FIELD OF THE INVENTION

The invention relates to a telephone system which comprises one or more telephones and in which internal calls between telephones and other functions provided by a private branch exchange telephone system are available.

BACKGROUND OF THE INVENTION

The most usual way of constructing a telephone system for a company or the like is to purchase a private branch exchange and connect individual telephones to its extensions. The private branch exchange, in turn, is connected to the nearest telephone exchange by a subscriber line. At present, the subscriber line is almost always digital, the transfer mode being PCM. When the telephone traffic density grows, a necessary number of PCM time slots can thus be flexibly allocated to the subscriber. This most frequently used method is illustrated in Figure 1A. Each one of the telephones (there may be even hundreds of them) is connected by a dedicated line to a branch exchange, which may be either a PBX (Private Branch Exchange) or a PABX (Private Automatic Branch Exchange). In the case of a PBX, it is possible, but not necessary, to use an operator who switches an incoming call from the telephone network to a telephone connected to the private branch exchange system, and vice versa. A PABX is fully automatic: when a number is dialled, the call is automatically switched. It is possible to have direct dialling numbers and a variety of other facilities offered by a private automatic branch exchange, such as call waiting, call forwarding, conference calling, and automatic providing of statistical information on calls. Private branch exchanges of both types allow charge- free internal calls. If a company is geographically widely spread, private branch exchanges within different areas can be interconnected by rented lines, whereby it is possible to form a private branch exchange network where internal calls are charge-free.

If the user does not wish to invest in his own private branch exchange, he can buy private branch exchange service from a telephone operator, Figure B. Individual telephones are connected by a subscriber line to the centrex service of a telephone exchange. The centrex service is a service offered by the teleoperator in addition to the basic services. In the centrex

service, a certain number of subscriber connections of a telephone exchange are assigned to an organization, such as a company. The centrex, which is associated with a telephone exchange and provided by the operator, offers the same services as a PBX or PABX of a company, whereby the company does not have to purchase and maintain a private branch exchange of its own. At first, only subscribers under the same telephone exchange could belong to the same centrex service, wherefore internal calls and call forwarding were possible only from one subscriber line through the telephone exchange to another subscriber line connected to the same telephone exchange. Later it became possible to provide an expanded service in which subscribers who are located geographically far from one another and who are thus connected to different telephone exchanges belong to the same centrex group. This was possible only if the user of the service rented a private line for interconnecting the different telephone exchanges. A characteristic feature of the system of Figures 1A and 1 B is that the cabling of the telephone sets requires a star-configuration network, wherein the common connection point is the private branch exchange or the main distributing frame in the telephone room of the building.

Before the introduction of private branch exchanges, a key telephone system of Figure 1C was used. In that system, the external lines from a telephone exchange are wired in parallel to all the telephones, and a pair of wires extends between each telephone in the system, allowing thus internal calls to be made. A typical feature of the telephone is that it comprises a large number of buttons with a light next to them. In the case of an incoming external call, the light of the external line button flashes in all the telephones, whereby it is possible to switch to the external line from any telephone by pressing the respective button. Correspondingly, an internal call is set up by pressing the button of the line extending to the called telephone. Heavy cabling is typical of a key telephone system, since each external and internal line requires a separate pair of wires, and it is hardly possible to provide any private branch exchange services.

A system is improved significantly if a private branch exchange is connected to an ISDN network and ISDN telephones are used. Figure 2 is a general view of ISDN telephones. A telephone is connected to a private branch exchange or a network terminal by an ISDN interface part 12 which provides a 2B+D interface. The line is matched by a transformer 11 to a

telephone comprising a processor 13, software 14, a memory 15, a keyboard, and a display 16. The handset is connected to the processor by an adapter 17; in addition, a separate adapter 18 may be provided for connecting a general-purpose computer PC to the telephone. Through the telephone, the computer may thus use the ISDN connection, which is much faster than an analogue connection. On account of the services offered by the ISDN network and the terminal software supporting them, an ISDN telephone has extremely many uses as compared with an analogue telephone.

