Tài liệu Mạng và viễn thông P15 ppt

15 Mobile Telephone Networks Being away from a telephone, a telex or a facsimile machine has become unacceptable for many individuals, not only because they cannot be contacted, but also because they may be deprived of the opportunity to refer to others for advice or information. For these individuals, the advent of mobile communications promises a new era, one in which there will never be an excuse for being ‘out-of-touch’. This chapter discusses modern mobile radio communication technologies, covering the principles of ‘radio telephone service’, ‘trunk mobile radio (TMR)’ cordless telephones, the ‘global system for mobilecommunication (GSM)’, as well as describing telephone communication with ships, aircraft, and trains and the emerging satellite mobile networks (e.g. Iridium and Globalstar). Mobile datacommunication networks are covered in Chapter 24. 15.1 RADIO TELEPHONE SERVICE The first radio telephone services were manually operated in the high frequency radio band. These supported international telephone services, as well as communication with ships and aircraft. Immediately after World War 2, a new breed of VHF (very high frequency) radio transmitters and receivers (transceivers) were developed. First used for applications such as the police, the fire service and in taxis, later development lead to their use as full radio telephones, connected to the public switched telephone network (PSTN) for the receipt and generation of ordinary telephone calls. The technology used a radio mast located on a hill and equipped with a powerful multi-channel radio transceiver. The mobile stations were weighty but were nonetheless popular for commercial ‘car telephone’ service, which grew rapidly in popularity in the mid-1970s. For calls made to or from the radiotelephone user, the public telephone network is connected via mobile switching centres (MSCs) to a number of transmitter base stations which emit and receive radio signals from the mobile telephones. Two radio channels (using different frequencies) are needed to connect each telephone during conversation, one radio channel for each direction of conversation. Figure 15.1 illustrates a typical automatic radio telephone system of the late 1970s. 297 Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) 298 MOBILE TELEPHONE NETWORKS swltchlng Mobile centre (MSC) Public switched telephone network LOO circutts to PSTN Figure 15.1 A simple radio telephone system Figure 15.1 illustrates a single mobile switching centre and a single transmitter base station, serving 3600 mobile customers within an area of 1200 km2, within a radius of about 20 km from the base station. Calls are made or received by the mobile telephone station by monitoring a particular radio channel, called the signalling or call up channel. When making a call, the mobile station signals a message to the base station in much the same way as an ordinary telephone sends an off-hook signal and a dialled digit train (Chapter 7). The base station next allocates a free pair of radio channels to be used between the base station and the mobile for the subsequent period of conversation, and the two switch over to the allocated channels simultaneously. At the end of the call the radio channels are released for use on another call. Incoming calls to the radio telephone connect in the same way. An ordinary telephone number is dialled by an ordinary customer connected to the public switched telephone network (PSTN). A particular area code within the normal telephone numbering plan is usually allocated to identify the mobile network, and all calls with this dialled area code are routed to the mobile switching centre and via the appropriate base station transmitter to reach the desired mobile receiver. A hurdle to be overcome in the design of radio telephone systems is the need for incoming calls to be routed only to the nearest base station. Failure to do so causes the failure of incoming calls, because the mobile cannot guarantee to be within the coverage area of a particular transmitting base station. Early radio telephone systems overcame the problem by requiring the caller to know the whereabouts of the mobile that he wished to call. The same customer number would be used on all incoming calls, but a different area code would have to be used, depending on which base station the caller wished the call to be routed to (i.e. according to which zone the caller thought the mobile was in). Figure 15.2 illustrates this principle. We see here that if the mobile customer’s number is ‘12345’, then when a caller believes that the mobile is in zone 1, the number 033 1 12345 should be dialled. Similarly for zones 2 and 3, the numbers 0332 12345 and 0333 12345 are appropriate. Some more advanced CELLULAR RADIO 299 Dial area code Mobile as below plus zone 2 0332 zone 3 0 333 Figure 15.2 Area code routing for incoming radio telephone calls radio telephone systems were developed with the ability to ‘remember’ the last location from which a call was made and route the call there. Others used ‘trial-and-error’ methods, but these systems were all