GB2166622A - Cordless telephone system - Google Patents

Abstract

A cordless telephone system includes a base station (15) connected to extension lines (L101 to L124) and a PABX (11) and a cordless handset (H101 to H124) corresponding to each exchange line. The base station includes ring detectors (119); a control unit (117), switches (121) and basesets (B101 to B108). In a system with 24 extension lines 24 handsets and eight basesets, the basesets transmit and receive signals over respective frequency channels, and three handsets share the channel of each baseset. With dynamic channel allocation, all of the handsets (H201 to H224) are adapted to be frequency agile and can communicate with any baseset (B201 to B208). Each baseset is allocated a different frequency channel. An idle marker tone generator (231) in the base station, under the supervision of a control unit (217), causes an idle tone to be transmitted from a free baseset. All free handsets tune to this tone; the first incoming or outgoing call seizes the free channel and the idle tone is transmitted over a new free channel. <IMAGE>

Description

SPECIFICATION Cordless telephone system The present invention relates to cordless telephone systems.

Cordless telephones have a handset which is capable of transmitting and receiving signals, usually carried by frequency modulated radio waves, and a baseset, also having a transmitter and receiver.

Signals are transmitted to the handset from the baseset at one frequency band (the frequency band currently used in this country is around 1.6 MHz) and from handset to baseset at another frequency band (currently this band is around 47 MHz).

The two frequencies used within these bands for a particular cordless telephone (handset-baseset pair) provide for full duplex working and are referred to as a "frequency channel". The main advantage of cordless telephones is that the user can be mobile: normally the baseset is stationary whilst the handset is portable and may be used anywhere in range of the baseset.

It has been proposed in W083/02380 (Ericsson) to have a wireless telephone set associated with a stationary unit in a home or office, which is connected to a telephone line. There may be other units in the same area, having access to two or more speech channels. A common, unvarying radio signalling channel which assigns an idle speech channel to a wireless telephone set -- stationary unit pair to enable a call to be made or received. This allows a greater density of subscribers to be served without interference.

Many businesses have a private branch exchange (PBX) connected to public telephone network and wired to a number of telephones at fixed locations; a typical PBX is the applicants' PABX (private automatic branch exchange) "Monarch" (Registered Trade Mark). The cost of wiring-up a suite of offices to a PBX can be high, and once wired-up, the extensions are then fixed.

The term "private branch exchange" (PABX) used in this specification and claims means any exchange (eg in an office or other premises) which is connected via one or more telephone lines to the public switched telephone network, and has a larger number of extension lines for users. A PBX therefore provides for concentration so that relatively few lines connected to the public network need to be rented, and yet the premises can be provided with access to the public network from many more extensions, if desired. A PBX also allows extension to extenion calls to be connected via the PBX without using the public network. A PBX may be, for example, a manual exchange (PMBX), an automatic (or semi-automatic) exchange (PABX), or an electronic PABX (EPABX) or computerised branch exchange (CPBX).

The present invention provides a cordless telephone system comprising a plurality of transmitting/receiving basesets operable over respective frequency channels and a larger number of transmitting/receiving handsets operable over said frequency channels, wherein the basesets are arranged for connection to a private branch exchange for communication between handsets, and access to or from the public network via the branch exchange.

Such a system has certain advantages. Facilities could be provided very quickly in new buildings, possibly as a temporary measure until completion of a wired installation. In many cases, it would be very useful to be able to provide a PBX system where telephones are needed for a short period only, such as for an exhibition or conference. A cordless system permits office reorganisation without the need for expensive rewiring. The system is especially convenient where users are likely to be moving around the building, for example from desks to computerterminal.

The system can accommodate a larger number of handsets than basesets. One alternative is for each baseset to operate over one frequency channel and for handsets (one, two or more) corresponding to that baseset to operate over the same channel.

Another possibility is for each handset to be operable over all the frequency channels and for dynamic channel allocation means to be provided so that a handset may select or be allocated a free channel when required.

The present invention also provides a base station for a cordless telephone system, comprising a number of transmitting/receiving basesets operable over respective frequency channels, a controller for controlling calls to and from a larger number of transmitting/receiving handsets operable over said frequency channels, and switching means for switching signals between handsets and respective extension lines to a private branch exchange.

The invention will now be described, byway of example, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of one embodiment of a cordless telephone system according to invention; Figure 2 is a simplified diagram of an embodiment of the base station shown in Figure 1; Figure 3 is a flow diagram giving information on the operation of a control unit for the embodiment of Figures 1 and 2; Figure 4 is a simplified diagram of a second embodiment of a base station according to the invention; Figure 5 is a diagram showing the handset configuration for a handset for use in conjunction with the base station of Figure 4; and Figures 6 to 8 are flow diagrams showing the sequence of events during call set-up and cleardown in an embodiment of a handset suitable for use with the baseset of Figures 4 and 5.

