Mạng và viễn thông P24

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Mạng và viễn thông P24

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Mobile and Radio Data Networks Just as computers and datacommunication are revolutionizing office life, so mobile and radio data networks are enabling corporate computer networks be extendedto every part of the comto pany’s business, including the mobile sales force, the haulage fleet and the travelling executive. Data network techniques can now also be usedto trace and pinpoint trucks on the roador ships at sea.

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  1. 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) 24 Mobile and Radio Data Networks Just as computers and datacommunication are revolutionizing office life, so mobile and radio data networks are enabling corporate computer networks be extendedto every part of the com- to pany’s business, including the mobile sales force, the haulage fleet and the travelling executive. Data network techniques can now also be usedto trace and pinpoint trucks on the roador ships at sea. This chapter discusses someof the most recent radio data network technologies, covering the principles of ‘radiopaging’, mobile data networking, ‘wireless LANS’, as well as describing radiodetermination services. 24.1 RADIOPAGING Radiopaging was the first major type of network that enables transfer of short data messages to mobile recipients. Initially it was a method of alerting an individual in a remote or unknown location (typically by ‘bleeping’him) to the fact that someone wishes to converse with him by phone. Subsequently, the possibility to send a short text message to the mobile recipient became commonplace. To be paged an individual needs to carry a special radioreceiver, called a radiopager. The unit is about thesize of a cigarette box, and designed to be worn on abelt or clip- is ped inside a pocket. The person carrying the pager may roam freely and can be paged provided they are within the radiopaging service area. The service may provide a full nationwide coverage. Figure 24.1 illustrates a typical radiopaging receiver. The initial radiopagers were allocated a normal telephone number as if they were standard telephones.Pagingwasachieved by diallingthisnumber, as if makinga normal telephone call. Instead of being connected through to the radiopager the caller either speaks to a radiopaging service operator, or hears a recorded message confirming that the radiopager has been paged. Paging is done by sending a radio signal to the radiopager, causing it to emit an audible ‘bleeping signal’ to alert its wearer. The simplest types of radiopager, even today, provide no further information to the wearer than thebleep. Having been alerted, the wearer must find a nearby telephone and 425
  2. 426 NETWORKS DATA MOBILE AND RADIO Figure 24.1 Messagedisplayradiopager. A relativelysophisticatedradiopager,allowingnot only bleeping facilities, but also the conveyance of a short textual message. (Courtesy of British Telecom)
  3. RADIOPAGING 421 ring a pre-arranged telephone number (say the radiopaging operator, or the wearer’s own secretary) to be given the message or the telephone numberof the caller whowishes to speak with him. Thus for a caller to page an individual they must first inform the intermediary office (the radiopaging operator or the ‘roaming individual’s’ secretary). The caller leaves either a message for the paged person, or a telephone number to be called. Figure 24.2 gives a general schematic view of a radiopaging network. The key elements of a radiopaging system are the paging access control equipment ( P A C E ) , thepaging transmitter andthe paging receiver. ThePACE,containsthe electronics necessary for the overall controlof the radiopaging network. It is the PACE which codes up the necessary signal to alert only the appropriate receiver. This signal is distributed to alltheradiotransmittersservingthewhole of thegeographicarea covered by theradio-pagingservice.On receiving itsindividualalertingsignalthe receiver bleeps. A special code is used between the PACE andall the paging receivers.It enables each receiver to be distinguished and alerted. The earliest codes used a discrete signal tone, modulated onto a radio frequency to identify each receiver. Such systems, available in the late 1950s, could address a small number receivers. Two-tone systems rapidly followed of in the 1960s, as the popularity radiopaging grew. In the two-tone system,to around of up 70 tones are used, any two of whichare sent in consecutive short bursts, allowing up to 70 X 70 = 4900 receivers to be alerted individually. Two-tone systems were used for on- site paging applications, such as summoning medical in a large hospital. staff Two-tone systems were too small in capacity to be considered for use over a wide area covering large citiesor a nation. Hence followed the development of systems using of more bursts of tone. A number proprietary five-tone systems were developed typically Transmitter aerial Pagingserviceoperator !takes message and pages roaming individual ‘ 1 Pager bleeps ‘t (individual \ recalls \ operator for message) Directlink fo! ‘bleep-only \ , \. Public pagers switched telephone Caller network Figure 24.2 Radio paging a ‘roaming individual’