However, none of the telephone systems described above allows the subscriber to be mobile. Limited mobility can be provided by replacing an extension line of a private branch exchange with a radio path. This is illustrated in Figure 1 D. The system employs a conventional PABX/PBX exchange, the extension side of which is connected to a radio branch exchange. A plurality of base stations is connected to the radio branch exchange by cabling of star configuration. Each of the base stations may simultaneously communicate with several radio telephones. The solution of the figure is particularly suitable for use in a time division multiplexed radio system, where a radio channel consists of correspondingly numbered time slots of successive frames. When the transmission and reception take place on the same frequency, it is possible with one frequency to form as many channels as there are time slot pairs in the frame. An example of such a time division system is the DECT system (Digital European Cordless Telephony), which will be described briefly with reference to Figures 3A and 3B.

The 20 MHz frequency band allocated to the DECT makes it possible to have 10 frequency channels, when the channel separation is 1.728 MHz. In Figure 3A, each frequency channel is divided in the time domain into frames of 10 ms on the TDMA principle, and a frame is divided into 24 time slots, the first twelve time slots F1 ,...,F12 being downlink time slots, and the other twelve time slots R1 ,...,R12 being uplink time slots. The time slots form pairs in such a manner that time slots with corresponding numbers, e.g. F5, R5, form a duplex channel. A channel is thus unambiguous¬ ly determined by the time slot and the frequency. One channel can transfer data at the rate of 32 kbit/s, but it is possible to allocate a plurality of channels to a telephone according to the need; the transfer rate achieved when all the time slots are used will be 384 kbit/s, which is sufficient for instance for duplex video transmission in accordance with the standard H.320. The possibility of

allocating time slots (channels) and thereby transfer rate according to the need is a significant advantage of the DECT. One time slot is sufficient for the services of a PSTN network, and 1 to 5 time slots are needed for an ISDN network, whereas all of the 12 time slots are required when the transfer rate of a local area network (LAN) is utilized. Access to existing and future networks, such as PSTN, ISDN and GSM networks, has been taken into account in the definition of the DECT; to allow private services to be used, the DECT also includes access to a PABX and X.25 networks.

The medium access control layer MAC defined in the DECT specification ETS 300 175-3 of the ETSI has two main tasks. First, it selects the physical channels (time slot, frequency), and then allocates and releases these channels. Second, it multiplexes and demultiplexes control information, higher level information, and error control information into a packet suitable for a time slot. These tasks are used for providing three independent services: broadcast service, connection-oriented service, and connectionless service.

Figure 3B illustrates the contents of a data packet sent during one time slot. The packet contains at first a 4-byte sequence of synchronization bits, then 6 bytes of signalling data, 2 bytes for error control, 40 bytes of digitized speech or data, and finally 4 bits for error control. The known telephone systems described above have some drawbacks. Both the private branch exchange solution and the centrex solution are expensive particularly for small offices and home offices SOHO. The price becomes even higher if the extension lines are replaced with a radio system, such as DECT. On the other hand, in order to obtain the advantages of a private branch exchange, such as internal calls, direct dialling numbers, etc., a small office has no other alternative than to buy a private branch exchange or centrex service. Since there is little staff in a small office, and the staff is usually busy, the internal and external communication of such an office should be arranged extremely economically and with light investments. In such an office, efficient communication would also require the possibility of using wireless connections.

To solve this problem, EP 0242030, WO 88/05240 and WO 86/06570 disclose a telephone system where the required intelligence is distributed to telephones interconnected by a common broadband transmission line with a number of relatively narrow-band channels. Each telephone possesses intelligence and switching capacity to the extent that it

can switch itself through the common transmission line to any other telephone to allow bidirectional traffic. The conventional centralized switching system of star configuration is thus avoided. Such a system requires, however, a considerable amount of designing relating to the complicated channel structure, protocol and signalling used on the common transmission line, and to the software and circuit solutions required by the intelligence of the actual telephone. This as well as the complicated and expensive construction of the telephones make the system more expensive, which reduces its competitive¬ ness as compared with the conventional centralized telephone systems. It would also be difficult to introduce wireless connections.

It is an object of the present invention to provide a telephone system which has all the advantages of a private branch exchange system, but in which a private branch exchange is not required at all. A further object is to utilize wireless telephones without a separate radio branch exchange and a PBX connected thereto. Yet another object is to provide a system which has low initial costs and expanding costs, and which can be easily expanded from a system of a few telephones to a system of dozens of telephones.

