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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)
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 network. Much interest, in particular, has been
his
channelled into radio technologies(so-called‘radio-in-the-localloop’ or ‘wireless localloop’,
WLL), as these are seen as aquickandeconomic way to create newaccess 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
cordlesstelephone technology a
asmeans providing
for access to fixed
‘limited mobility’
networks.
16.1 THE DRIVE FOR RADIO IN THE LOCAL LOOP
It was historically the casethat 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 manyin countries,
there not
has necessarily been a
relaxation of the transmission network monopoly. In consequence, the new telephone
carriers(networkoperators)maybedependentontheirstrongestcompetitorsfor
the supply of all transmission links. Thankfully for thenew operators, if a little slowly,
the national transmission monopolies are alsobeing removed. Unfortunately, however,
thisdoes not immediatelyremovethedependence 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
- 320 CORDLESS
TELEPHONY
AND RADIO IN THE (RILL)
LOCAL
LOOP
16.2 FIXED NETWORKS BASED ON RADIO TECHNOLOGY
Figure 16.1 illustratesthetypicalconfiguration of a new telephonenetworkbased '
largely on radio transmission links.
Traditional point-to-point ( P T P ) microwave radio technology ideally suitedfor the
is
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 resolvedwithrespect to
on a
licensing of the required radio frequencies, but as the frequencies are requiredstrict
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 end
of
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 ( P M P ) , 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 h a n d - o f ) but the radio designis complicated by the
physical properties of available radio bands (most having relatively short range and
maybe requiring line ofsight (LOS)).Further problems are posed the difficult radio
by
operating conditions of urban environments (radio shadows, multipath, interference).
For heavily used lines with bitrates above 2 Mbit/s, point-to-point ( P T P ) microwave
remainsthepredominantmethodbecause of thestrong signalstrengthneededto
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
cordless
station
Figure 16.1 New telephone network structure based on radio technology
- FIXED NETWORKS
BASED TECHNOLOGY
ON RADIO 321
bands at 18 GHz, 23 GHz and ‘38GHz are nowallocated 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 manyunallocated radio frequency ranges above 40 GHz, but therelatively
short range of radio signals at these frequencies and theneed for unimpeded line o sight
f
between the antennae (because the radio waves, unlike at lower frequencies, are less
capable of even slight dzfrraction around corners and pastobstacles). Much attention is
thus focussed on the radiorange between 400 MHz and about40 GHz. There have been
threedistincttechnologicalapproaches, butthe different approachesare likely to
converge. The three approaches are
0 cordless
telephony
0 wireless ISDN
0 shorthaul point-to-point ( P T P ) and point-to-ntultipoint ( P M P ) 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 ( j i x e d ) telephonenetwork, but in which thehandsetcommunicates 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
telephonesinadjacentcustomers’premises do not interferewithone anotherand
cannot be maliciously overheard. A customer is not prepared to pay for the next-door-
neighbour’s calls, made on thewrongbasestation.Thismayhappen if ahandset
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 basestation. They are thususeless away from home, but make
the customer more mobile about his own premises.
- 322 CORDLESS
TELEPHONY AND RADIO IN THE LOOP
LOCAL (RILL)
k ::1 :
:
\\M
W
:
‘cradle’
U
Telephone
( base statlon 1
’
I
I
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 shifted to
is
the public network operator’s site. First came telepoint or CT2 (2ndgeneration cordless
telephony). DECT (digital European cordless telephony) followed.
16.4 TELEPOINT OR CORDLESSTELEPHONE 2 (CT2)
An extension of the ideaofcordlesstelephones is theconcept of telepoint, French
pointel or wideareacordlesstelephone. In telepoint a new type of digitalcordless
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 withthe base stations of the public network. Standing
within50-200mofatelepoint,acallerwithatelepointhandset is able tomake
outgoing calls into the publicswitched 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 connectionof 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. (Thereis one exception to this, and thatis 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.)
- EAN
(DIGITAL DECT 323
Other base
stations
Handset
‘fixed’network)
Basestation
Coverage (outgoing
area calls only)
Figure 16.3 Telepoint service
As we have seen, telepoint or second generation cordless telephone (CT2) as it is also
known is capableonly of makingoutgoingcalls.Thetechnologicalproblems 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 notprovide for a hand-oflprocedure for moving
between base station zones,so users were forced to stay in rangeof 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 atcommercial service of CT2 in several countries, theCT2 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 DECTstandards, developed by ETSI,havegrownto be asophisticatedset,
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 ( C M ) for digital
wide area cordless telephones. Along the way, a number of other features have been
built in
e securitymeasuresagainstunauthorized use of thehandsetandoverhearing of
conversation
- 324 CORDLESS
TELEPHONY AND RADIO IN THE LOOP
LOCAL (RILL)
0 mobile stationtracking so that incomingcalls can be forwardedtotheDECT
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 functioningof ISDN data
terminals over DECT
0 OS1 compatibility of the DECT protocols
Figure 16.4 illustrates the reference model of the DECT system.
