WO1996039785A2

WO1996039785A2 - Improvements of or relating to cordless telephony

Info

Publication number: WO1996039785A2
Application number: PCT/SE1996/000699
Authority: WO
Inventors: Peter Olanders
Original assignee: Telia Ab
Priority date: 1995-06-06
Filing date: 1996-05-30
Publication date: 1996-12-12
Also published as: WO1996039785A3; SE9502052D0; SE9502052L; SE514244C2

Abstract

The present invention relates to improvements of or relating to cordless telephony and allows a DECT-system to be established at a comparatively low cost by use of the existing public telecommunications network. It further allows that the construction of the radio base stations are simplified and their costs reduced by placing certain functions in a local telephone exchange and to share these functions between radio base stations. Transmission between radio base stations and a local telephone exchange is simplified by the use of an ISDN-protocol, i.e. a transmission format based on the ISDN-standard which utilizes two B-channels with 64 kbit/s, and one D-channel with 16 kbit/s, over one single twisted copper connection. In a WACA-system according to the present invention, much of the higher control layers, for instance NWK and DLC, are placed in a central control unit, for instance located to a local telephone exchange. In this way the system layers can be made comparatively powerful. This makes it possible that subscriber equipment which introduces a radio base station can be simplified for instance by making the MAC-layer thinner. Consequently, the subscriber equipment is not quite independent since its fixed unit is at least partly remote-controlled.

Description

TITLE OF THE INVENTION: IMPROVEMENTS OF OR RELATING

TO CORDLESS TELEPHONY

The present invention relates to a cordless telecommunications system and a procedure to construct it.

Cordless telephony is a well nown technology and includes a number of different types of systems inclusive, at one extreme, digital mobile cell systems with complete international roaming, such as the GSM-system, and at the other extreme, simple cordless telephones for use at home which operate according to the ordinary air interface standard CT2. Typical standards for cordless telephony include CT2, CT3, DECT, Bellcore Framework and PHP. A brief explanation of some of these standards can be found in "Communications Technology Handbook" by Geoff Lewis, published by Newnes in 1994. For a more detailed explanation of these standards references should be made to different standard specifications published by such bodies as ETSI (The European Telecommunications Standards Institute) .

The present invention relates to cordless telephony based on TDMA/TDD of the general type which is specified in the DECT- standards. Though the invention is described with relation to a DECT-system, it has obvious applications at similar systems which fulfils other standards.

DECT can be used for a field of system applications, inclusive home- and plant telephony, for instance cordless telephones and radio-exchanges for companies, public access systems such as Telepoint, and network infrastructure, such as Cordless Local Loop. The present invention can be applied at all four of the above mentioned system applications.

A basic DECT-system includes one or more radio base stations, one or more portable stations, and possibly a number of repeaters or slave stations. Transmission between the radio base stations and the portable stations utilizes TDMA/TDD which is used to create a total -number of 120 duplex channels on 10 carrier frequencies, at which each carrier frequency carries 12 uplink channels and 12 downlink channels. The operation frequency band is between 1880 and 1900 MHz.

CONFIRMATION COP Typically the radio base stations have a radio coverage of about 100 m radius, and the effective range of a given base station can be extended by the use of repeaters. Handover can be performed between different channels and between different radio base stations. Handover between different DECT-systems, however, can only be performed by means of functions which are included in the public switched telecommunications network. DECT-systems can be used for private communication, for instance in a cordless telephone, and for public access systems, for instance in a Telepoint-system. Some of the problems which are related to the establishing of DECT-systems are briefly sketched below: it is important for a network operator to achieve acceptable surface radio coverage as soon as possible; network operators need to utilize their existing wire networks to achieve installation economy; and

DECT-systems must be sufficiently attractive to subscribers in the public access system so a sufficent customer base is created to support the system. The present invention aims at solving at least some of the above outlined problems.

