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 o
Trang 1Cordless 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
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
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,
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)
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long point-to-point links between switching centres (i.e between local exchanges and
relatively small
customers by the monopoly network players, so that new effort needs to be applied to
endpoints are fixed, so this removes the need for much of the complexity of the GSM
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
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
Trang 3FIXED NETWORKS BASED ON RADIO TECHNOLOGY 321
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
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
converge The three approaches are
We discuss each in turn
Cordless telephony is the term used to describe telephone sets connected to the ordin-
the handset, which also acts as a radio transceiver The base station is connected to the
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,
signalling system A major hurdle in the design of cordless telephones is ensuring that
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,
within range of their own base station They are thus useless away from home, but make
the customer more mobile about his own premises
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W
( base statlon 1
Mobile
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
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,
locations, such as airports, stations, and street corners (much as public payphones are
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
To make a call, the handset sends a signal, including a special handset identity code,
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,
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.)
Trang 5DECT (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
tracking the mobile handset location were not solved by CT2, so that incoming calls to
receivers into CT2 handsets, so that the users could be paged with a displayed telephone
telephone handsets
A further problem was that CT2 did not provide for a hand-oflprocedure for moving
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,
D ECT (digital European cordless telephony)
16.5 DECT (DIGITAL EUROPEAN CORDLESS TELEPHONY)
development grew from the desire to develop a common air interface ( C M ) for digital
built in
conversation
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telephone user, no matter where he is
terminals over DECT
Figure 16.4 illustrates the reference model of the DECT system
tofixed radio terminations ( F T ) (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 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
radio radio
terminal terminal
D4
application application
I ra;io I 1 ra;io I
terminal terminal
I application portable 1 I application portable 1
DECT network
fixed fixed radio radio
‘F( termination termination
Figure 16.4 DECT reference model
Trang 7DECT HANDOVER 325
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
bases ( H D B ) and visitor data bases ( V D B ) , which perform similar functions to the home location register ( H L R ) and visitor location register ( V L R ) of the GSM system
controlled handover ( M C H O ) , 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 ( M A H O ) of GSM
information provided by the mobile The decision to initiate handover in the DECT
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
kilometres have been achieved), there has been a need to find a means of extending the
FRS = fixed relay station MRS = mobile relay station
PT = portable terminal RFP = radio fixed part
Figure 16.5 DECT fixed and mobile relay stations
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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
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
Trang 9THE 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)
~~
10 ms, 24 slots, 11520 bits 4
-l
- Slot
-
basic
packet P32
- - - - _ _ _
- - - - _ _ _
A-field TA Q1 BA Q2 A-field info R-CRC
a0 a3 a4 a7 a8 a47 a48 a63
Figure 16.7 TDMA frame structure in DECT
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
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Table 16.2 DECT signalling parameters (A-field)
BA
B-field information type
protocol information
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
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
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
technical system performance
MICROWAVE RADIO
multipoint ( P M P ) systems of shorthaul microwave radio for use in the microwave band
basis These systems may not tap the initial market for ISDN radio in the local loop,
including radio in the local loop for A T M (asynchronous transfer mode)