superseded by the advent of cellular radio, which has eliminated the market for old-style radio telephones used by roaming users. Another drawback of the early radio telephone systems was their low user capacity, and their ‘unfriendly’ usage characteristics. Not only did the automatic systems rely on the caller to know where the mobile was, but some demanded the ‘press-to-speak’ action of the mobile user. The systems allowed the mobile user to continue to move about during the call, but if he or she moved out of the coverage area of the base station, the call would be terminated. There were no facilities for transferring to other base stations during the course of a call. Together, these drawbacks lead to the demise of radiotelephony in its original guise and to its replacement with cellular radio, discussed next. The technology has, however, survived for localized radio network coverage, such as required by taxi companies or regional haulage companies. The terms private mobile radio (PMR) or trunk mobile radio (TMR) (in Germany Bundelfunk) are more commonly used nowadays. Some interest has been shown to extend the usage and life of the technology, as demonstrated in the recent development by ETSI of new standards for TETRA (trans-European trunk radio), which aims to provide relatively low speed integrated voice and data networking capability to and from mobile stations located within relatively large geographical radio coverage areas. TETRA is discussed more fully in Chapter 24. 15.2 CELLULAR RADIO A difficulty with early radio telephone systems was the small capacity in number of users. This arose because the early systems were designed to have very wide area coverage, but had a limited number of radio channels available within each zone. Furthermore, the re-use of radio channels in other zones was precluded by the risk of 300 MOBILE TELEPHONE NETWORKS interference except where base stations were separated by large distances. Because the 1970s saw a boom in radio telephone demand, and because radio channel availability was (and still is) a limited resource, there was pressure for development of revised methods, more efficiently using the radio bandwidth, thereby greatly increasing the available user capacity. The system that evolved has become known as cellular radio. The basic components of cellular radio networks are mobile switching centres (MSCs), base stations, and mobile units, as Figure 15.3 shows. Cellular radio networks make efficient use of the radio spectrum, re-using the same radio channel frequencies in a large number of base stations or cells. Oriented in a 'honeycomb' fashion, each cell is kept small, so that the radio transmitting power required at the transmitting base station can be kept low. This limits the area over which the radio signal is effective, and so reduces the area over which radio signal interference can occur. Outside the interfering zone of the transmitter, i.e. in a non- adjacent cell at a sufficient distance away from the first, the same radio channel frequencies may be re-used. Figure 15.5 illustrates the interfering zone of a given cell base station, and shows another cell using the same radio channel frequencies. A whole honeycomb of cells is established, re-using radio channels between the various cells according to a pre-determined plans. A seven cell re-usepattern is shown in Figure 15.6. Seven different radio channel frequency schemes are repeated over each cluster of seven hexagonal cells, each cell using a different set of frequencies. By such planning the same radio frequency can be used for different conversations two or three cells away. Figure 15.6 shows a 7-cell re-use pattern. Other patterns, some involving as few as three cells, and some more than thirty cells, can also be used. Large repeat patterns are necessary to cater for heavy traffic demand in built-up areas where small non-adjacent cells may still interfere with one another. Each cell is served initially by a single base station at its centre and is complemented as traffic grows with directional antennas and more radio channels. Using direc- tional antennas helps to overcome radio wave shadows. For example, to locate three Cell coverage area / 'Cells' ? / MSCs or centres; /\" I/ /" Mobile handset Other base statlons Figure 15.3 The basic components of a cellular radio network CELLULAR RADIO 301 Figure 15.4 Cellular radio carphone: mounted and in use in a car Cell channels re-using 1-400 v Figure 15.5 Cellular radio channel interference and re-use. BS = Base Station 302 MOBILE TELEPHONE NETWORKS Transmitter 'Cluster'of seven cells Adjacent 'cluster' W Figure 15.6 Cellular radio in re-use pattern obstacle Radio 'shadow' Mobile station \ Alternative radio path Figure 15.7 Location of multiple base stations directional antennas, one at each alternate corner of the cell, helps to overcome the shadow effects that might otherwise occur near tall buildings by giving an alternative transmission path, as Figure 15.7 shows. A feature of cellular radio networks is their ability to cope with an increasing level of demand first by using more radio channels and