The present invention allows a DECT-system to be established at a comparatively small cost by use of the existing public telephone network. It further makes possible that the construction of the radio base stations can be simplified and their cost be reduced by locating certain functions to a local telephone exchance and^' share these functions between radio base stations. Transmission between radio base stations and a local telephone exchange is simplified by the use of an ISDN-protocol, i.e. a transmission format based on the ISDN-standard which uses two channels with 64 kbit/s, and a D- channel with 16 kbit/s over a simple twisted pair copper connection, It should be emphasized that the present invention does not use an ISDN-system for communication between radio base stations and a local telephone exchange, only an ISDN-transmission format.

The use of ISDN in connection with DECT is known. For instance is ETSI at present developing a DECT/ISDN-cooperation profile. Moreover, a number of architectures for systems of DECT-type, connected to ISDN, have been suggested in the patent literature. Examples of such systems are briefly mentioned below. US patent 5 297 193 describes a cordless telephone network which has a number of fixed sections which by radio is linked to portable stations. The fixed stations are connected to an ISDN-system. The invention principally relates to the providing of a centralized support service for the system. According to the description the system relates to the provision of ISDN-circuits at a radio telephone system, rather than the use of an ISDN-protocol to transmit speech and signals between a radio base station and a local telephone exchange. In US patent 5 040 177 is described a Telepoint-service, in which radio base stations are connected to a public ISDN-network. Each radio base station can manage up to 40 speech channels. The use of radio base stations which manage a mixture of private and public traffic is not described. As with US patent 5 297 193 the system relates to the provision of ISDN-circuits at a radio telephone system more than use of an ISDN-protocol to transmit speech and signalling between a radio base station and a local telephone exchange.

PCT-application W093/21719 describes a DECT-system which has the ability to cooperate with an ISDN-system. A DECT-format with double slot is modified to allow the DECT-system to at the same time transmit an ISDN B-channel and a D-channel separately. This allows that ISDN outband signalling can be managed by a DECT-system. The invention relates to the cooperation between DECT and ISDN rather than the use of an ISDN-protocol for transmission between a local telephone exchange and a radio base station.

US patent 5 325 419 describes a PCS with complete ISDN- interface. The invention utilizes intelligent radio base stations. None of the documents relating to the prior art, which has been discussed above, describes the use of an ISDN-protocol to transmit at least three independent speech channels in an ADPCM-format, per ISDN-channel, between a radio base station and a local telephone exchange. Other characteristics of the invention which do not exist in the previous technology described above are: - that radio base stations manage a mixture of private and public traffic; that the control functions of the system, inclusive encryption, decryption, encoding and decoding are localized to a local telephone exchange; that radio base stations are supplied with power from a local telephone exchange; data compression, interleaving and signal filtering to reduce the bit rate of the D-channel; and that repeaters are linked to the radio base stations via the public telecommunications network. One of the ideas according to the invention which lies behind a WACA-system according to present invention is that much of the higher control layers, for instance NWK and DLC, can be placed in a central control unit, for instance located in an LX. In this way these system layers can be made comparatively powerful. This allows that subscriber equipment which includes a radio base station can be simplified, for instance by reduction of the MAC-layer. Consequently the subscriber equipment will not be completely independent because its fixed part FP at least partly is remote-controlled.

According to a first aspect of the present invention is provided a cordless telecommunications system for transmission of speech, inclusive at least one radio base station and at least one portable station, in which data, inclusive signalling and encoded speech, is transmitted between said at least one base station and said at least one portable station by use of TDMA/TDD, and in which the speech is encoded, by use of ADPCM, characterized in that said radio base station is linked to a local telephone exchange, in that signalling and speech is transmitted between said radio base station and said local telephone exchange with an ISDN-protocol, in that speech is transmitted between said radio base station and said local telephone exchange in the same data format as said speech is received by said radio base station from a portable station or transmitted from said radio base station to a portable station, and in that at least three speech channels per ISDN-protocol channel are accessible for transmission between said radio base station and said local telephone exchange.

