Field Of The Invention

The present invention relates generally to a method and apparatus for cellular payphone signalling. More specifically, the present invention relates to a cellular payphone signalling method and apparatus for accurately determining the start of a telephone call originated at a cellular payphone.

Background Of The Invention There is an increasing need for cellular payphone service, particularly in regions without highly developed wire-based telephone networks. The implementation of payphone service within a conventional cellular telephone system, however, entails certain difficulties due to the nature of operation of conventional cellular systems.

Upon placing a call from a cellular payphone, the caller first listens to a ring-back tone while waiting for the called party to answer. When the called party does answer, the only indication provided to the calling cellular telephone is of an audible nature; i.e., the ring-back tone ceases and the called party begins to speak. The cellular switching equipment provides no indication to the calling cellular telephone, or to the base station of the cell from which the call was made, that the call has been answered. Even though such signalling is available from the wire-based public switched telephone network, it is either ignored by the cellular switching equipment entirely, or not forwarded on to the cellular base station and ultimately to the call-originating cellular telephone.

For calls originating from a cellular payphone, the above-described limitation in the operation of conventional cellular systems creates the problem of

determining when and if to charge the caller for the call. A payphone caller does not expect to be charged for a call which is not answered, and if the call is answered, the caller expects to be charged beginning with the answering of the call and not sooner.

One approach to the above-described problem has been to use voice-recognition means in a cellular payphone to detect the beginning of a conversation. The call is considered answered, and thus charging commences, when a voice signal is detected on the line. This approach, however, is not entirely accurate due to the occurrence of random events which could be mistaken for the start of a two-way conversation, for example, as when the calling party speaks before the called party answers. As such, the need exists for reliably and accurately signalling a cellular payphone of the answering of a call originating from the payphone so that the calling party is properly charged for the call.

Summary Of The Invention

An object of the present invention is to provide a means for conveying signalling to a cellular payphone indicating that a call originating from said payphone has been answered, thereby enabling said payphone to reliably and accurately charge the calling party for the call.

In accordance with the present invention, a cellular payphone signalling interface circuit is provided between a cellular switch and a public switched telephone network (PSTN) for providing to a call- originating cellular telephone, signalling information indicating that the call has been answered by the called party. The interface circuit of the present invention is particularly intended for providing such signalling information to cellular pay-telephones, although it can be used in conjunction with any kind of telephone.

In an embodiment of the present invention, the interface circuit operates in conjunction with specially

adapted cellular payphones. In this embodiment, before transmitting to the cellular telephone system a call set¬ up message with the telephone number of the called party, the payphone adds a predetermined prefix to the telephone number. The prefix indicates to the cellular switch handling the call that the call originated from a cellular payphone. The cellular switch, in turn, routes the call to an outgoing trunk coupled to the signalling interface circuit of the present invention. The interface circuit forwards the call to the PSTN which ultimately routes the call to the called party. The PSTN then generates an audible, in-band ring-back signal which is sent back to the payphone as an indication to the caller that his call has been routed to the desired destination and to await the answering of the call by the called party.

When the called party answers the call, the PSTN provides out-of-band signalling, over a signalling channel of the trunk handling the call, which indicates that the call has been answered. The signalling interface circuit of the present invention, which monitors said signalling channel, is thereby informed that the call has been answered. The interface circuit, in turn, generates an audible in-band activation message, comprised of a sequence of predetermined dual-tone multi- frequency (DTMF) signals, which message is sent to the payphone from which the call originated. The payphone receives and decodes the sequence of DTMF signals, and upon determining that the received DTMF signals represent the activation message, then enables two-way voice communication and begins to charge the caller for the call.

The apparatus and method of the present invention therefore provide a reliable and accurate in- band indication to a cellular payphone of the answering of a call originating from said payphone.

Brief Description Of The Drawings

Figure 1 is a block diagram showing the major components and events involved in processing a cellular payphone call in accordance with the present invention. Figure 2 is a block diagram showing the major functional blocks of an exemplary embodiment of a cellular payphone signalling interface circuit in accordance with the present invention.