These objects are achieved with a telephone system of the invention, which is characterized by what is disclosed in the independent claims. Preferred embodiments of the invention are disclosed in the dependent claims.

BRIEF DESCRIPTION OF THE INVENTION

A telephone system consists of telephones and a bus interconnecting the telephones. The channel structure of the bus and data transmission through the bus are in compliance with a radio standard. The discovery made by the applicant, that the baseband channel structure and protocols of radio standards are directly applicable to a cable bus which inter¬ connects telephones, allows the bus to be implemented without any significant extra designing efforts. Furthermore, circuits and solutions manufactured for the standard in question can be used for implementing the telephones, which lowers the price of the telephones considerably. In addition, part of the bus can be implemented as a radio path using radio parts of the standard in question for adapting baseband channels and radio channels. One of the most significant advantages of the invention is that it allows a telephone to operate as a wireless base station using radio parts of a radio standard for adapting baseband channels and radio channels. Since the same channel structure and

protocol are used on the bus and at the radio interface, no special arrangements are required in the telephone. This allows a wireless branch exchange or a combination of a wireless and a fixed branch exchange to be provided. Furthermore, since the radio interface is in compliance with a radio standard, the wireless subscriber station may be any terminal manufactured for this radio standard. The invention also allows new functions added to the radio standard to be easily introduced into the telephone system.

According to the basic principles of the invention, the bus carries baseband signalling of a radio system, and depending on the radio system used, the bus has a plurality of time division duplex channels TDD or a plurality of frequency division duplex channels FDD. Each channel can transfer one speech or data connection. At least one channel may be allocated as a signalling channel to which all the telephones listen continuously, or each channel may contain a signalling portion. One or more telephones of the system may be connected to a PSTN or ISDN network through an analogue, or preferably digital, multichannel connection, and this telephone converts the channels received from the bus according to the baseband signalling of the radio system into a form suitable for the subscriber line used, and vice versa.

A telephone comprises at least two basic parts: an actual telephone part, which is preferably an ISDN telephone, and a baseband part, which adapts speech channels of a public switched telephone network to the channels on the bus, and vice versa. A telephone may also comprise a radio part, if the baseband bus is replaced with a radio path and/or if the telephone operates as a base station for a hand-held mobile phone. Any telephone can independently allocate desired channels of the bus to itself, establish and maintain a connection, and finally release the channels. When allocating or releasing channels, a telephone sends allocation/release messages, as signalling on the bus, to other telephones connected to the bus for their information. The signalling messages on the bus can be transmitted for instance on the signalling channel or in the signalling part of the bus channels.

According to an embodiment of the invention, each telephone may have access to an up-to-date centralized or distributed database containing the individual addresses of the telephones. The addressing and numbering scheme are the same as in a public switched telephone network; in the case of an ISDN, ISDN numbering is followed. The database may also contain

information on the status of outgoing connections to a PSTN or ISDN, and information on the status of the channels of the bus interconnecting the telephones. The signalling messages transmitted on the bus update the information stored in the database in real time. The connections of the telephone system and the channels of the bus themselves may contain the above-mentioned status information, for example in a message transmitted on the signalling channel or in the signalling part of a channel, whereby it is not necessary to store the status information in the database.

When an internal call is made from one telephone of the system to another, subscriber A dials the number of subscriber B. The telephone searches for an idle channel and allocates it. Simultaneously it sends a paging to the telephone of subscriber B as signalling on the bus, whereby the telephone of subscriber B switches itself to the allocated channel. An internal call connection is thus established. Correspondingly, when an external call is made, the telephone searches for and allocates an idle channel and external line. The telephone connected to the telephone network switches the channel of the bus to the selected external line, whereby a connection is established.

According to a preferred embodiment, part of the bus is replaced with a radio path, whereby at least two telephones comprise a radio transceiver unit, which converts a channel structure used on a fixed bus into a form suitable for transmission on a radio frequency, and vice versa.

According to the preferred embodiment, baseband signalling of a general radio system, such as the CT2, DECT or GSM standard, is used on the bus. The DECT standard is recommended, since it allows 12 duplex channels to be formed on one bus when one frequency is used. Each telephone can perform the functions of the MAC layer.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail by means of preferred embodiments and with reference to the accompanying drawings, in which

Figures 1A to 1 D illustrate telephone systems of the prior art, Figure 2 is a block diagram of an ISDN telephone, Figure 3A illustrates the frame structure of the DECT system, Figure 3B illustrates the contents of a packet to be sent in a time slot,

Figure 4 illustrates a telephone of the invention, and Figure 5 illustrates a telephone system of the invention.