Referring to the drawings, Figure 1 shows a cordless PBX system according to an embodiment of the invention and Figure 2 shows some features of the base station of that system. The system comprises a standard PABX 11, such as the "Monarch" PABX connected to the public network via line 13. PABX 11 is also connected via twentyfour standard two-wire telephone extension lines L101 to L124 to a base station or adaptor 15. The base station 15 comprises eight basesets B101 to B108, each having two aerials (not shown), one for transmitting and one for receiving radio signals.

Each of the basesets operates over a different frequency channel. The basesets are suitably shielded from one another so that their signals do not interfere. Base station 15 need not exist physically, but the basesets B101 to B108 are conveniently located in a housing together with the other elements of the base station. The term base station is to be understood to include a number of separate basesets and other elements associated with a particular PBX.

Lines L101, L102 and L103 are connected to the first baseset B101 via a three-way switch (one of switches 121 shown in Figure 2), lines L104, L105 and L106 are similarly connected to the second baseset B102 and so forth, with three extension lines to each baseset. Twenty four handsets H101 to H124 correspond respectively to lines L101 to L124.

Each baseset and its associated handsets operate over a particular frequency channel, different from that of the other basesets and handsets. The particularfrequencychannel is assigned on setting the system up and does not vary during operation.

Note that traffic on each channel is not multiplexed.

Each handset is provided with an aeriai (not shown).

Suitable handsets are the applicants' "Hawk" (Registered Trade Mark) cordless telephones. The handsets may incorporate various optional features of cordless or ordinary wired telephones if desired.

Base station 15 may take the form illustrated in a simplified version in Figure 2. The station has a control unit 117, ring detectors 119 and an array of eight three-way switches 121. Only one baseset B101 and its thee corresponding handsets H101, H102 and H103 and lines L101, L102 and L103 are shown for clarity. In fact all twenty four lines L101 to L124 are connected to switches 121 and the ring detectors 119. The eight basesets B101 to B108 are connected to respective switches 121 and to control unit 117. Switches 121 thus provide for 3:1 concentration of the lines from the PABX.

Control unit 117 comprises on Intel 8085 microprocessor system. As indicated above the eight basesets and also switches 121 and ring detectors 119 are interfaced to the control unit. The control unit memory stores details of the switch states of switches 121. The unit also includes circuitry to detect when one of the handsets H101 to H124 is calling. Handsets H101, H102 and H103 are assigned a particular channel over which to transmit and receive, and the other 21 handsets operate in similar groups of three on the remaining seven channels. Each handset has a unique identifying security code, and the twenty-four codes are stored in the control unit memory in a look-up table for conversion to and from the telephone numbers of the handsets.The memory in unit 117 also maintains a look-up table of busy/free channels, indicating which of the eight frequency channels is busy at any time.

When a user actuates a handset H102, say, to make an outgoing call, the transmitter of that handset transmits the appropriate security code over the allocated channel by frequency shift keying to the receiver of baseset Bi 01. The code will be corrupted if the channel is already busy; if it is not the security code is forwarded to control unit 117 for verification to check that it is a valid code. The control unit also checks whether the first channel is free in the busy!free table of channels. On confirmation that the code is correct and the channel is free, a signal is sent from the control unit to switch 121 to connect baseset B101 via line L102 to PABX 11, and signals may be sent over the public network to the called equipment.The control unit then marks the channel as busy in the table. Dial tone is transmitted to the handset and the user can dial. If the call is answered, conversation takes place over the public network via the PABX and the radio frequency channel. Termination of the call is indicated by a "cleardown" signal from handset H102, which is noted by the control unit 117 and the busy/free table is altered accordingly so that any of handsets H101 to H103 may make or receive a call.

If the channel is busy (ie handsetH101 or H103 is already making a call) H102 will send its code; there will be a short burst of interference to the call taking place, and the code from handset 102 will be corrupted. There will be no verification-and no dial tone will be transmitted to handset H102. For an incoming call from an outside caller for a particular handset, H103 say, ringing current is sent via the PABX to line L103 and is detected by the appropriate ring detector 119. fling detector 119 signals to the control unit which consults the look-up table to convert the number for extension line Li 03 to the code for handset H103. The control unit 117 also checks that the first channel is free.If it is not the caller will continue to hear the ringing tone, although ringing is not passed to the handset. If desired queuing may be provided for, or the call may be answered with a loop and busy tone or a suitable announcement. If the channel is free, then a path is connected between switch 121 and baseset B101, and ringing is transmitted from the baseset to handset H103. The first channel is marked as busy in the busy/free table until the call is cleared. If the handset answers, conversation takes place via the public network, the PABX and the radio channel.