  4. 428 NETWORKS DATA RADIO MOBILE AND using a repertoireof ten different tone frequencies,and allowing up to 25 bursts of tone per second. This amounts to a system capacity of 10 X 10 X 10 X 10 X 10 = 100 000 receivers, and a calling or paging rate of 25/5 calls per second. However, even five-tone systems were unable to cope with the explosion in demand thatmany of the radiopaging operators saw in the late 1970s, and new digital coding systems for paging became necessary. The digital codes were not as sensitive as their predecessors, but they had enhanced performance capabilities in terms of overall calling rate and capacity. Furthermore, they offered the scope for short alphanumeric messages to be paged to thereceiver and promised lower overall unit costs, both of the PACE and of the individual receivers. A number of digital codes were developed in the late 1970s and early 1980s, among them the Swedish MBS code (1978), the American GSC code (1973), and the Japanese NTT code (1978). The most important code, now common throughout the world, is that stimulated by theBritishPost Office. Known as the POCSAG code,afterthe advisory group that developed it (the Post Ofice code standardisation advisory group), it was developed the over period 1975-1981 and was accepted by the CCIR (ConsultativeCommittee for InternationalRadio, theforerunnerto I T U - R ) asthe first international radiopaging standard.It has a capacityof 2 million pagers (per zone) and a paging rate of up to 15 calls per second. Furthermore, it has the capability for transmitting short alphanumeric messages to the paging receiver. It works by trans- mittingaconstantdigitalbitpattern of 512 bitspersecond. The bit pattern is segregated into batches, with each batch sub-divided into eight frames. A particular pager willbe identified by a 21-bit radioidentitycode, transmitted within one (and always the same one)of the eight frames. It is this code, when recognized by the paging receiver, that results in the alerting bleep. An extra feature of the POCSAG code is that an extra two bit code can be used to provide four different bleeping cadences in each pager. These can be assigned with different telephone numbers for paging, and they correspond to four different recall telephone numbers. This may be useful for a user whofor most of the time is contacted by a small number of different people, because it potentially removes the need for the intermediary. Figure 24.3 shows how each caller uses a different telephone number to page the roaming individual, and produce a distinctive bleeping cadence. Caller Caller calls Roaming individual Recall telephone number no. telephone hears bleep cadence number 1 A ......... 6 2 1 1 1 (corresponds to caller 1 2 B .-. - - . 72372 3 C ---- 04923 I D ..-..- 53224 Figure 24.3 Different paging cadence identifies appropriate recall number
  5. MOBILE DATA NETWORKS 429 Text messages (consisting of alphanumeric characters) were initially conveyed by the radiopaging operator. Itis nowadays sometimes also possible input the to message using videotext or asimilar data networkservice. The most advanced receivers, when used in a suitably equipped radiopaging network, capableof messages up to80 characters long. are The pager itself is a small, cheap and reliable device. Most are battery-operated, but if the pager were to be on all of the time, the battery life would be very short, so a technique of battery conservation has become standard. We have already described how the radio identity code is always transmitted in the same frame of an eight frame batch to a particular receiver. This means that receivers need ‘look’ only for their own identity code in one particular frame, and can be ‘switched off for seven-eighths of the time. This prolongs battery life. Paging receivers include a small wire loop aerial, and because of the low battery power can only detect strong radio signals. This fact needs to be taken into account by the radiopaging system operator when establishing transmitter locations and deter- mining transmitted power requirements, and by the user when expecting important calls. The radio fade nearlargebuildings can beamajor contributorto the low probability of paging success. The paging access control equipment ( P A C E ) stores the database of information to determine which zones the customer has paid for, and to convert the telephone numbers dialled by callers into the codenecessary to alert the pagers, and in addition it performs the coding of textual alphanumeric messages. The PACE also has the ability to queue up calls if the incoming calling rate is greater than that possible for alerting receivers over the radio link. Furthermore, the PACE prepares records of customer usage, for later billing and overall network monitoring. The most advanced modern paging radiopaging systems are satellite paging systems. These work in exactly the same way as terrestrial radiopaging systems, except that the transmittedsignal is relayed via asatellite to achieveaglobalcoverage area.This enables the roaming individual to receive his messages wherever he is in the world. 