Preferably four compressed speech channels with 32 kbit/s are accessible per ISDN-protocol channel for transmission of speech between said radio base station and said local telephone exchange. Said radio base station can be arranged to manage both private traffic and public traffic.

Said radio base station can be connected to said local telephone exchange with a local loop connection in the public telecommunications network.

Preferably a control device for said system is located at said local telephone exchange.

Signalling between said control device and said base station can be digitally compressed. Alternatively the signalling between said control device and said base station, intended for or received from different portable stations, can be interleaved.

Alternatively said radio base station can be arranged to, from the signalling between said control device and said base station, delete each part of it which is only needed for operation of said radio base station, which by that reduces the signal traffic levels between said radio base station and said control device.

Functions for encryption, decryption, encoding and decoding are located in said local telephone exchange and arranged to be shared between a number of radio base stations.

According to a second aspect of the present invention is provided a method to construct a cordless telecommunications system, as described above, characterized in the steps: to localize a control device for said cordless telecommunications system at a local telephone exchange; to localize radio base stations at selected subscribers; and to connect said local telephone exchange to said radio base stations via an existing public local loop network, . by use of an ISDN-transmission protocol.

Said method can include the step to, at a subscriber, install a radio base station arranged to serve a number of subscribers. Preferably a sufficient number of radio base stations are installed, localized in a way that their radio coverages overlap each other, to provide essentially complete radio coverage, over a predetermined geographical area, substantially larger than a radio coverage for one single radio base station. Said method can include the steps: to install radio base stations at some subscribers; to, at other subscribers, install repeater stations; to link repeater stations to radio base stations by connections established over an existing public local loop. According to a third aspect of the present invention is provided a cordless telecommunications system which has a number of radio base stations, which each is arranged to provide telecommunications services to a number of portable stations, characterized in that said cordless teleconmmunications system is arranged to at least in some radio base stations combine functionality with public access and functionality with private access, resulting in that one single base station can be used to provide private home service and a public Telepoint-service.

Embodiments of the invention will now be described as examples with references to enclosed drawings, in which:

Figure 1 illustrates the relation between the cost for providing cordless telephony and a number of subscribers, for small and big cell units.

Figure 2 is a schematic illustration of a communication link established with a cordless telephone system, in accordance to the the present invention, between a portable station and the public telecommunications network via a radio base station and local telephone exchange.

Figure 3 is a schematic illustration of the frame structure for a DECT-system.

Figure 4 is a schematic illustration of the radio coverage provided by a cordless telecommunications system in accordance to the present invention.

Embodiments of the present invention which is described after this use the standard DECT for cordless telephony. It is quite obvious that the invention is not restricted to the DECT-standard, besides what is specifically indicated in the claims in this patent application, and can be implmented by use of other standards for cordless telephony which are compatible with the use of TDMA/TDD digital radio transmission. In accordance to accepted practice in the field of telecommunication acronyms and abbreviations are to a large extent used in this application. In order to give the reader assistance is below presented a list of terms which are used:

A/D Analog/digital conversion ADPCM Adaptive Differential Pulse Code Modulation.

B-channel 64 kbit/s ISDN-channel.

BER Bit Error Rate. CFP Central Fixed Part.

D/A Digital/analog conversion.

D-channel 16 kbit/s ISDN-channel.

DECT Digital European Cordless Telecommunications.

DLC Data Link Control. ETSI European Telecommunications Standards Institute.

FP Fixed Part.

ISDN Integrated Services Digital Network.

IWP Inter-Working Profile.

LX Local Telephone Exchange. MAC Medium Access Control.

NWK Network layer.

OSI Open Systems Interconnection.

PCS Personal Communications System.

PHL Physical Layer. PP Portable Part.

PSTN Public Switched Telephone Network.

RFP . Radio Fixed Part.

TDD Time Division Duplex.