PREFERRED EMBODIMENT OF THE INVENTION

Figure 4 shows the essential parts of a telephone according to the invention. The basic part, which could be called the basic telephone, consists of a telephone set known per se. The telephone may be analogue, but in that case it should have good telecommunication properties. Therefore the basic telephone is preferably a conventional ISDN telephone, which already has a sufficient amount of processing capacity for the purposes of the invention. Such a telephone was described above in connection with Figure 2. At least the telephones from which the system is connected to an ISDN network comprise an ISDN interface part 12, which connects the telephone to an ISDN basic rate interface (2B+D). This provides a sufficient amount of external line capacity. All programmes needed in the telephone system of the invention are positioned in the software block 14 of the telephone. This will be discussed in greater detail below.

The ISDN telephone serving as the basic part is provided with a baseband module comprising at least a memory 41 , a processor 42, a block 44 performing the ISDN-radio system conversion, a block 46 performing multiplexing/demultiplexing, and a bus adapter 45. The radio system used is preferably the known DECT system, wherefore the baseband module is in practice a DECT telephone without a radio part and without a user interface. In the description below, it will be assumed that the radio system is DECT. The baseband module performs the functions of the MAC layer: allocating and releasing channels consisting of time slots, processing speech and data carried on channels from the ISDN part of the telephone, i.e. the public switched telephone network, to comply with the radio system used, building packets, and multiplexing them into correct time slots of the frame. The corresponding functions to be performed are demultiplexing speech and data inserted in time slots of frames, i.e. in packet form, received from the bus 47, converting information contained in the packet to the format of the ISDN standard, and finally supplying it to the ISDN part of the telephone for further processing and for forwarding it to the public switched telephone network.

In this manner the telephone converts channels of the radio system of the bus 47 into ISDN channels of the telephone network, and vice versa. Another alternative is that information received from the bus is supplied to a

PC 19 connected to the telephone, and vice versa, and a speech signal received from the handset 110 is converted into packets to be transmitted on a channel of the bus, and vice versa.

In addition to a baseband module, the ISDN telephone may comprise a radio part 43. In this case, it may serve as a base station for one or more radio telephones. When one carrier wave is used, up to 12 DECT telephones may communicate with the base station simultaneously. The fixed or dynamically adjustable part of the twelve channels of the DECT is used for the bus and the rest is used for the radio channels to DECT or GSM/DECT telephones. A multiplexer 46, for example, separates two time slots from the frame for the radio part 43, whereas the other ten are supplied to the bus 47.

The baseband module and radio part together form a known DECT telephone, the components of which may be positioned on the same board as the ISDN part. All telephones of the telephone system may be as described above.

If desired, it is possible to omit the ISDN interface part 12 and the matching transformer 11 from the telephones which have access only to the bus.

All telephones are connected to the bus 47, and the communication between telephones, and the communication between a single telephone and the public switched telephone network take place through the bus 47. Baseband signalling of the radio system standard employed is used on the bus, and the baseband part is responsible for the control, processing, synchronization, etc. The bus may be a single twisted pair cable, whereby 12 simultaneous calls can be transmitted on it when DECT is used. If the switching capacity is to be increased, a new twisted pair cable corresponding to a different frequency on the radio path is connected in parallel. Since the DECT has ten frequencies, it is possible to construct a bus with 120 channels. A telephone system may thus comprise several hundreds of telephones. The baseband module must then be expanded correspondingly to allow it to process all frequencies. In the DECT system this is fairly easy to implement.

Figure 5 shows the entire telephone system of the invention. Telephones 51 , 54, 55 and 56, which are essentially in accordance with Figure 4, are connected through their bus adapter to a common baseband bus, where the channels are formed in compliance with the radio system. Subscriber lines extend from telephones 51 and 56 through network terminals 57 to a telephone exchange 50 of a public switched telephone network. The

subscriber line is an ISDN basic rate interface (2B+D). Telephones 54 and 55 do not comprise a radio part, whereas telephones 51 and 56 comprise one. In this case, telephone 56 serves not only as a telephone with a handset but also as a base station for radio telephones d, e, and f. The bus between telephones 51 and 52 is not implemented with a fixed cable but as a radio bus. The telephones must therefore comprise a radio part (part 43 in Figure 4). The transmission powers of the DECT system allow telephones to be located at a distance of 50 to 300 metres from each other. The bus between telephones 52 and 53, in turn, is implemented with a cable, wherefore the channels are baseband channels. Telephones 53 and 52 also serve as base stations for radio telephones.

