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Cordless Telephony and Radio in
the Local Loop (RILL)
The rapid deregulation of telephone network services taking place during the
1990s
has brought a
large number of new public network operators to the market, each of which has an interest in
optimizing the cost of customer connection to his network. Much interest, in particular, has been
channelled into radio technologies (so-called ‘radio-in-the-local loop’ or ‘wireless local loop’,
WLL),
as these are seen as a quick and economic way to create new access infrastructure,
bypassing the dependence on the established monopoly operators for ‘last-mile’ connections. In
this chapter we discuss some of the most important technologies in this sector. We also discuss
cordless telephone technology as a means for providing ‘limited mobility’ access
to
fixed
networks.
16.1
THE DRIVE
FOR
RADIO
IN
THE
LOCAL LOOP
It was historically the case that a monopoly existed on both the public telephone service
and the construction and operation of telecommunications transmission networks. The
state-owned monopoly carrier had the sole right to
lay
cables in the street or construct
radio transmission links. Although competition
in public telephone network services
may have been introduced in many countries, there has not necessarily been a

relaxation of the transmission network monopoly. In consequence, the new telephone
carriers (network operators) may be dependent on their strongest competitors for
the supply of all transmission links. Thankfully for the new operators, if a little slowly,
the national transmission monopolies are also being removed. Unfortunately, however,
this does not immediately remove the dependence of the new operators on the ex-
monopoly carrier, because the large base of established lineplant and investment is
difficult for the new carriers to duplicate quickly. The best hope for them lies in the
rapid construction
of
an overlay, radio-based infrastructure.
319
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)
320
CORDLESS TELEPHONY AND
RADIO
IN
THE LOCAL LOOP (RILL)
16.2
FIXED NETWORKS BASED
ON
RADIO TECHNOLOGY
Figure 16.1 illustrates the typical configuration of a new telephone network based
'
largely
on
radio transmission links.
Traditional

point-to-point
(PTP)
microwave radio technology is ideally suited for the
long point-to-point links between switching centres (i.e. between local exchanges and
regionalswitching centres and for the trunks between regional switching centres). There
may be some regulatory and administrative matters to be resolved with respect to
licensing of the required radio frequencies, but as the frequencies are required
on
a strict
point-to-point basis and not over wider areas, this should be achievable both from a
regulatory and planning point of view, because the number of links
of
this type is
relatively small.
By contrast, there has been relatively little attention paid to radio connection of end
customers by the monopoly network players,
so
that new effort needs to be applied to
develop economic technology and to finding suitable radio frequency bands for this new
application. Here, the nature of the connection is
point-to-multipoint
(PMP),
requiring
potential cdnnection of many thousands of endstations with dynamic allocation of
radio bandwidth. The radio path length need only extend to about
5
km and the
endpoints are fixed,
so
this removes the need for much of the complexity of the GSM

system (e.g. for
roaming
and
hand-of)
but the radio design is complicated by the
physical properties of available radio bands (most having relatively short range and
maybe requiring
line ofsight
(LOS)).
Further problems are posed by the difficult radio
operating conditions of urban environments
(radio shadows, multipath, interference).
For heavily used lines with bitrates above
2
Mbit/s,
point-to-point
(PTP)
microwave
remains the predominant method because of the strong signal strength needed to
support high bitrates reliably over appreciable distances. This is best achieved with
highly directional antennas, focussing the radio signal along a single path. Frequency
regional
switching
centre
local
2
or
34 Mbitls
repeater
local

loop
up
to
5
km
station
64kbit/s-2 Mbitls
regional
,
I
\
switching
,
centre
,
station cordless
Figure
16.1
New telephone network structure based on radio technology
FIXED NETWORKS BASED ON RADIO TECHNOLOGY
321
bands at 18 GHz, 23 GHz and
‘38
GHz are now allocated for so-called
shorthaul
microwave
radio systems. The range of systems drops dramatically with higher
frequency, so that while 15 km range is realistic within much of Europe for 18 GHz
systems, 5-7 km is the reckoned range at 38 GHz.
There are many unallocated radio frequency ranges above

40
GHz, but the relatively
short range of radio signals at these frequencies and the need for unimpeded
line
of
sight
between the antennae (because the radio waves, unlike at lower frequencies, are less
capable of even slight
dzfrraction
around corners and past obstacles). Much attention is
thus focussed on the radio range between
400
MHz and about 40 GHz. There have been
three distinct technological approaches, but the different approaches are likely to
converge. The three approaches are
0
cordless telephony
0
wireless ISDN
0
shorthaul
point-to-point
(PTP)
and
point-to-ntultipoint
(PMP) microwave radio
We discuss each in turn.
16.3
CORDLESS
TELEPHONES