A call between two handset extensions is dealt with in a similar manner. Two basesets and two frequency channels will be involved, as two handsets sharing a channel cannot communicate with one another.

Figure 3 is a flow diagram showing some of the functions of control- unit 117. On supplying power to the control unit 117, the unit enters the idle state.

Incoming calls ("Exchange Req" in Figure 3) are detected by the ring detectors 119, which signal the control unit. The control unit responds to a signal from one of detectors 119 by translating the identity of the called line to the security code of the appropriate handset. The control unit checks that the frequency channel for that handset is free and then causes the handset to transmit the security code to the handset over its allocated channel. The "waiting for answer" state is then entered.

Reception of the security code causes the called handset to ring; the ringing signal continues until the handset answers, the caller clears down, there is time out (after three minutes of ringing) or there is some other signal, usually an erroneous one.

When the handset answers, its security code is transmitted to the baseset and the control unit verifies the received security code according to the answering scheme which has been selected (see below). If the code is acceptable, the control unit operates the three-way switch (121) to connect the calling line to the baseset and proceeds to the "waiting for action" state. If the code is not verified, the control unit continues in the ringing state until time out.

The control unit remains in the "waiting for action" state during the duration of a call until the caller clears after failing to get a reply. If the calling line clears during the speech phase, the baseset is not cleared to the idle state until the called handset clears. After clear down, or if the received carrier is lost for more than two seconds (for example if the called party moves out of range of the baseset) the control unit waits for a short time ("time out") to check for a fake clear, or a recovery of a lost carrier.

After the time out, switch 121 is opened and the control unit returns to the idle state.

If there is an incorrect ("other") signal, the control unit returns to the "waiting for action" state.

For outgoing calls ("Handset Req"), the control unit first verifies the code of the calling handset. If the code is valid, the appropriate switch (121) is operated, connecting the extension line for the calling handset to the baseset, and the control unit enters the "waiting for action" state. If the handset code is not valid, the handset request is ignored and the control unit returns to the idle state. The "waiting for action" state is terminated as for incoming calls. Any digits ("RXD") dialled by the calling handset during the duration of a call, to signal recall or requests for special services or facilities, are relayed to the calling line and to the PABX. The control unit then returns to the "waiting for action" state.

When in the "waiting for action" state, there are two call answering options. An option is selected by operating a hardware switch (not shown), which selects the appropriate software subroutines in the control unit's stored program and alters the way in which the received security code is validated when a handset answers. In "selective answering", only the called handset is allowed to answer and attempts to answer using other handsets are ignored. In "open answering", only the called handset is caused to ring, but any handset sharing the same frequency channel may answer. (In cases where large numbers of handsets share the same channel, the control unit may be programmed to permit only one or some of the handsets to answer calls to a particular handset.) With many PABXs, the clearing of the call by the calling party returns dial tone to the called party.

This could present a drawback in an open answering scheme, as a handset which has answered for another extension could originate calls on that extension. This is avoided by providing software in the control unit to prevent dial tone being sent to a handset which has answered for another extension.

The above system can be provided relatively cheaply, and offers considerable savings in cabling costs compared with a wired system having the same number of extensions. There is also the freedom to reorganise without rewiring costs and the advantaye of mobility for users. Only minor modifications to standard cordless telephones are needed, and the provision of simple three-way switches, a control unit and ring detectors. The base station can be provided as an adaptor which can be connected to a standard PABX to provide cordless handsets with the advantages of concentration, exchange to exchange calls, and any other services normally offered by the PABX to wired telephones.

It has already been mentioned that the system described above cannot provide communication between handsets sharing a channel. The facility of communication between any pair of handsets can be provided by dynamic channel allocation (DCA).

Instead of being capable of operating on a single frequency channel, each handset can operate on two or more channels. For example, if the system of Figures 1 and 2 were modified for DCA, each handset could utilise any of the eight channels. A group of handsets such as H101 to H103 would not need to be associated with a particular baseset. Any ofthe exchange lines L101 to L124 could be switched to any baseset in order to communicate with the handset corresponding to the particular exchange line.