All telephones of the system are provided with the same software, stored in a software module 14. The software is used by the processor 13 of the ISDN part, the processor 42 of the baseband part, and any other processors, Figure 4. The software allows each telephone to allocate any idle channels independently and thus to establish a call to another telephone of the system or to the public switched telephone network. All telephones listen to maintenance messages of the system continuously on the signalling channel or in the signalling part of the bus channels. According to an embodiment of the invention, each telephone has access to a database containing real-time data on each channel between the telephones, and on the channels leading from telephone 51 to the telephone exchange 50 and their busy/idle status. The database is, for instance, distributed to the memory of each telephone. The signalling messages of the bus update the data contained in the database, and thus each telephone always has up-to-date information on the state of the system.

In another embodiment of the invention, information on the channels of the system and their busy/idle status is provided in connection with signalling on the channels so as to be available to each telephone. A telephone system consisting preferably of ISDN telephones therefore requires no separate database.

In the case of an incoming call to the system, telephone 51 examines to which telephone the call is addressed, searches for and allocates an idle channel, and transmits the paging on the signalling channel of the bus or in the signalling part of the allocated channel. When the call is set down, the calling, or optionally the called telephone releases the channel.

The telephone system of the invention allows all functions offered by a conventional private branch exchange telephone system to be implemented without a private branch exchange. It can be stated that the private branch exchange functions are distributed to the telephones of the system, and each telephone may serve as a private branch exchange.

In the minimum version, the system of the invention comprises only two telephones, both of which may be connected to the public switched telephone network. At the same time, these telephones may serve as base stations for radio telephones, which makes internal calls between the radio telephones possible. In this way, it is possible to implement a partly wireless telephone system of a small office.

It is possible to obtain more telephones and to interconnect them by the bus structure of the invention, which may be a physical line or a DECT radio path. A fixed or dynamically adjustable part of the twelve channels of the DECT are used for the bus, and the rest of them are used for radio channels to DECT or GSM/DECT telephones. Each telephone may be connected to the public switched telephone network either directly through an external line connected to the telephone, if there is one, or via the bus through an external line connected to another telephone. The private branch exchange function distributed to telephones performs all switching operations without that the caller has to take any measures.

The telephone system of the invention has many advantages. Since the private branch exchange function is distributed to telephones, the system is less expensive than conventional private branch exchange solutions, when the number of telephones does not exceed about 20. It is not expensive to expand the system, since it requires only a new telephone to be connected to the bus. Unlike in the conventional cabling systems in a star configuration, only one cable is required, and no special skill is required in installing the cable. Since a private branch exchange is not required, the system takes up less space.

With a general-purpose computer, the telephone can be used as a faster data transmission equipment replacing an analogue modem. DECT time slots can be allocated according to the need. When a computer is connected to more than one telephones, the computers may communicate with one another through the bus. No separate cabling interconnecting the computers is thus necessarily required.

Interconnecting telephones by a bus, for example in compliance with the DECT protocol, makes bidirectional transmission of speech in one twisted pair cable possible. A plurality of telephones may be connected in parallel to the twisted pair cable. If the cabling is complicated, part of the twisted pair cable bus may be replaced with a radio bus. If each telephone comprises a radio part, good radio coverage is achieved without any special base station designing. If the DECT protocol is used, large data transmission capacity per telephone is achieved, maximally 12x32 kbit/s. The wireless telephone is the system may be a DECT telephone or a GSM/DECT telephone, which will be introduced into the market in the near future. The cabling of an existing private branch exchange system can be utilized by connecting pairs of extension cables in parallel at the star point (the branch exchange).

It will be obvious to one skilled in the art that the basic idea of the invention can be implemented in many different ways. The invention and its embodiments are thus not limited to the examples described above, but they can be modified within the scope of the appended claims.