Cordless telephony is the term used to describe telephone sets connected to the ordin-
ary
(jixed)
telephone network, but in which the handset communicates with the
network by a radio transmission link instead
of
wires. The cradle part of a cordless
telephone terminal acts as a radio transceiver or
base
station
to connect a radio path to
the handset, which also acts as a radio transceiver. The base station is connected to the
public telephone network in the normal way. Figure 16.2 illustrates a typical cordless
telephone configuration. The maximum range of these systems is typically 50 metres.
Cordless telephones were popular for some time in North America and Japan before
they took off in Europe. The problem was that the European (CEPT) design specifica-
tions were more complex, making the products comparatively expensive. The exception
was West Germany, where cordless phones were rented out by the Bundespost at little
more than the rental cost of ordinary telephones.
Cordless telephones are very simple in comparison with cellular radio telephones,
comprising
a
(duplex) two-way conversational radio channel, with a relatively simple
signalling system. A major hurdle in the design of cordless telephones is ensuring that
telephones in adjacent customers’ premises do not interfere with one another and
cannot be maliciously overheard.
A
customer is not prepared to pay for the next-door-
neighbour’s calls, made on the wrong base station. This may happen if a handset
interferes

with the base station next door, and was the main reason for the very strict
CEPT specifications.
The advantage of cordless telephones is the freedom to carry them about the house,
down the garden, around the workshop,
so
saving users from being away from the
phone and not hearing the phone ring. Simple cordless telephones can be used only
within range of their own base station. They are thus useless away from
home,
but make
the customer more mobile about his own premises.
322
CORDLESS TELEPHONY AND RADIO
IN
THE LOCAL LOOP (RILL)
k
1::
:::
W
U’
I
\\M
Telephone ‘cradle’
I
\
(
base
statlon
1
Mobile

handset
I
up
to
50
metres
Figure
16.2
A
cordless
telephone
From basic cordless telephony (radio path within the end customer’s premises) have
evolved second and third generation technologies in which the base station is shifted to
the public network operator’s site. First came
telepoint
or
CT2 (2nd generation cordless
telephony). DECT (digital European cordless telephony)
followed.
16.4
TELEPOINT OR CORDLESS TELEPHONE
2
(CT2)
An extension of the idea of cordless telephones is the concept of
telepoint,
French
pointel
or
wide area cordless telephone.
In

telepoint
a new type of digital cordless
telephone is used with a number of base stations. Besides the base station in his house,
the customer has access to public
telepoint
base stations situated in well populated
locations, such as airports, stations, and street corners (much as public payphones are
located today). A
common air interface
(CAZ)
ensures compatibility of mobile handsets
from various manufacturers with the
base
stations of the public network. Standing
within 50-200m of a telepoint, a caller with a telepoint handset is able to make
outgoing calls into the public switched telephone network in a similar way to a cellular
radio customer making an outgoing call, except that he may not move from one base
station to another during the call. Incoming calls, however, are not possible other than
at the home base station (i.e. the subscriber’s home). Telepoint hardware includes a
mobile handset and a number of base stations, each connected directly to the public
switched telephone network, as Figure
16.3
illustrates.
To
make a call, the handset sends a signal, including a special handset identity code,
over a control channel to the base station, which confirms the identity and authorization
of the user, and then allocates a radio channel in a way similar to that used in cellular
radio. Onward connection of the call is made directly via the
PSTN,
applying dial tone,

collecting dialled digits, etc., while the base station records call details for later billing of
the customer. (There is one exception to this, and that is when the customer has installed
a private base station in his own premises. In this case the customer pays for public
network calls in the normal way as recorded by the
PSTN
operator.)
DECT (DIGITAL EUROPEAN CORDLESS TELEPHONY)
323
Other base
stations
Handset
‘fixed’network)
Base station
Coverage area (outgoing calls only)
Figure
16.3 Telepoint
service
As we have seen, telepoint or second generation cordless telephone (CT2) as it is also
known is capable only of making outgoing calls. The technological problems of
tracking the mobile handset location were not solved by CT2,
so
that incoming calls to
users in roaming locations were not possible. There was talk of building radiopaging
receivers into CT2 handsets,
so
that the users could be paged with a displayed telephone
number to dial when next he was near a telepoint base station, but this would have
made the cost of the handsets and their ongoing operation more than that of cellular
telephone handsets.
A

further problem was that CT2 did not provide for a hand-oflprocedure for moving
between base station zones,
so users were forced to stay in range
of
a single base station
for the duration of each call. Thus a car needed to be parked in a telepoint car park, it
could not be on the move.
Despite attempts at commercial service of CT2 in several countries, the CT2 standard
failed, but the basic ideas and the technology survived in a third generation version,
DECT (digital European cordless telephony).
16.5
DECT (DIGITAL EUROPEAN CORDLESS TELEPHONY)
The DECT standards, developed by ETSI, have grown to be a sophisticated set,
starting to rival
GSM
in terms
of
the degree of complexity. The initiative for their
development grew from the desire to develop a common air interface (CM) for digital
wide area cordless telephones. Along the way, a number of other features have been
built in
e
security measures against unauthorized use
of
the handset and overhearing of
conversation
324
CORDLESS TELEPHONY AND RADIO IN
THE
LOCAL LOOP (RILL)

