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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)
426
MOBILE AND RADIO DATA NETWORKS
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
(PACE),
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
!takes message and pages
roaming individual
‘1

‘t
\
\
Direct link
fo!
,
‘bleep-only
\.
pagers
\
Public
telephone
switched
network Caller
Figure
24.2
Radio paging a ‘roaming individual’
428
MOBILE
AND RADIO DATA NETWORKS
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
CCIR
(Consultative Committee for International Radio,
the forerunner to
ITU-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
Caller
calls
number
hears bleep cadence
telephone
no.
Roaming individual
Recall telephone number
1
62
11
1
(corresponds
to
caller
1
. . . .
.
.
.
.
.
A
2
B
53224
D

I
04923

C
3
72372
.
-
.
- -
.

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
(PACE)
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
430
MOBILE
AND RADIO DATA NETWORKS
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
(PTMR)
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 hed
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
PA3-c
packet
‘terrestrial’ application
network
network
\
comp
4
-1
-
______-_
c-
-
asynchronous
or
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.
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 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)
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-standard format data in non-X.25-standard format
Teleservices
4.8
kbit/s speech
encrypted speech
432
MOBILE
AND RADIO DATA
NETWORKS
__
F
inter-system interface
switching and management
infrastructure
(SwMI)
line station
interface
I
line station

user
interface
termination station
ISDN
lerrnination interface
(to
data MTO or MT2
(to
data
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
(LS)
and a
mobile station
(MS)

via
a
base station
and
switching and management infrastructure
(SwMI).
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
(AI)
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 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
LAN)
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
Uplink: 380-390 MHz Downlink: 390-400 MHz
4
10-420 MHz 420-430 MHz
450-460 MHz 460-470 MHz
870-888 MHz 915-933 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
434
MOBILE
AND
RADIO
DATA NETWORKS
V
WIRELESS
LANS
435
OS1
reference
model
HIPERLAN
protocol layers
higher layers
data link layer (layer
2)
I
physical layer (layer
1)
I

higher layers
logical link control (LLC)
IEEE802.2
medium access control (MAC)
channel access control CAC)
I
radio medium
I
Figure
24.8
HIPERLAN
protocol reference
model
reference model. Note that the use
of
the standard
IEEE
802.2
(IS0
8802.2) logical link
control
(LLC)
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 (MAC)

sublayer, a
channel access
control
(CAC) sublayer is also used. This is necessary to accommodate the control
mechanisms necessary for the radio channel.
The
logical link control
(LLC)
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 (MAC)
sublayer provides for the delivery
of the information to the correct endpoint, providing for LAN addressing, data
encryption and relaying as necessary. The
channel access control
(CAC)
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
(NPMA).
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
a priority resolution phase
a contention resolution phase
8
a transmission phase
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 POSITIONING SYSTEM (GPS)
In July
1985
the
Federal Communications Commission (FCC)
of the
USA
authorized
the use of a new satellite radio service to be called the
radiodetermination satellite service
(RDSS).
It has widespread benefit in the 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,
GPS).
GPS
is a set of techniques combining radio and computer capabilities which is
capable of determining precise geographical locations of points on the ground. It is used
for applications such as tracking ships 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 by the velocity of light.
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.