TDMA Time Division Multiple Access. U-plane User plane.

VPN Virtual Private Network.

VPN+ Virtual Private Network with mobility support, sometimes called M-VPN.

WACA Wide Access Cordless Access. A cordless telecommunications system in accordance to the present invention can be described as a WACA, i.e. the system provides cordless access to telecommunications services over a wide geographical area, for certain an area which is considarably larger than the radio coverage for one single radio base station. The invention, however, is not restricted to WACA-systerns. At construction and installation of WACA-systems a telecommunications network operator must ensure that acceptable surface coverage is produced by the system as rapidly and economically as possible. As is evident from Figure 1 the initial costs for small cell systems are essentially larger than for large cell systems, i.e. when the number of users of the system is small. By increase of the number of users or subscribers the costs for small cell systems however remain essentially constant, whereas the costs for large cell systems grows in proportion to the number of users. The fact is that the costs per user can be regarded as approximately proportional to the cell size. Provided that a system rapidly can attract a comparatively large number of users the small cell alternative consequently is economically to prefer. DECT- systems are generally small cell systems. Typically a DECT radio base station has a radio coverage of about 100 m radius or less. It is cost effective for a telecommunications operator which establishes a new system, to maximally utilize installed network infrastructure. At installation of a WACA-system one should ideally utilize the existing twisted copper network to establish connection between radio base stations and local telephone exchanges. In this way the need to install a DECT-connection network is avoided. Figure 2 illustrates in schematic form the way in which a portable DECT-station can be connected, via a radio base station an local telephone exchange, to the public telecommunications network in the so called U-plane. A system can be illustrated schematically in a number of ways, for instance by use of block diagrams or circuit diagrams. The user plane is a simplified schematic illustration of a system which shows how a user "sees" or experiences the system in operation. The PP, i.e. the portable unit, is physically located to a portable station or cordless handset. Th PP includes a number of layers - NWK, DLC, MAC and PHL. A radio connection can be established between the physical layer PHL of the portable handset and the physical layer of a fixed radio unit RFP. The fixed radio unit is of course physically located to a base station or a radio base station. A wire connection which utilizes a ISDN-protocol can then be established between the physical layer of the fixed radio unit and the physical layer of the central fixed unit CFP. The CFP is normally located to a radio base station, but the present invention suggests that it can be located to a local telephone exchange. The CFP is then connected via the local telephone exchange LX to the public telecommunications network.

It should be noticed that the introduction of the CFP in a radio base station significantly adds to the costs for a radio base station. The CP performs a great variety of functions, such as for instance related to encryption, decryption, encoding and decoding. By including these functions in a local telephone exchange they can be timeshared between different radio base stations. This in its turn will reduce the total cost of the system and allows construction of especially cheap and simple radio base stations which have small need for power. Radio base stations can be located at the subscriber and connected to the local telephone exchange via the existing local wire network. This implies for the DECT-system according to the present invention that the RFP and the CFP are connected with twisted copper pairs which are used in the local network. (Other connection media can be used if they are available, for instance optical fibre) . The connection between the RFP and the CFP must be able to transfer a 32 kbit/s ADPCM-signal, as has been specified in the DECT-standards, together with the belonging DECT-signalling.

Ideally the protocol and the signalling between the RFP and the CFP should be digital. This facilitates the use of repeaters. The use of digital transmission further avoids the need for analog/digital conversion between the transport network of the headbit of the public network and the subscriber, which consequently reduces the unavoidable distorsion and the noise which is produced at each A/D or D/A-step.

The protocol which is used at the present invention for transmission between the RFP and the CFP is an ISDN-protocol which is utilized for communication between station equipment and subscriber equipment. ISDN can successfully be transmitted via twisted copper pairs between a local telephone exchange and subscriber equipment, and is consequently suitable for use in the present invention to link the RFP at a radio base station to the CFP at a local telephone exchange. The ISDN-protocol includes two B- channels, i.e. two speech channels with 64 kbit/s, and one D- channel, i.e. a channel with 16 kbit/s, which is used for signalling and normally is used in packet transmission mode. It should definitely be observed that the link between the RFP and the CFP does not constitute a part of an ISDN-system but that this is only a suitable protocol. The fact is that each B-channel can be used in the present invention to carry two speech channels instead of the only speech channel which is normally used in ISDN.