0
mobile station tracking
so
that incoming calls can be forwarded to the DECT
telephone user, no matter where he is
0
full handover of mobile stations from one cell to the next
0
64 kbit/s traffic carrying capability, to provide for correct functioning of ISDN data
terminals over DECT
0
OS1 compatibility of the DECT protocols
Figure 16.4 illustrates the reference model of the DECT system.
The most important part
of
DECT is the radio common air interface, D3. This allows
for the connection ofportable radio terminals
(PT)
(the portablepart,
PP,
of the system)
tofixed
radio terminations
(FT)
(being the.fi.xedpart,
FP,
of the system using a cordless
(i.e. radiolink) connection. This interface can be used on its own in a similar manner to
the
CM

(common air interface) of CT2. Thus straightforward cordless telephones for
home or office use are already being marketed for use by a single customer in his own
premises. Meanwhile, for those with a public DECT network service subscription, use
of
the handsets in the wide area may also be possible.
The advantage of the DECT interface over predecessing cordless telephone
technologies is the high speech quality afforded by
a
digital radio connection and the
extra security measures added to guard against overhearing and unauthorized use, be it
radio radio
1
ray0
i
raiio
terminal terminal
D4
portable
portable
application
application
I
ra;io
I
1
ra;io
I
terminal terminal
I
portable

1
I
portable
1
application application
DECT
network
fixed fixed
radio radio
‘F(
termination termination
Figure
16.4
DECT
reference model
DECT
HANDOVER
325
malicious or unintended. The extra security is afforded by means of data
encryption
and by
smart card
(so-called
DECT authorization module, DAM)
user identification in
a
manner similar to that employed by the GSM system (Chapter
15).
16.6
DECT HANDOVER

The interfaces D1 and D2 and the functions
HDB (home data base)
and
VDB (visitor
data base)
are additional to those available in CT2. These support the ability to receive
incoming calls in a wide area DECT network, also support
roaming
between cells. Each
fixed radio termination
(FT)
controls a cell within a DECT radio network. Roaming
between cells is controlled by a local network function comprising
home data
bases (HDB)
and
visitor data bases (VDB),
which perform similar functions to the
home
location register (HLR)
and
visitor location register (VLR)
of the GSM system
(Chapter
15).
Unlike GSM, however, the
handover
in DECT is by means of
mobile
controlled handover (MCHO),

in which the mobile station alone decides when to
handover and controls the process. This is claimed to lead to faster and more reliable
handover. This method compares with the
mobile assisted handover (MAHO)
of GSM
in which the
mobile switching centre
and
base stations
control the handover based on
information provided by the mobile. The decision to initiate handover in the DECT
system
is
based upon the mobile unit’s measurement of the
RSSI
(received signal
strength indicator),
CjI
(carrier to interference)
and
BER
(bit error rates)
of alternative
signals.
16.7 THE RADIO RELAY STATION CONCEPT IN DECT
As
the range of a single hop within the DECT system is relatively limited (typically
200
metres, although under ideal conditions with specific technical configurations several
kilometres have been achieved), there has been a need to find a means of extending the

range. The radio relay station concept allows for relaying of connections (i.e. conca-
tenation of several radio links) to allow the
portable radio termination
(PT) to stray
FRS
=
fixed relay station
MRS
=
mobile relay station
PT =
portable terminal
RFP
=
radio fixed part
Figure
16.5
DECT fixed and mobile relay stations
326
CORDLESS TELEPHONY AND
RADIO
IN
THE LOCAL LOOP (RILL)
somewhat further away from the
jixed radio termination
(or
radio jixed part,
RFP).
Relay stations may be either
jixed relay stations,

