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).