The use of an ISDN protocol link between the RFP and the CFP is shown in Figure 2. The ISDN-protocol allows simultaneous transmission of four ADPCM speech channels with 32 kbit/s over the two ISDN B-channels. There is however one problem with the signalling and this is discussed later in this application. The separation of the RFP and the CFP and the use of an ISDN-link means that the MAC-layer, see Figure 2, in the RFP can be made very "thin", because it only needs to manage the separation of the signalling and the speech and distribute these between the two B- channels and the D-channel in the ISDN-protocol. (The MAC-layer distributes and makes free physical channels depending on what higher layers request) . The MAC-layer, or the media access layer, exists in most telecommunications systems, but not in OSI-based systems. The MAC-layer is located above the physical layer which it principally controls. The "thickness" of the MAC-layer, i.e. its content, is decided by the amount of controlling it has to perform. In the present invention the MAC-layer is "thin" because of the amount of controlling which is performed in higher layers, for instance NWK and DLC, and the demands from PHL.

DECT-specific signalling is transmitted over the ISDN-channel which connects the RFP and the CFP. The frame structure which is used for DECT-transmission is illustrated in Figure 3. From this we conclude that a data packet is preceded by a synchronization sequence of 32 bits, followed by a data packet which contains 388 bits. The data packet can be followed by four bits which repeates the last four bits in the data packet, to facilitate detection of slot drift and by that avoid packet collision. The data packet includes 48 bits signalling data, followed by 16 error detection/correction bits. The packet includes 320 bits user data, followed by further four error detection bits. Consequently are 48 bits signalling data per channel produced per time frame. The length of the time frame is 10 milliseconds. This corresponds to 4,8 kbit/s signalling traffic per speech channel, i.e. totally 19,2 kbit/s signalling per four speech channels. It should be noticed that the synchronization sequence need not be transmitted continuously between the RFP and the CFP, because the ADPCM time structure itself can be used to produce a time controlling clock signal. The demand for 19,2 kbit/s signalling, however, still exceeds the capacity of an ISDN D-channel with 3,2 kbit/s. Unless further measures are not taken, the capacity of the D-channel will restrict the capacity of the ISDN-protocol link to three DECT speech channels.

The ISDN-link can however be arranged to support four DECT speech channels by: using data compression; - signal interleaving; coordination and reduction of the signal needs; or a combination of the above mentioned. The DECT-signalling does not utilize data compression technologies. The data compression level which is required to compress the 4,8 kbit/s DECT-signalling to the accessible 4 kbit/s on the ISDN-channel is not high - 1:1,2. Because BER for wire connections is essentially lower than on radio links, with a number of magnitudes, the low data compression which is required to squeeze in the DECT-signalling on the ISDN D-channel does not cause any problems. A lot of suitable data compression technologies to achieve this data compression level is well known to experts in the field. The greatest signalling need at DECT-signalling is at roaming, during handover betweeen cells, and during the call connection. These processes are relatively short and comparatively seldom occurring and do not require a continuous need for signal capacity. This means that signal interleaving can be used to reduce the average signalling need on the D-channel. Different signals can be interleaved, i.e. signalling for four simultaneous calls which are transferred on the D-channel are interleaved with dynamic displacement/delay, according to need.