Detailed Description

Figure 1 is a block diagram showing the major components involved in the processing of a call originating from a cellular payphone, in accordance with the present invention. As shown in Figure 1, a call originating from a payphone 1 is first routed to a cellular transceiver 2 in radio communication with a cellular base station 3. The cellular transceiver 2 can either be integrated into the payphone 1 or exist as a separate unit (as shown) with an interface to the payphone 1. The cellular transceiver 2 and the base station 3 communicate in a known manner, using a conventional cellular telephony protocol. The cellular base station 3 is coupled to a cellular switch 5, typically over one or more intervening radio links 4. Typically, there are multiple cellular base stations coupled to the cellular switch 5 and multiple payphones distributed among the cells covered by the base stations. For the sake of simplicity, only one payphone 1 and one base station 3 have been shown in Figure 1. The cellular switch 5 routes calls from the base station 3 to at least one outgoing trunk 6a which is coupled to a cellular payphone signalling interface circuit (CPSIC) 10, in accordance with the present invention. The CPSIC 10 is typically located in close proximity to the cellular switch 5, such as in the same equipment cabinet, for instance. An exemplary embodiment of the CPSIC 10 will be described in greater detail below.

All voice and signalling information on outgoing trunk 6a is coupled through by the CPSIC 10 to a trunk 6b which is coupled to a public switch telephone network (PSTN) 8, typically over one or more radio links 7. While the CPSIC 10 monitors signals on the trunks 6a and 6b and inserts signals onto the trunk 6a, as described more fully below, the CPSIC 10 is essentially transparent to the conventional signalling and voice data transmission which occurs on the trunks 6a and 6b. Once the signalling pertaining to the call originating from the payphone 1 is transmitted to the PSTN 8, the PSTN 8 ultimately routes the call to a called subscriber telephone 9.

In addition to the major components involved in the processing of a cellular payphone call in accordance with the present invention, Figure 1 also depicts the major events occurring in the processing of such a call. As shown in Figure 1, at step 1, a user of the payphone 1 dials a telephone number to which he wishes to place a call. At step 2, the payphone 1 adds a prefix, for example, the signals representing the dialpad key sequence *#*, to the number dialed by the caller. A number of different prefixes could be used so long as the prefix used is not a prefix used by other services or other call-originating telephones and would not be confused as originating from anything other than a cellular payphone. Modifying a conventional cellular payphone to include the capability of adding such a prefix to each number dialed is a straight-forward matter, which in some cases may simply entail the modification of software or firmware in the payphone.

The payphone 1 sends the dialed telephone number with the prefix to the cellular transceiver 2, on to the cellular base station 3, and eventually to the cellular switch 5. The communication of the dialed number and prefix from the payphone 1 to the cellular switch 5 is carried out in a known way, using a known cellular telephone protocol and format.

At step 3, the cellular switch 5 analyzes the received dialed number and prefix and determines from the prefix that the call originated at a cellular payphone. The cellular switch 5 is programmed to route all calls which it determines to have originated from a payphone to the outgoing trunk 6a which is coupled to the CPSIC 10. The CPSIC 10 monitors the trunk 6a and passes it on, over trunk 6b, to the PSTN 8. In addition to a plurality of voice data carrying channels, the trunks 6a and 6b include a signalling channel for carrying signalling data indicating such events as the answering of a call carried by the trunk. An example of such a trunk is the El type trunk.

It should be noted that the prefix added by the payphone 1 is used only by the cellular switch 5, which strips the prefix from the dialed number before forwarding the dialed number over the outgoing trunk 6a. Any modern cellular switch can be programmed to route calls destined for telephone numbers bearing a predetermined prefix to a particular outgoing trunk or group of trunks.

As stated above, the call is then routed through the CPSIC 10, and via the trunk 6b to the PSTN 8, which ultimately routes the call to the called subscriber telephone 9. As indicated in step 4a, the called subscriber telephone 9 rings, while the PSTN, in step 4b, sends an audible ring-back tone back to the calling payphone 1. At step 5, the caller hears the ring-back tone and awaits the answering of his call by the called subscriber.

