also be able to use lheir telephones over the enti re GSM service area．Roaming is fully automatic between and withi rall countries covered by GSM system．In addition to international roaming，GSM provides new services，such as high—speed data communication，Facsim_le and short message service．The GSM technicaI specifications are<br/>designed to work in concert with other standards，e．g．ISDN．Interworkinc between the standards is in this way assured．In the long term perspective cellular systems，using a digital technology，will become the universa method of telecommunication．<br/> The third generation mobile communication system cu rrently being developed。in Europe is intended to integ rate all the different services osecond generation systems and cover a much wider range of broadbanc services(voice，data，video and multimedia)consistent and compatible<br/>with technology developments taking place within the fixed te Jeoommunica-tion networks.</p><p> Ourrent Situation and the Future in the Telecommunication Future World<br/> Today’s telecommunication networks a re cha racterized by speciali-zation．This means that for every individual telecommunication service at least one network exists that transports this service．A few examples of existing public networks are described below：<br/> ·A telex network transports telex information，i．e．messages of cha racters，transported at very low speed．The cha racters are coded based on a specific 5-bit code．<br/> ·POTS(plain old telephone service)is transported via the public switched telephone network(PSTN)．This ubiquitous network offers the customers classical two—way voice conversation．<br/> ·Computer data are transported in the public domain either by apacket switched data network based on X．25 protocols，or in a very limited number of countries by a circuit switched data network based on X．21 protocols．<br/> ·Television signals can be transported in th ree ways：broadcast viaradio waves using ground antenna，by the coaxial tree network of commu-nity antenna TV network or recently via a satellite，using the so—called direct broadcast system．<br/> ·In the private domain．computer data are mainly transported by LANs(LocaI Area Network)．The most famous ones are Ethernet，token bus and token ring(IEEE 802 series)．<br/> Each of these networks was specially designed for that specific serv-ice and is often not at all applicable to transport another service．For instance，the original CATV networks did not allow the transportation of POTS；or the PSTN does not t ransport TV signals~or the transfer of voice<br/>over an X．25 network is very problematic because Of too la rge end-to-enddelay and jitter on this delay．<br/> Only in limited and special cases can service types other than the one the network was originally designed for be transported over it。．This is for instance the case for the PSTN which is capable of transporting computer data at a Iimited speed。if modems a re provided at both ends of the net-work．<br/> An important consequence of this service specialization is the exist-ence of a large number of often world—wide independent networks，each requiring its own design phase，manufactu ring and maintenance4．In addi-tion，the dimensioning of each network must be done for every individual<br/>service type．Even if resources are freely available in one network，they cannot be used by another service type For example，the peak hours in the telephone network are between 9 a．m．and 5 P．m．，whereas the peak hours in the CATV network are during evening．Since resou rce pooling is<br/>impossible each network must be dimensioned for its worst case traffic conditions which is the peak hour traffic．<br/> A fi rst step，albeit a limited one towards a single universal network，is the introduction of NISDN in which voice and data are transported over a single medium This network cannot transport TV signals due to its limited bandwidth capabilities，so a special TV network is still required．Even in NISDN the integration of narrowband services such as data and voice can<br/>be considered as being rather limited：the user access to the network is fully integ rated，either by a basic access or primary rate interface．Howev-er，inside the network there wiII stiII exist for some time a packet switched and a circuit switched network as two overlay networks incapable of trans-porting other traffic types and each dimensioned either for voice or X．25 data．<br/> Another important consequence of this service specialization is the inability of the network to benefit highly from the progress made in technolo—gy and coding algorithms．For instance，current digital NISDN switches are designed for 64 kbit／s voice channels．However，with the cu rrent prog ress in speech coding and chip technology．bit rates of 32 kbit／s,13 kbit／s and<br/>even lower will be used in the future．The existing switches and transmis-sion systems are not directly suited and thus need an adaptation，or will not efficiently use their InternaI resou rces tor these Iower speed bit rates．<br/> When designing the future BISDN network，one must take into account all possible existing and futu re services．Suppose a network is capable of transporting a specific service，e．g．a ci rcuit switched service with a channel rate of 70 Mbit／s．Suppose also that it is specifically designed to<br/>transport this bit rate．Some years later a new teleservice of，for example，40 Mbit／s appears on the scene．This would mean that the network de-signed for that service(i．e．70 Mbit／s)will be capable of transporting the new teleservice，but with a la rge inefficiency：only 40 out of the 70 Mbit／s available wiII be used．This example is not un realistic．It is very Iikely that<br/>in the future new services will emerge which have not yet been identified，and of which the requirements are unknown today．<br/> As can be concluded from the above examples。the networks of todayare very specialized and suffer from a large number of disadvantages，the most important being：<br/> ·Service Dependence<br/> Each network is only capable of transporting one specific service for which it was intentionally designed．Only in a limited number of cases and by using additional equipment(e．g a modem)and with an inefficient use Of its resources can it be adapted to other services6．<br/> ·Inflexibility<br/> Advances in audio，video and speech coding and compression algo-rithms and progress in Very Large Systems Integration(VLSI)technology influence the bit rate generated by a certain service and thus change the service requirements for the network．In the futu re，new services with<br/>unknown requirements will appear．For the time being it is yet unclear,e．g．what the requi rements in terms of’bit rate for HDTV will be．A spe-cialized network has great difficulties in adapting to changing or new serv-ice requi rements．<br/> ·Inefficiency<br/> The internal available resou rces are used inefficiently Resources which are available in one network cannot be made available to other net-works．<br/> Taking into account all these considerations on flexibility，service depe-dence and resource usage，it is consequently very important in the future that only a single network exists and that this network of the future(B-IS-DN)is service—independent．This implies a single network capable of<br/>transporting all services，sharing alI its available resources between the different services．<br/> A single service-·independent network will not suffer from the disad-vantages described above，but it will have the following main advantages：<br/> ·Flexible and future-safe<br/> Advances in the state of the art of coding algorithms and VLSI technol—ogy may reduce the bandwidth of existing teleservices．A network capable of transporting all types of services will be able to adapt itself to changing or ReW needs．<br/> ·Efficient in the use of its available resources<br/> All available resou rces can be shared between aII services，such that an optimal statistical sharing of the resources can be obtained<br/> ·Less expensive<br/> Since only one network needs to be designed，manufactured and maintained,the overall costs of the design, manufacturing operations and maintenance w…be smaller．</p>
发表于 2007-4-23 17:32:00