The signalling need can be reduced by coordination and reduction of the total signal traffic. Parts of the radio specific signalling need not be transmitted to the CFP. Parts of the radio specific signalling and other signalling can be shared between speech channels. This can be used to reduce the requirements on the D- channel, making it land up within the transmission capacity. Embodiments of the present invention have a number of advantages, inclusive: low costs of production for the RFP; simple connection of the RFP; sufficiently simple for a subscriber to connect the RFP him/herself; the provision of services to a number of subscribers, up to four, over the same connection;

- system which utilize repeaters, i.e. master/slave systems, can easily be implemented making handover between adjacent cells easy, which by that adds to the radio surface coverage; and - the provision of mixed systems.

In a mixed system a subscriber which has a radio base station installed at him/her place, keeps one or possibly two DECT-channels for his/he own use. The remaining three (or two) channels will then be accessible for public use. Overlapping cells from adjacent locals can in this way build up comparatively good radio coverage. Figure 4 illustrates the way in which the radio coverages, which correspond to the individual radio base stations, are combined to give almost total radio coverage for a given area. This means that a Telepoint- system can be built up by use of existing local loops and radio base stations located at subscribers. Use of radio base stations at subscribers also facilitates the accessibility to Telepoint on private property, for instance an industrial plant, to advantage both to the network operator and the subscriber.

The construction of a large number of radio base stations at subscribers implies that not all subscribers need to have a dedicated RFP installed at his/her own place.

Embodiments of the present invention can be applied in order to at the same time in a system provide:

- home telephony, i.e. telephony with private access/restricted access; telephony with public access, for instance Telepoint;

- VPN and VPN+ "teleworking", by dynamic allocation of the accessible traffic capacity between data and speech, with mobility. The present invention provides a DECT-based system in which comparatively high traffic capacity, two speech channels with 64 kbit/s, plus a signalling channel with 16 kbit/s, can be made accessible to subscribers. Different developments of the present invention will be realized be experts in the field.

Speech encoders and encryption equipment can be centrally placed in the CFP. This dramatically reduces the costs and the power consumption for the RFP. This makes it possible that the RFP can be supplied with power from a central power source, for instance located at the local telephone exchange.

Slave base stations, i.e. repeaters, can be connected to master base stations via the existing local loop wire network. In this connection it should be noticed that a large number of houses are equipped with a number of telephone outlets. The use of repeaters will add to the radio surface coverage. Handover can be controlled from a master base station with comparatively small additions. Control of handover between a radio base station and a repeater, from the radio base station, will reduce the load on the CFP.

Use of the present invention will facilitate system updatings to complete ISDN/DECT-cooperation. Since the RFP is constructed to operate with an ISDN-protocol, it is especially simple to arrange that the RFP operates directly with ISDN in acordance to DECT/ISDN IWP. In other words, ISDN-services can be distributed from the RFP via a DECT-radio interface and/or a "fixed" outlet, i.e. the common ISDN-outlet. In the latter case (fixed outlet) the requirements on the RFP is increased due to, among other things, the need to include a speech encoder. In this case the RFP becomes a part of an ISDN- terminal. It should be noticed that the function of an RFP, as an

ISDN-terminal, falls outside the scope of the present invention even if the characteristics of the present invention facilitates an upgrading to complete ISDN-operation.

Claims

PATENT CLAIMS

1. Cordless telecommunications system for transmission of speech, including at least one radio base station and at least one portable station, in which data, inclusive signalling and encoded speech, is transmitted between said at least one base station and said at least one portable station by use of TDMA/TDD and in which speech is encoded by use of ADPCM, c h a r a c t e r i z e d in that said radio base station is linked to a local telephone exchange, in that signalling and speech is transmitted between said radio base station and said local telephone exchange with an ISDN-protocol, in that speech is transmitted between said radio base station and said local telephone exchange in the same data format in which said speech is received by said radio base station from a portable station or transmitted from said radio base station to a portable station, and in that at least three speech channels per ISDN protocol channel are accessible for transmission between said radio base station and said local telephone exchange.

2. Cordless telecommunications system according to patent claim 1, c h a r a c t e r i z e d in that four compressed speech channels with 32 kbit/s are accessible per ISDN protocol channel for transmission of speech betweeen said radio base station and said local telephone exchange.