24.2 MOBILE DATA NETWORKS Mobile radio is an awkward medium for carrying data. Interference, fading, screening by obstacles, and the hand-off procedure betweencells all conspire to increase errors, so althoughthedigital fixed telephonenetworkmayexpect to achieve errorrates no greater than 1 in 105bits, the error rate over mobile radio can be as high as 1 in 50 bits. Very basic systems with slow transmission speeds (say 300 bit/s) have been used. At theserates few dataare lost andconnectionsthatare lost can be re-established manually. However, for more ambitious applications error-correcting procedures must be used, normally a technique employing forward errorcorrection (FEC) and auto- matic re-requestretransmission. In this technique sufficient redundant information is sent for data errors tobe detected and the original data reconstructed even if individual bits are corrupted during transmission. Typical speeds achieved are 2.4-4.8 kbit/s. The appearance of mobile data networks was largely stimulated by the taxi industry. Press to speak private mobile radio systems first appeared in taxis as a means for controlling taxi fleet movements. A taxi customer calls a telephone number, where a
  6. 430 DATA MOBILE AND RADIO NETWORKS number of operators act to accept ordersand despatch available taxisto pick clients up. The despatching process occursby radio. After each ‘drop-off’ a taxi driver registers his position and receives instructions about where he can ‘pick-up’ his next client. By the mid-1980s the press to speak despatch systems had become unable to cope with the size of some of large metropolitan taxi despatch consortia. becoming the It was difficult to be able reliablyto contact all the drivers,and wearing on the drivers always to have to listen out for calls. Computer despatch systems were being introduced for the automation of taxi route planning, and the natural extension was direct computer readout to the individual drivers their planned activities. computer automation it of By became possible to ensure despatch of a client order to a particular taxi driver, who could be automatically prompted acknowledge its receiptand his acceptance of the to order. Simple confirmation by the driver ensures precise computer tracking pick-up of time and a successfully completed fare, Subsequent computer analysisof journey time statistics couldfurther help future journey planning. Now there was a need data networking viaradio. Most of the systems developed for to answer this need evolved from the previous press-to-speak trunk mobile radio private (PTMR) systems used in the taxi regional haulage business beforehand. a result and As they tendto use a similar radio frequency range operation, and a similar 12.5 kHz for or 25 kHz channel spacing. The derived user bitrates achievable are typically around data 7200 bit/s per connection, but once the overheads necessaryto ensure the reliableand bit error free transport of the user data are removed, the effective data rate of some systems doesnot exceed 2400 bit/s. Miserable, you might think, when compared e d to h network data applications runningat 64 kbit/s or even higher rates,but quite adequate for the short packet (i.e. around 2000 byte packet messages (approximately 2000 char- acters)) for which the systems were developed.. Thethreebestknownmanufacturers of speed low mobile data networksare Motorola (its Modacorn system), Ericsson (Eritel’s Mobitex system) and ARDIS. The systems find their main application in private network applications within metropolitan or regional operations (for haulage or taxi companies) or on campus sites, essentially providing radio-basedX.25 packet networks, as Figure24.4 shows. There have been a radio data network I application P A 3 - c ‘terrestrial’ packet network \ comp 4 asynchronousor proprietary mode - 1 c - - ______-_-------- standard X.25 - transmission Figure 24.4 Typical arrangement of a mobile data network
  7. (TRANS-EUROPEAN TETRA TRUNKED RADIO SYSTEM) 431 number of attempts at providing commercial nationwide and even international public service networks, but these have not been a great success. 24.3 TETRA (TRANS-EUROPEAN TRUNKED RADIO SYSTEM) Despite the relatively low interest in low speed mobile data networks, and the emerg- ence of the GSM and DECT systems (Chapters 15 and 16) as overpowering competitors both for voice service via trunk mobile radio and data carriage via Modacom-like low speed mobile data networks, there has been continued affort applied by ETSI to agree the TETRA (tuns-European trunkradio)series of standards. These are intended to provide for harmonization of trunk mobile radio networks across Europe, opening the way for pan-European services and the use of identical equipment. Work on the TETRA standards startedin ETSI in 1988, when a system to be called mobile digital frunk radiosysfem (MDTRS) was foreseen. This was renamed TETRA in 1991. A series of standards have now been published, which can be classified into two different broad system categories 0 TETRA V+D is a system for integrated voice and data 0 TETRA PDO is a system for packet data only The first of these systems is intended as an ISDN-like replacement for analogue trunk mobile radio systems (Chapter 15). The second system is a standardized version of the Modacorn-like systems, but with higher data throughput capabilities. Table 24.1 lists the bearer and teleservices planned to be made available. Table 24.1 Bearer and teleservices supported by the various TETRA standards TETRA V + D (voice and data) TETRAPDO (packet data only) Bearer Services 7.2-28.8 kbit/s circuit-switched voice or data (without error control) 4.8-19.2 kbit/s circuit-switched voice or data (some error control) 2.4-9.6 kbit/s circuit-switched voice or data (strong error control) connection-oriented (CONS) connection-oriented (CONS) point-to-point packetdata (X.25) point-to-point packet data (X.25) connectionless (CLNS) connectionless (CLNS) point-to-point packet data (X.25) point-to-point packet data (X.25) connectionless (CLNS) connectionless (CLNS) point-to-point or broadcast packet point-to-point or broadcast packet data in non-X.25-standardformat in data non-X.25-standard format Teleservices 4.8 kbit/s speech encrypted speech