more antennas in the cell, and then by reducing the size of cells, splitting the old cells to form a multiplicity of new ones. Only a limited number of radio channels can be made available in a cell at the same time, and this limits the number of simultaneous telephone conversations. By increasing the number of cells, the overall call capacity can be increased. The number of channels needed in a given cell is determined by the normal Erlung formula (Chapter 30). The total call demand during the busy hour of the day depends on the number of callers within the cell at the time. Reducing the size of the cell has the effect of reducing the number of mobile stations that is likely to be in it at any time, and so relieves MAKING CELLULAR RADIO CALLS 303 R / 1 / Metropolitan l zone \ \ \ ‘ A Country zone Figure 15.8 Cell splitting to increase cell capacity congestion. Figure 15.8 shows a simple splitting of cells and a gradual reduction in cell size in the transitional region between a low traffic (country) area and the high traffic region surrounding a major metropolitan zone. When splitting cells in this manner, due care needs to be taken when allocating radio frequencies to the new cells, and a new frequency re-use plan may be necessary to prevent inter-cell interference. 15.3 MAKING CELLULAR RADIO CALLS One or more control orpuging radio channels are used to make or receive calls between the mobile station. If a mobile user wants to make a call, the mobile handset scans the pre- determined channels to determine the strongest control channel, and monitors it to receive network status and availability information. When the telephone number of the destina- tion has been dialled by the customer and the send key has been pressed, the mobile handset finds a free control channel and broadcasts a request for a user (i.e. radio telephone) channel. All the base stations use the same control channels, and monitor them for call requests. On the receipt of such a request by any base station, a message is sent to the nearest mobile switching centre (MSC), indicating both the desire of the mobile station to place a call and the strength of the radio signal received from the mobile. The MSC determines which base station has received the greatest signal strength, and, based on this, decides which cell the mobile is in. It then requests the mobile handset to identify itself with an authorization number that can be used for call charging. The authorization procedure eliminates any scope for fraud. Following authorization of an outgoing call, a free radio channel is allocated in the appropriate cell for the carriage of the call itself, and the call is extended to its destination on the public switched telephone network. Incidentally, the appropriate cell need not necessarily imply the nearest base station; more sophisticated cellular net- works might also choose to use adjacent base stations if this will help to alleviate 304 MOBILE TELEPHONE NETWORKS New stronger signal radio path 7 New path / I switching Old cell l centre) Pat h re1 eased on hand - off Figure 15.9 ‘Hand-off’ during a call channel congestion. At the end of the call, the mobile station generates an on-hook or end of call signal which causes release of the radio channel, and reverts the handset back to monitoring the control and paging channel. Each mobile switching centre controls a number of radio base stations. If, during the course of a call, the mobile station moves from one cell to another (as is highly likely, because the cells are small), then the MSC is able to transfer the call to route via a different base station, appropriate to the new position. The process of changeover to the new cell occurs without disturbance to the call, and is known as hand-oflor handover. This is one of the most important capabilities of a mobile telephone network. Hand-off is initiated either by the active base station, or by the mobile station, depending upon the network and system type. The relative strengths of signals received at all the nearest- base-stations are compared with one another continuously, and when the current station signal strength falls below a pre-determined threshold, or is surpassed by the signal strength available via an adjacent base station, then hand-ofSis initiated. The mobile switching centre establishes a duplicate radio and telephone channel in the new cell, and once established, the call is transferred to the new radio path by a control message to the mobile handset. When confirmed on the new channel and base station, the original connection is cleared, as Figure 15.9 illustrates. 