3. Cordless telecommunications system according to either patent claim 1 or patent claim 2, c h a r a c t e r i z e d in that said radio base station is arranged to manage both private traffic and public traffic.

4. Cordless telecommunications system according to any of the preceding patent claims, c h a r a c t e r i z e d in that said system fulfils the DECT-standard for cordless telephony.

5. Cordless telecommunications system according to any of the preceding patent claims, c h a r a c t e r i z e d in that said radio base station is connected to said local telephone exchange with a local loop connection in the public telecommunications network. 6. Cordless telecommunications system according to any of the preceding patent claims, c h a r a c t e r i z e d in that a control device for said system is placed at said local telephone exchange.

7. Cordless telecommunications system according to patent claim 6, c h a r a c t e r i z e d in that signalling between said control device and said base station is digitally compressed.

8. Cordless telecommunications system according to patent claim 6, c h a r a c t e r i z e d in that signalling between said control device and said base station, intended for or received from different portable stations is interleaved.

9. Cordless telecommunications system according to patent claim 6, c h a r a c t e r i z e d in that said radio base station is arranged to, from the signalling between said control device and said base station, delete any part thereof which is only required for operation of said radio base station, which by that reduces the signal traffic levels between said radio base stations and said control device.

10. Cordless telecommunications system according to any of the patent claims 6-9, c h a r a c t e r i z e d in that functions for encryption, decryption, encoding and decoding are located in said local telephone exchange and arranged to be shared between a number of radio base stations.

11. Cordless telecommunications system according to any of the patent claims 6-9, c h a r a c t e r i z e d in that said radio base station is supplied with power from said local telephone exdhange.

12. Cordless telecommunications system according to any of the preceding patent claims, c h a r a c t e r i z e d in that it includes at least one repeater linked to a radio base station by a local loop connection in the public network. 13. Method to construct a cordless telecommunications system according to any of the patent claims 1-12, c h a r a c t e r i z e d in the steps: to place a control device for said cordless telecommunications system at a local telephone exchange; - to place radio base stations at selected subscribers; and to connect said local telephone exchange to said radio base stations via an existing public local loop network by use of an ISDN transmission protocol. 1 . Method according to patent claim 13, c h a r a c t e r i z e d in the step to at a subscriber install a radio base station arranged to serve a number of subscribers.

15. Method according to patent claim 14, c h a r a c t e r i z e d in installing a sufficient number of radio base stations, placed in a way that their radio coverages overlap each other to provide essentially complete radio coverage over a predetermined geographical area, substantially larger than a radio coverage of one single radio base station. 16. Method according to any of the patent claims 13-15, c h a r a c t e r i z e d in that at a subscriber install a radio base station arranged to provide a private service to this subscriber and a public service accessible to other subscribers within the radio coverage of this radio base station. 17. Method according to any of the patent claims 13-16, c h a r a c t e r i z e d in compressing signalling, interleaving signalling and/or reducing signalling in a way that all signal traffic between a radio base station and a local telephone exchange can be transmitted over an ISDN D-channel. 18. Method according to any of the patent claims 13-17, c h a r a c t e r i z e d in installing said radio base stations in a way that said radio base stations are supplied with power by transmission of electrical current over an existing public local loop network. 19. Method according to any of the patent claims 14-18, ^• c h a r a c t e r i z e d in the steps: to, at some subscribers, install radio base stations; to, at other subscribers, install repeater stations; and to link repeater stations to radio base stations by connections established over an existing public local loop. 20. Cordless telecommunications system having a number of base stations, each arranged to provide telecommunications services to a number of portable stations, c h a r a c t e r i z e d in that said cordless telecommunications system is arranged to, at least in some radio base stations, combine public access functionality and private access functionality in a way that one single radio base station can be used to provide private home service and public Telepoint- service.