  8. 432 DATA MOBILE AND RADIO NETWORKS __ Finter-systeminterface switching and management infrastructure (SwMI) line station interface I line station station user termination ISDN interface lerrnination interface MT2 (toMTO or data (to data terminal) terminal] MTO, mobile termination type0 provides a non-standard terminal interface MT2, mobile termination type provides a TETRA standard R,-interface 2 Figure 24.5 Basic architecture of the TETRA system Figure 24.5 illustratesthebasicarchitecture of the TETRA system.Theconcept foresees a normal connection between aline station ( L S ) and a mobile station ( M S ) via a base station and switchingand management infrastructure ( S w M I ) . Thus a typical example would be a taxi computer despatch centre as a line station connected to the fixed ISDN network, accessing oneor more(typically many) mobile stations. Similar to the DECT system, the data base is conceived to take over home data base and visitor data base functions, to allow roaming of mobile stations between different base stations and even between different TETRA networks. The inter-system interface (ZSZ) allows for interconnection of TETRA networks operated by separate entities. The various c-plane and u-plane air interfaces ( A I ) are designed to conform with OSI. Table 24.2 presents a brief technical overview of the TETRA system. 24.4 WIRELESS LANS The idea of wireless LANs ( WLANs) has been around for aslong as LANs themselves. Indeedthefirst LAN, developed by theXerox company based ontheALOHA- protocol, which became the basis of ethernet, was based on a radio medium. There are two main benefits wireless LANs when compared with cable-based LANs 0 ability tosupport mobile data terminals(forexample,employeesusing laptop computers at various different desk locations within a given office building) 0 ability to connect new devices without the need to lay more cabling Two standards for wireless LANs have been developed. These are the IEEE 802.1 1 standard and the ETSI HZPERLAN (high performance L A N ) standard. We describe here the ETSI HIPERLAN system.
  9. WIRELESS LANS 433 Table 24.2 Technical overview of the TETRA system Radio Bands Uplink: 380-390 MHz Downlink: 390-400 MHz MHz MHz 420-430 4 10-420 450-460 MHz 460-470 MHz 870-888 MHz 915-933 MHz Channel separation 25 kHz Channel multiplexing V + D: TDMA (time division multiple access), with S-ALOHA on the random access channel PDO: S-ALOHA with data sense multiple access (DSMA) Duplex modulation FDD (frequency division duplex), 10 MHz spacing Frame structure V + D: 14.17ms/slot, 510 bits per slot, 4 slots per frame PDO: 124 bit block length with forward error correction (FEC). Continuous downlink transmission, burst uplink ALOHA Modulation 7r/4 DQPSK (differential qauternary phase shift keying) Connection set-up time circuit switched connection, less than 300ms connection-oriented data, less than 2 S Propagation delay V + D: less than 500 ms for connection-oriented services 3-10 seconds for connectionless services PDO: less than 100ms for 128 byte packet In a wireless LAN each of the devices to be connected to the LAN is equipped with a radio transmitter and receiver suited to operate at one of the defined system radio channel frequencies. For the HZPERLAN system, five different channels are available, either in the band 5.15-5.30GHz or in the band 17.1-17.3GHz, but only one of the channels is used in a single LAN at atime. The radio channel has a total bitrate closeto 24Mbit/s but the maximum user data throughput rate is around 10-20Mbit/s, i.e. of similar capacity to a cable-based ethernet or token ring LAN. When a device wishes to send information, this is transmitted in a mannersimilar to that used in an ethernet LAN. In other words, the information is simply transmitted to all other terminals in the LAN, as soon as the radio channel is available. All devices participating in the LAN‘listen’ to the radio channel at all times, but only ‘pick up’and decode data relevant to themselves. The structure of the LAN is therefore very simple, as Figure 24.6 illustrates, but all devices must lie within about 50 metres of one another, because of the 1 Watt maximum radio transmit power allowed. The multiple radio frequencies (five per band) defined in the HIPERLAN standard allow multiple LANs to exist beside one another and even overlapping one another. Without multiple frequencies different LANs in adjacent offices might not be possible, and multiple LANs in the same office certainly not. The 50 metre maximum diameter of the LAN could also be a major constraint in some circumstances. For this reason, the radio MAC (mediumaccess control) provides a forwarding (or relay) function. When the forwarding function is configured into the