of
The most important part DECT is the radio common air interface, D3. Thisallows
for the connection ofportable radio terminals ( P T ) (the portablepart,PP, of the system)
tofixed radio terminations ( F T ) (being the.fi.xedpart, FP, of the system using a cordless
(i.e. radiolink) connection. This interface can beused on its own in a similarmanner to
the C M (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 theDECT interface predecessing
over 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 be it use,
DECT network
fixed fixed
radio radio radio
termination
‘F(
termination
1 i ray0
terminal terminal
D4
raiio I ra;io
terminal terminal
I1 r;
ao
i I
portable
application
portable
application I application
portable 1I application
portable 1
Figure 16.4 DECT referencemodel
- 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, D A M ) 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 V D B (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 ( F T ) controls a cell within a DECT radio network. Roaming
between cells is controlled by a local network function
comprising homedata
bases ( H D B ) and visitor data bases ( V D B ) ,which perform similar functions to the home
location
register ( H L R ) and visitorlocation register ( V L R ) of the GSM system
(Chapter 15). Unlike GSM, however, the handover in DECT is by means of mobile
controlledhandover ( M C H O ) , in which themobilestationalone 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 ( M A H O ) 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 uponthe mobile unit’s measurementofthe RSSI (receivedsignal
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 ( P T ) t o stray
FRS = fixed relay station
MRS = mobile relay station
PT = portable terminal
RFP = radio fixed part
Figure 16.5 DECT fixed and mobilerelay stations
- 326 CORDLESS TELEPHONY AND RADIO IN THE (RILL)
LOCAL
LOOP
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 used to connect a
are
single connection or call, making it impracticable for high trafEc volume networks. In
addition, the connection qualityis 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. Thelowerlayermanagemententityisthesetofnetworkmanagement
functions providedto monitor and reconfigure the protocols as necessary for network
operation.
The characteristics the physical layerof the radio interfaceare listed in Table
of 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 packetPO0 (for ashort signalling burst)or two half slots (low capacity physical
packet P08).
U-plane C-plane
Figure 16.6 DECT protocolreferencemodel
- THE DECT AIR INTERFACE
(D3-INTERFACE) 327
Table 16.1 DECTairinterface,physicallayer
~~
Radio band 1880-1900MHz
Number of radio channels 10
Radio channel separation 1.728 MHz
Transmitter power (max) 250 mW
Channel multiplexing TDMA (time division multiple access)
Duplex modulation TDD (time division duplexing)
TDMA frame duration 10 ms
Timeslots per TDMA frame 24
Modulation GFSK (Gaussian frequency shift keying)
Total bit rate 1 152kbit/s per cell
User channels B channel: 32 kbit/s (user)
A channel: 6.4 kbit/s (signalling)
10 ms, 24 slots, 11520 bits 4-
l
Slot
-
-
basic
phvsical S-field D-field 2
packet P32
so s31:
d387 d383 d64a0 d63 I
D-field B-field A-field X
----___
bo - - - - - - - _ _ -_- _ _- - - _ _ _
- b319
A-field TA Q1 BA Q2 A-field info R-CRC
a47a4a0
a48 a3
a63
a8
a7
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 theD-field (for carriage of data). TheD-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(RILL)
LOCAL
LOOP
Table 16.2 DECT signalling parameters(A-field)
Parameter Purpose
A-field TA information type
Q1, Q2 Quality control bits used as handover criteria
BA B-field information type
A-field information
The used for carriage of MAC and layer
field higher c-plane
protocol information
R-CRC redundancy
Cyclic
check (for error detection)
Full 64 kbit/s ISDN bearer channels (i.e. user information channels for ISDN kbit/s
64
data) may be transmitted over DECT networks by the occupation of two B channels.
16.9 OTHER ISDN WIRELESS LOCAL LOOPSYSTEMS
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 presently (1997) only a European standard, othersystems
is
have also been developed for ISDN wireless local loop, aiming to provide fortelephone
and full 64 kbit/s connection service. These systems use a variety of different and as yet
unstandardizedtechniques.Exampletechnologiesincludethose of Ionica (a British
company aiming to offer a full scale telephonenetworkacrossthe UK - based on
technology called Proximity i developed conjunction
in with Northern Telecom,
N O R T E L ) , Airspan (a system developed by D S c 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 manufacturershavestarteddevelopments of point-to-
multipoint ( P M P ) systems of shorthaulmicrowave radio for use in the microwave band
above1OGHz.Thesedevelopments foresee dynamicallocation of radiobandwidth
within a cell, allowinga fixed or maybe even mobileend user to requestvarious
differentbitrates (64 kbit/s or multiplesthereof) 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 inalatergenerationtheymaybetheobviouschoicefor broadband services,
including radio in the local loop for A T M (asynchronous transfer mode).
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