Tài liệu Mạng và viễn thông P24 pptx

This chapter discusses some of the most recent radio data network technologies, covering the principles of ‘radiopaging’, mobile data networking, ‘wireless LANS’, as well as describing r

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 to be extended to every part of the com- pany’s business, including the mobile sales force, the haulage fleet and the travelling executive

Data network techniques can now also be used to trace and pinpoint trucks on the road or ships

at sea This chapter discusses some of 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 radio receiver, called a radiopager

The unit is about the size of a cigarette box, and is designed to be worn on a belt or clip-

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 Paging was achieved by dialling this number, as if making a 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 the bleep Having been alerted, the wearer must find a nearby telephone and

425

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)

Figure 24.1 Message display radiopager A relatively sophisticated radiopager, allowing not only bleeping facilities, but also the conveyance of a short textual message (Courtesy of British Telecom)

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 number of the caller who wishes

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 ) , the paging transmitter and the paging receiver The PACE, contains the

electronics necessary for the overall control of the radiopaging network It is the PACE which codes up the necessary signal to alert only the appropriate receiver This signal is distributed to all the radio transmitters serving the whole of the geographic area

covered by the radio-paging service On receiving its individual alerting signal the

receiver bleeps

A special code is used between the PACE and all 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 of receivers Two-tone systems rapidly followed

in the 1960s, as the popularity of radiopaging grew In the two-tone system, up to around

70 tones are used, any two of which are 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 staff in a large hospital

Two-tone systems were too small in capacity to be considered for use over a wide area covering large cities or a nation Hence followed the development of systems using more bursts of tone A number of proprietary five-tone systems were developed typically

Transmitter aerial

Pager bleeps

(individual

operator for recalls

message)

Paging service operator

! t a k e s message and pages roaming individual ‘ 1

‘t

\

\

Direct link fo! ,

‘bleep-only \

pagers

\

P u b l i c telephone switched network Caller

Figure 24.2 Radio paging a ‘roaming individual’

using a repertoire of 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 that many 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 the receiver 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 the British Post Office Known as the POCSAG code, after the

advisory group that developed it (the Post Ofice code standardisation advisory group),

it was developed over the period 1975-1981 and was accepted by the C C I R (Consultative Committee f o r International Radio, the forerunner to I T U - R ) as the first international radiopaging standard It has a capacity of 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- mitting a constant digital bit pattern of 512 bits per second The bit pattern is

segregated into batches, with each batch sub-divided into eight frames A particular

pager will be identified by a 21-bit radio identity code, 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 who for 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

C a l l e r

c a l l s number hears bleep cadence telephone

no

Roaming individual Recall telephone number

1 A 6 2 1 1 1 (corresponds to caller 1

5 3 2 2 4

D

I

0 4 9 2 3

- - - -

C

3

7 2 3 7 2

- - - - -

Figure 24.3 Different paging cadence identifies appropriate recall number

MOBILE DATA NETWORKS 429

Text messages (consisting of alphanumeric characters) were initially conveyed by the radiopaging operator It is nowadays sometimes also possible to input the message using videotext or a similar data network service The most advanced receivers, when used in a suitably equipped radiopaging network, are capable of messages up to 80 characters long 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 near large buildings can be a major contributor to 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 code necessary 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 transmitted signal is relayed via a satellite to achieve a global coverage 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 between cells all conspire to increase errors, so although the digital fixed telephone network may expect to achieve error rates no greater than 1 in 105 bits, 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 these rates few data are lost and connections that are lost can be re-established manually However, for more ambitious applications error-correcting procedures must

be used, normally a technique employing forward error correction (FEC) and auto-

matic re-request retransmission In this technique sufficient redundant information is sent for data errors to be 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

number of operators act to accept orders and despatch available taxis to pick clients up The despatching process occurs by 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 the large metropolitan taxi despatch consortia It was becoming difficult to be able reliably to 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 of their planned activities By computer automation it became possible to ensure despatch of a client order to a particular taxi driver, who could be automatically prompted to acknowledge its receipt and his acceptance of the

order Simple confirmation by the driver ensures precise computer tracking of pick-up time and a successfully completed fare, Subsequent computer analysis of journey time statistics could further help future journey planning

Now there was a need for data networking via radio Most of the systems developed

to answer this need evolved from the previous press-to-speak private trunk mobile radio

( P T M R ) systems used in the taxi and regional haulage business beforehand As a result they tend to use a similar radio frequency range for operation, and a similar 12.5 kHz or

25 kHz channel spacing The derived user data bitrates achievable are typically around

7200 bit/s per connection, but once the overheads necessary to ensure the reliable and bit error free transport of the user data are removed, the effective data rate of some systems does not exceed 2400 bit/s Miserable, you might think, when compared to h e d

network data applications running at 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

The three best known manufacturers of low speed mobile data networks are

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-based X.25 packet networks, as Figure 24.4 shows There have been a

radio data I P A 3 - c packet

‘terrestrial’ application

4 asynchronous or - 1 c - - _ _ _ _ _ _ - _ - - - standard X.25 - proprietary mode

transmission

Figure 24.4 Typical arrangement of a mobile data network

TETRA (TRANS-EUROPEAN 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

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 trunk radio) 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 started in ETSI in 1988, when a system to be called mobile digital frunk radio sysfem (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) TETRA PDO (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 packet data (X.25) point-to-point packet data (X.25)

point-to-point packet data (X.25) point-to-point packet data (X.25)

point-to-point o r broadcast packet point-to-point or broadcast packet data in non-X.25-standard format data in non-X.25-standard format

Teleservices 4.8 kbit/s speech

encrypted speech

_ _ F inter-system interface

switching and management infrastructure (SwMI)

line station interface

I

line station

user

interface termination station ISDN lerrnination interface

terminal) terminal]

MTO, mobile termination type 0 provides a non-standard terminal interface

MT2, mobile termination type 2 provides a TETRA standard R,-interface

Figure 24.5 Basic architecture of the TETRA system

Figure 24.5 illustrates the basic architecture of the TETRA system The concept

foresees a normal connection between a line station ( L S ) and a mobile station ( M S ) via

a base station and switching and 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 one or 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

The idea of wireless LANs ( WLANs) has been around for as long as LANs themselves

Indeed the first LAN, developed by the Xerox company based on the ALOHA-

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 to support mobile data terminals (for example, employees using 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

WIRELESS LANS 433

Table 24.2 Technical overview of the TETRA system

Radio Bands

Channel separation

Channel multiplexing

Duplex modulation

Frame structure

Modulation

Connection set-up time

Propagation delay

4 10-420 MHz 420-430 MHz

25 kHz

V + D: TDMA (time division multiple access), with S-ALOHA PDO: S-ALOHA with data sense multiple access (DSMA) FDD (frequency division duplex), 10 MHz spacing

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

on the random access channel

7r/4 DQPSK (differential qauternary phase shift keying) circuit switched connection, less than 300ms

connection-oriented data, less than 2 S

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 a time The radio channel has a total bitrate close to 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 manner similar 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 (medium access control) provides

a forwarding (or relay) function When the forwarding function is configured into the

V