The apparatus for a DCA system could be similar to that shown in Figure 1, with 24 extension lines from a PABX, eight basesets at a base station and 24 handsets each associated with a particular extension line. Figures 4 and 5 are simplified diagrams of the configuration of a base station and handset, respectively, adapted for DCA. 24 lines L201 to L224 are connected to a base station comprising a single 24 x 8 matrix cross-point switch 221 which acts as a concentrator for lines to L201 to L224, eight basesets B201 to B208 and 24 ring detectors 219. The basesets, switch and ring detectors are all corrected to a control unit 217 similar to that of the embodiment described with reference to Figures 1 to 3, but with some modifications.

In addition to these elements, the base station includes and idle channel marker tone generator 231 which generates an audio tone which can be transmitted on any one of eight channels to indicate that that channel is free, and a tone switch 233 connected by eight lines 235 to switch 221 and one line 237 to each of the basesets B201 to B208. Each baseset is preset to operate on one of the eight channels by selection of an appropriate crystal at installation. Each baseset B201 to B208 operates on a different frequency channel. Handsets H201 to H224 correspond respectively to lines L201 to L224 and each handset is adapted to operate on any one of the eight channels.

The configuration of one of the handsets is shown in Figure 5. All of the handsets are frequency agile and comprise a transmitting frequency generator 249 and a receiving frequency generator 251, connected to a handset transmitter 253 and receiver 255 respectively. Generators 249 and 251 suitably comprise a crystal for each frequency to be received or transmitted. The transmit frequency generator 249 supplies one of the eight channel transmit frequencies to the transmitter 253 and the receive frequency generator 251 supplies a corresponding receive frequency to the receiver 255 for conversion of an incoming received signal to the frequency of the receiver.The handsets include handset control logic 257 for co-ordinating channel changing, idle marker signal detection and security code sending and validating, means for sending and validating the security codes, 259 and 261 respectively, and an idle channel marker tone detector 263.

The control unit 217 of the base station holds a busy/free table of channels and maintains an idle tone on the first free channel by operating the tone switch 233 to permit the idle marker tone generator 231 to send the marker tone via the appropriate line 237 to the first free baseset for transmission over the channel of that baseset to the handsets. This tone indicates to the handsets that they may make calls on that channel. If no free channel is available, no free channel tone is generated. Since the tones are transmitted on the eight speech channels, all of the channels are available for speech, and no separate control channel is needed.

In the idle condition, the first free baseset emits an idle channel marker tone on its radio channel. All handsets not already busy scan all the channels to locate the tone and, once they have located it tune to the free channel. The transmitter and earpiece of each handset are disabled during scanning to prevent interference with the conversations taking place on other channels, and possible overhearing.

Handsets are prevented from transmitting if they do not detect an idle marker tone. This would normally happen if all channels are busy and no tone is being transmitted.

For an incoming call to a handset H210, say, ringing current from PABX is detected by the ringing detectors 219 on the extension line L210 corresponding to handset H210 and a signal is sent to control unit 217 which actuates switch 233 to remove the idle marker tone from the baseset transmitting it, B202, say, and marks baseset B202 as busy in the busy/free table. The control unit then prompts baseset B202 to transmit the security code corresponding to L210, thus signalling handset H210 to commence ringing. Control unit 217 identifies the next free baseset, B204, say, and actuates switch 233 to send the marker tone to baseset B204 via the line 237 between switch 233 and baseset B204.t On detecting an incoming call request (on the channel to which they are already tuned) all of the free handsets test whether the transmitted security code matches their respective internal codes.All handsets except handset H210 will not detect a match and they then search for the new idle channel marker signal transmitted from baseset B204 and locate the new idle channel. If no channel was free and no tone was being transmitted, the handsets would continue to search until a channel became free. Handset H210 detects a match, stays in the channel and begins to ring. If the call is answered, the handset sends its security code. After receipt by baseset B210 and validation by control unit 217, a speech path is set up between the caller and called party.

The procedure for setting up an incoming call as detailed above is iilustrated in Figure 6.

Figure 7 shows the procedure for an outgoing call.

A handset, H112 say, seizes the free channel to which it is already tuned by transmitting its security code which is received by the baseset transmitting the idle channel marker tone, B208 say. As soon as the control unit 217 starts to receive the code, it actuates switch 233 and causes the idle channel tone to be transmitted from the next free baseset, eg B201. This withdrawal of the tone from the original channel causes the remaining inactive handsets to search for the new idle channel. The channel of baseset B208 is marked as busy in the busy/free table.

If two handsetswereto attempt to seize a channel simultaneously, the code received by the control unit 217 would be corrupted and no dial tone would be transmitted to either handset. Dialled digits cannot be transmitted until the dial tone has been received, so the handset transmitters would be switched off and on again for a further attempt to seize a free channel. The chances of a second simultaneous attempted seizure are extremely low.