At step 6, the called subscriber telephone 9 goes off-hook, thus answering the call originating from the payphone 1. In response to the going off-hook of the called subscriber telephone 9, the PSTN 8, at step 7, sends an answering supervision signal over the signalling channel of the trunk 6b, which signal is received by the CPSIC 10, at step 8. Upon receiving the answering supervision signal, the CPSIC 10, at step 9, transmits to

the payphone l, in-band, an activation message comprised of a preselected sequence of DTMF signals. The payphone 1 receives the DTMF sequence at step 10 and upon determining that the received sequence represents the activation messages, enables two-way voice conversation, at step 11, and performs the necessary steps to commence charging for the call.

In order to detect the activation message DTMF sequence from the CPSIC 10, the payphone 1 must include a DTMF detector and the associated circuitry for recognizing the correct DTMF sequence and for enabling the voice paths and commencing charging. The DTMF detector that is used in the payphone 1 must have sufficient sensitivity to correctly detect DTMF signals transmitted from the CPSIC 10, which could be located at a substantial distance, spanning several links, from the payphone 1. Moreover, such a DTMF detector must be capable of rejecting spurious signals of short duration, such as bursts of speech, which could be mistaken for DTMF signals.

In an exemplary embodiment of the present invention, the predetermined DTMF sequence consists of the DTMF signal for the "A" key, followed by a silent interval, followed by the DTMF signal for the "C" key. The "A" and "C" keys are represent by two of the sixteen possible combinations of the two sets of four tones which make up the set of DTMF signals. The DTMF signals for the "A" and "C" keys are preferred for the activation message DTMF sequence because those keys are not included on a typical telephone dialpad. Moreover, using two DTMF signals, as opposed to only one, reduces the possibility of mistaking spurious tones or noises for the activation message DTMF sequence.

An exemplary embodiment of the cellular payphone signalling interface circuit (CPSIC) 10 of the present invention will now be described in greater detail with reference to Figure 2, which is a block diagram showing the major functional components of such a device.

As described above, the CPSIC 10 is coupled to the cellular switch 5 via one or more trunks 6a and to the PSTN 8 via one or more trunks 6b. The embodiment of Figure 2 is shown to interface with only one trunk 6a and one trunk 6b, for the sake of simplicity, but it should be readily apparent how to modify the circuit to handle multiple trunks.

In the exemplary embodiment, the trunks 6a and 6b are El type digital trunks, with each El trunk carrying 30 64Kbps voice channels, signalling, and synchronization information in a known format, at an overall bit rate of 2.048Mbps. It should be clear that the present invention can also be adapted for use with other known digital trunk formats, such as the Tl format. The CPSIC 10 is coupled to each of the trunks

6a and 6b via coupling transformers liar and Hat, and coupling transformers llbr and llbt, respectively. The coupling transformer pairs 11a and lib are coupled, respectively, to PCM trunk interface devices 12a and 12b, which are in turn coupled to a digital matrix 13, described more fully below. The coupling transformers liar and llbr are used to couple trunk signals received by the CPSIC 10, while coupling transformers Hat and llbt are used to couple trunk signals transmitted from the CPSIC 10. Suitable commercially available transformers which can be used for the receive transformers liar and llbr, and the transmit transformers Hat and llbt, are respectively the model TFS2574-4 and TFS2915-0 transformers manufactured by Filtran. Of course, the actual coupling transformers used will depend on the characteristics of the trunk and interface devices.