  10. 434 MOBILE AND RADIO DATA NETWORKS V
  11. WIRELESS LANS 435 OS1 reference model HIPERLAN protocol layers higher layers higher layers 2) data link layer (layer logical link control (LLC)IEEE802.2 medium access control (MAC) channel access controlC A C ) Iphysical (layer layer 1) I medium I radio I Figure 24.8 HIPERLAN protocolreference model reference model. Note thatthe use of the standard IEEE 802.2 ( I S 0 8802.2) logical link control ( L L C ) enables HIPERLAN to be used as a one-for-one replacement of an existing LAN. Unlike a normal LAN, however, two further sublayers are used beneath the LLC layer. In addition to a medium access control ( M A C ) sublayer, a channel access control ( C A C ) sublayer is also used. This is necessary to accommodate the control mechanisms necessary for the radio channel. The logical link control ( L L C ) sublayer of OS1 layer 2 provides for correct and secure delivery of information between two terminals connected to the LAN (error detection, correction, etc.). The medium access control ( M A C ) sublayer provides for the delivery of theinformationtothecorrectendpoint,providingfor LAN addressing, data encryption and relaying as necessary. The channel access control ( C A C ) sublayer codes the MAC information into a format suitable for transmission across a radio medium. The technique used is called non-pre-emptive priority multiple access ( N P M A ) . NPMA breaks up the radio channel into a number of channel access cycles, each of which is further sub-divided into three cycle sub-phases 8 apriorityresolutionphase acontentionresolutionphase 8 atransmissionphase In the priority phase, any stations which do not currently claim the highest priority transmission status, are refused permission to transmit. The remaining stations compete for use of the radio channel during the next phase and any contention is resolved. The remaining transmission phase is then allocated to successful stations surviving both the priority resolution and contention resolution phases. This is the phase when user data are transmitted. Priorities will change from one cycle to the next to ensure that all stations have an equal ability to send data. So much for the strengths of wireless LANs. The greatest difficulty is in achieving complete radio signal coverage throughout an office. Multipath effects, interference and propagation difficulties can lead to blackspots suffering very deep radio-fade (i.e. poor transmission). For static devices, the problem of a fade caused by multipath of inter- ference can be solved by moving the device only a small distance. For mobile terminals continuous good quality transmission may not be possible.
  12. 436 MOBILE AND RADIO DATA NETWORKS 24.5 RADIODETERMINATION SATELLITE SERVICES (RDSS) AND THE GLOBAL POSITIONINGSYSTEM(GPS) In July 1985 the Federal Communications Commission ( F C C ) of the USA authorized the use of a new satellite radio service to be called theradiodetermination satellite service (RDSS). It has widespread benefitthe in field of navigation and in personal communication technology. The technical and operational standards adopted by the FCC became the basis for worldwide standards agreement under the auspices of ITU (the global positioning system, G P S ) . GPS is asetoftechniquescombiningradio andcomputer capabilitieswhich is capable of determining precise geographical locations of points the ground. is used on It forapplicationssuchastrackingships at sea or truck fleets on land. Its potential includes the scope for keeping track of individuals in support of global cellular radio services. Indeed some claim that GPS offers a more efficient means of tracking cellular handsets than the current method of continual updating. The GPS system consists of a set of geostationary satellites, a control centre and a number of user terminals, called transceivers. The transceivers are typically quite small nowadays, even available in ‘handheld‘ form, as many yachtsmen will be familiar with. The control centre repeatedly sends an interrogation signal via the satellites to all the user terminals. Signals from the satellites are interpreted by each terminal and, if relevant, a response message is generated to 0 alert the control centre of current position (if requested) 0 reply to or request some other information from the control centre The relative position of the user terminal from one satellite is computed by the control centre from the round-trip alert-and-response signal time scaled the velocity of light. by The relative range from three different geostationary satellites enables the control centre to compute exactly the position of the terminal in three-dimensional coordinates of latitude, longitude and altitude. This information is then associated with the terminal identity (ID) code and can either be passed to a third party or transmitted back to the user terminal. Thus a fleet operator could trace a lorry on the road or a ship owner could determine a ship’s position at sea. Shipping companies will be familiar with the Navstar system.
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