15.4 TRACING CELLULAR RADIO HANDSETS Whenever the mobile handset is switched on, and at regular intervals thereafter, it uses the control channel to register its presence to the nearest mobile switching centre. This EARLY CELLULAR RADIO NETWORKS 305 enables the local mobile switching centre at least to have some idea of the location of the mobile user. If outside the geographical area covered by the base stations controlled by its home MSC, the local MSC (i.e. the nearest to the mobile user) undertakes a registration procedure, in which it interrogates the home MSC (or the intelligent network database associated with it) for details of the mobile, including the authoriza- tion number and other information. The information is held by the home MSC in a database called the home location register, or HLR. It contains the mapping informa- tion necessary for completing calls to the mobile user from the PSTN (its network identity, authorization and billing information). The local MSC duplicates some of this information in a temporary visitor location register or VLR, until the caller leaves the MSC area. Once the visiting location register has been established in the local MSC, outgoing calls may be made by the mobile user. The registration procedure is a crucial part of the mechanism used for tracing the whereabouts of mobile users, so that incoming calls can be delivered. Incoming calls are first routed to the nearest mobile switching centre (MSC) to the point of origin (i.e. the caller). This MSC interrogates the home location register for the last known location of the mobile user (this is known as a result of the most recent mobile registration). The call can then be forwarded to the mobile switching centre where the mobile was ‘last heard’, whereupon a paging mechanism, using the base station control channels, can locate the exact cell in which the mobile is currently located. A suitable free radio channel may then be selected for completion of the call. Both the handsets and the network infrastructure needed to support cellular radio are complex and expensive, although the increase in user demand is reducing the cost. The mobile handsets come in a number of different forms, from the traditional car-mounted telephone, to the pocket versions. The latter are expensive not only because of the feats of electronics miniaturization that has been necessary but also because the trends towards pocket telephones has necessitated advanced battery technology and the use of sophisticated battery conservation methods (which, in effect, turn much of the unit off for as much of the time as possible, to save power). The mobile switching centres (MSCs) rely on advanced computers capable of storing and updating large volumes of customer information, and also of rapidly interrogating other MSCs for the location of out-of-area, or roaming mobiles. The interrogation relies on the use of the mobile application part (MAP) and the transaction capability (TCAP) user parts of SS7 signalling (which we discussed in Chapter 12). 15.5 EARLY CELLULAR RADIO NETWORKS A number of different cellular radio standards have evolved, with the result that hand portables purchased for use on one system are unsuitable for use on another. The most common of the systems currently in use worldwide are as follows. AMPS (Advanced Mobile Telephone System) This was first introduced in Chicago in 1977 by Illinois Bell, at that time one of the operating companies of AT&T in the United States. Developed by AT&T’s Bell 306 MOBILE TELEPHONE NETWORKS Laboratories, this became the most commonly used system in North America up to the early 1990s, when digital systems began to take over. It operates in the 800 MHz and 900 MHz radio bands, at a channel spacing of 30 kHz. NMT (Nordic Mobile Telephone Service) This commenced service in the Nordic countries in 1981 and is used in other countries in Europe (Austria, Belgium, Czechoslovakia, France, Hungary, Netherlands, Spain, Switzerland). The original system was 450 MHz based but has been gradually extended and to some extent replaced after 1986 with the later NMT-900 version. C (Network C) Network C was a development of Network B, a digitally controlled mobile radio system introduced in the Federal Republic of Germany in 1971. Network B was 450 MHz based. Its replacement, Network C can be either 450 MHz or 900 MHz based (hence C-450 and C-900); it is still in use in Germany and Portugal, but being replaced by GSM systems. TACS (Total Access Communication System) This is a derivative of the American AMPS standard, modified to operate in the 900 MHz band, with a more efficient 25 kHz radio channel spacing. TACs was the system introduced into the UK and Ireland during 1985. ETACS or extended TACS is a com- patible derivative of TACs which gives greater channel availability, particularly in very congested areas like the metropolitan London area. The system is also used in Austria, Italy and Spain. Other mobile telephone standards include NAMTS (Nippon Automatic Mobile Telephone System used in Japan), Radiocom 2000 (used in France), RTMS (second generation Mobile Telephone used in Italy), UNZTAX (used in China and Hong Kong) and Comvik (used in Sweden). Table 15.1 Comparison of analogue cellular radio network types AMPS C 450 NMT 450 NMT 900 TACS (USA) (Germany) (Scandinavia) (Scandinavia) (UK) Uplink band 824-849 MHz 450-455 MHz 453-458 MHz 890-915 MHz 890-915 MHz Downlink 869-894 MHz 461-466 MHz 463-468 MHz 935-960 MHz 935-960 MHz band Channel 30 kHz 20 kHz 25 kHz 25 kHz 25 kHz spacing Multiplexing FDMA FDMA FDMA FDMA FDMA Modulation PSK FSK FSK FSK PSK Number of 833 222 180 1000 1000 channels