FRS,
or
mobile relay stations,
MRS,
as Figure
16.5
illustrates. Up to three relay stations may be traversed, but the topology
.
must be a star centred on the
W.
The drawback of DECT
relaying
is that multiple radio channels are used to connect a
single connection or call, making it impracticable for high trafEc volume networks. In
addition, the connection quality is likely to be degraded.
16.8
THE
DECT AIR INTERFACE (D3-INTERFACE)
The DECT air interface is designed to be OSI-compliant
(see
Chapter
9).
It therefore
comprises layered protocols for
physical layer, medium access control
and
data link
control
for both the
control-plane (c-plane)

and
user .plane (U-plane)
as Figure
16.6
illustrates.
The
c-plane
protocol stack, as we discussed in Chapter
7,
is used to .set up and
controlling connections (like telephone signalling). The
u-plane
protocol stack is that
used during the
conversation phase
of
a call or connection, to convey the user’s speech
or data. The lower layer management entity is the set of network management
functions provided to monitor and reconfigure the protocols as necessary for network
operation.
The characteristics of the
physical layer
of the radio interface are listed in Table
16.1.
The multiple access scheme is based on TDMA, as illustrated in Figure
16.7.
A single slot may comprise either a
basic physical packet
P32
(a full slot), a

short
physical packet
PO0
(for a short signalling burst) or two half slots
(low capacity physical
packet
P08).
C-plane U-plane
Figure
16.6
DECT
protocol reference model
THE DECT AIR INTERFACE (D3-INTERFACE)
327
Table
16.1
DECT air interface, physical layer
Radio band
Number of radio channels
Radio channel separation
Transmitter power (max)
Channel multiplexing
Duplex modulation
TDMA frame duration
Timeslots per TDMA frame
Modulation
Total bit rate
User channels
1880-1900MHz
10

1.728 MHz
250 mW
TDMA (time division multiple access)
TDD (time division duplexing)
10 ms
24
GFSK (Gaussian frequency shift keying)
1
152 kbit/s per cell
B
channel: 32 kbit/s (user)
A channel:
6.4
kbit/s (signalling)
~~
10
ms, 24
slots,
11520 bits
-4
l
-
Slot
-
basic
phvsical
2
D-field
S-field
packet

P32
so
s31:
a0
d63 d64 d383 d387
I
D-field
X
B-field A-field
___
bo

__
b319
___
___
A-field
R-CRC
A-field info
Q2
BA
Q1
TA
a0 a3 a4 a7 a8 a47 a48 a63
Figure
16.7
TDMA frame structure in DECT
The basic physical packet
P32
is

of
424 bytes length, subdivided into the
S-Jeld
(for
synchronisation) and the
D-field
(for carriage of data). The D-field is further subdivided
into
A-
and
B-fields,
whereby the
A-Jeld
is
a
permanent signalling channel (for
c-plane
protocol) and the
B-field
is the user data information filed
(u-plane
protocol). The
various parameters within the
A-field
have the functions listed in Table
16.2.
328
CORDLESS TELEPHONY AND RADIO IN
THE
LOCAL LOOP (RILL)

Table 16.2
DECT signalling parameters (A-field)
Parameter Purpose
TA A-field information
type
Q1,
Q2
BA
Quality
control
bits used as handover criteria
B-field information type
A-field information The field used for
carriage
of
MAC
and higher layer c-plane
protocol information
R-CRC Cyclic redundancy check
(for error detection)
Full 64 kbit/s ISDN
bearer channels
(i.e. user information channels for ISDN 64 kbit/s
data) may be transmitted over DECT networks by the occupation of two B channels.
16.9 OTHER ISDN WIRELESS LOCAL LOOP SYSTEMS
Partly due to the scepticism about the suitability of DECT as a means for large scale
mass market connection
of
fixed network customers to a telephone network, and partly
due to the fact that DECT is presently (1997) only

a
European standard, other systems
have also been developed for ISDN wireless local loop, aiming to provide for telephone
and full
64
kbit/s connection service. These systems use a variety of different and as yet
unstandardized techniques. Example technologies include those of
Ionica
(a British
company aiming to offer a full scale telephone network across the
UK
-
based on
technology called
Proximity
i
developed in conjunction with
Northern Telecom,
NORTEL), Airspan
(a system developed by
DSc
in cooperation with British Telecom)
and
Airloop
(a
Lucent Technologies
equipment developed by
Bell Laboratories
for use in
the deregulating

US
and Dutch markets). Which
of
these systems or DECT survives in
the long term remains to be seen. Critical will be the cost per user, as well as the
technical system performance.
16.10 SHORTHAUL POINT-TO-MULTIPOINT (PMP)
MICROWAVE RADIO
Meanwhile, the manufacturers
of
traditional point-to-point microwave radio systems
have not been idle. Several manufacturers have started developments of
point-to-
multipoint
(PMP)
systems of shorthaul microwave radio for use in the microwave band
above 1OGHz. These developments foresee dynamic allocation of radio bandwidth
within
a
cell, allowing a fixed or maybe even mobile end user to request various
different bitrates (64 kbit/s or multiples thereof) on an on-demand (i.e. call-by-call)
basis. These systems may not tap the initial market for ISDN radio in the local loop,
but in a later generation they may be the obvious choice for broadband services,
including radio in the local loop for
A
TM (asynchronous transfer mode).