When dial tone has been received from the PABX, the transmission of dialled digits to baseset B208 takes place and the digits are sent via switch 221 and line L21 2 to the PABX.

Calls are cleared as indicated in Figure 8 by the handset sending a "clear" signal to the base station on the channel it has been using, after completion of the call. This causes the carrier to be removed from the handset and when the baseset detects the absence of a carrier (either due to an intentional clear or due to the handset moving out of range of the baseset) it clears down the path and changes the busy/free table to mark the channel as free.

It will be appreciated that in this embodiment, any handset can call any other, provided there are at least two free channels.

If all the channels are in use, a further incoming call may be left ringing, put in queue or given a busy signal.

Other optional features may be provided if desired, by suitable programming of the control unit, basesets or handsets. For example, the control unit may be programmed to send an incoming call to another extension if there is no reply from the required extension. This is a useful additional feature for Strowger-type PBXs which do not have facilities for call-diversion. In some circumstances it may be useful to give an alarm signal if there is no reply from an extension. Supplementary facilities such as call-forward, ring again etc may be included if desired.

The two alternative call answering schemes mentioned above may be provided in the DCA system of Figures 4 and 8.

It will be noted that in the embodiments described with reference to the drawings the base station functions as an adaptor which may be a discrete unit and can be connected to any PBX having enough extension lines.

Frequency bands other than those mentioned above may be used for transmission between basesets and handsets in systems according to this invention. For example, the 900 MHz band could be used.

Frequency channels and transmitter/receivers currently used for cordless telephones typically have a range of 20-100 m. The area covered by a single system may be increased by providing multiple basesets connected to a primary base station comprising a baseset for each frequency channel used by the systerrt (together with ring detectors, switch and control unit if used). The range can be increased or tailored to a desired configuration by use of a leaky feeder connected to the transmitter/receiver of the basesets via a suitable combiner.

In the embodiment of Figures 1 to 8 there has been a concentration factor of 3:1, ie three handsets to one baseset (on average for the DCA embodiment). The concentration factor chosen will depend on the expected traffic, and could be, for example, 8:1 where relatively low traffic is expected.

The systems described above had eight channels available; in many cases smaller numbers of channels, for example three or four, may be acceptable.

Claims (13)

1.A A cordless telephone system comprising a plurality of transmitting/receiving basesets operable over respective frequency channels and a larger number of associated transmitting/receiving handsets operable over said frequency channels, wherein the basesets are arranged for connection to a private branch exchange.

2. A cordless telephone system as claimed in Claim 1, including a private branch exchange and an extension line corresponding to each handset between the basesets and the private branch exchange.

3. A cordless telephone system as claimed in claim 2, including a base station comprising said basesets.

4. A cordless telephone system as claimed in claim 3, wherein said base station includes a controller for controlling calls to and from the handsets.

5. A cordless telephone system as claimed in claim 4, wherein the base station includes switching means controlled, in use, by the controller for switching calls to and from the handsets between the corresponding extension lines and the basesets.

6. A cordless telephone system as claimed in claim 4 or claim 5, wherein the controller is adapted to maintain a busy/free table of channels.

7. A cordless telephone system as claimed in any one of the claims 3 to 6, wherein each handset has an identifying security code and the controller is adapted to check said security codes.

8. A cordless telephone system as claimed in any one of the claims 3 to 7, wherein the base station includes ringing detectors for detecting ringing signals.

9. A cordless telephone system as claimed in any preceding claim, wherein each baseset is adapted to operate over a single frequency channel and one or more handsets correspond to that baseset and are adapted to operate over the same frequency channel.

10. A cordless telephone system as claimed in any one of claims 1 to 8, wherein each baseset is adapted to operate over a single frequency channel and each handset is adapted to be selectively operable over any one of said frequency channels, and dynamic channel allocation means are provided to identify a free frequency channel for transmission or reception of a call.

11. A cordless telephone system as claimed in claim 10, wherein the dynamic channel allocation means includes a tone generator adapted to send signals for transmission by a free baseset over the channel of that baseset, and the handsets are adapted so that all free handsets tune to the channel of the free baseset for reception or transmission of a call via that baseset.

12. A base station for a cordless telephone system, comprising a number of transmitting/ receiving basesets operable over respective frequency channels, a controller for controlling calls to and from a larger number of transmitting/ receiving handsets operable over said frequency channels, and switching means for switching signals between handsets and respective telephone extension lines to a private branch exchange.

13. A cordless telephone system substantially as hereinbefore described with reference to Figure 1 to 3, or Figures 4 and 8 of the accompanying drawings.