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- 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
- 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)
- 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’
- 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
- 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
- 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
- (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
- 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.
- 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
- 434 MOBILE AND RADIO DATA NETWORKS
V
- 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.
- 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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