In the exemplary embodiment of Figure 2, the trunk interface devices 12a and 12b receive and transmit signals to and from the trunks 6a and 6b in a known

"HDB3" format and communicate with the digital matrix 13 over known "ST-BUS" format interfaces. On the ST-BUS interface, signalling and synchronization data, and voice

data are provided on different lines, whereas in the HDB3 format, signalling, synchronization, and voice data are provided in one serial bit stream. The trunk interface devices 12a and 12b convert HDB3 signals received from the trunks 6a and 6b to the ST-BUS format and transmit said converted signals to the matrix 13, and convert ST- BUS signals received from the matrix 13 to the HDB3 format for transmission to the trunks 6a and 6b. In the exemplary embodiment, an MH89790 integrated circuit, manufactured by Mitel Semiconductor of Kanata, Ontario, Canada is used for each of the PCM trunk interface devices 12a and 12b.

The digital matrix 13, under the control of a microprocessor 18, interconnects the outgoing trunk 6a from the cellular switch with the trunk 6b to the PSTN 8. In the exemplary embodiment, the digital matrix 13 is implemented using an MT8980D digital switch integrated circuit, manufactured by Mitel. The digital matrix 13 has a plurality of ST-BUS inputs, denoted ST _r , and a plurality of ST-BUS outputs, denoted ST ₀ . Additionally, the matrix 13 includes a bi-directional control bus interface which is coupled to a microprocessor 18. As explained more fully below, the microprocessor 18, with its associated ROM 19 and RAM 20, provides the intelligence for the operation of the matrix 13. Each of the ST-BUS inputs ST _X to the matrix 13 can handle a 2.048Mbps serial bit stream (the bit rate of an El trunk) organized as 32 64Kbps serial bit streams, or channels. Likewise, each of the outputs ST ₀ from the matrix 13 carries 32 64Kbps channels. Under the control of the microprocessor 18, the matrix 13 can route any 64Kbps channel received by any one of the S^ inputs to any 64Kbps channel output by any one of the ST ₀ outputs. As shown in Figure 2, voice and signalling data from the trunks 6a and 6b are provided to separate S^ inputs of the matrix 13 by the trunk interface devices 12a and 12b, respectively. The voice and signalling data input to the matrix 13 is then routed to ST ₀ outputs of the matrix 13

and provided to the interface devices 12a and 12b for transmission to the trunks 6a and 6b, respectively.

A further ST-BUS input S^ of the matrix 13 is coupled to the digital outputs of two A/D converters 16a and 16b. The analog inputs of the A/D converters 16a and 16b are coupled, respectively, to the outputs of DTMF generators 17a and 17b. The DTMF generator 17a is configured with the use of DIP switches, for instance, to generate the DTMF signal for the "A" key, whereas the DTMF generator 17b is configured to generate the DTMF signal for the "C" key. The A/D converters 16a and 16b digitize the analog DTMF outputs of the DTMF generators 17a and 17b, respectively. Under the control of a selector 21, each A/D converter alternately outputs its respective digitized DTMF signal to the matrix 13 as a serial 64Kbps channel. The selector 21 enables the A/D converter 16a to provide its digitized DTMF output on one of the 32 serial channels of the STi input of the matrix 13 to which the A/D converters are coupled, while the selector 21 enables the A/D converter 16b to provide its digitized DTMF output on another of the 32 channels of said STi input.

Suitable commercially available codecs such as the MT8965, manufactured by Mitel, can be used for the A/D converters 16a and 16b. A suitable commercially available DTMF generator is the TCM5089N, manufactured by Texas Instruments, Inc. of Dallas, Texas.

In addition to being able to route any channel of an ST _A input to any channel of an ST ₀ output, the digital matrix 13 can also make any input channel readable at the control bus interface and any output channel writable from the control bus interface. The microprocessor 18, which as described above is coupled to the control bus interface of the matrix 13, is programmed to monitor the signalling data on the trunk 6b in order to determine from the signalling data whether a call carried on one of the trunk's 30 voice channels has been answered.

Upon determining that a call originating from the cellular payphone 1 has been answered, the microprocessor 18 controls the digital matrix 13 to route, in the sequence of the activation message, the digitized DTMF signals from the A/D converters 16a and 16b to the appropriate voice channel on the trunk 6a for transmission to the payphone 1. In other words, the microprocessor 18 controls the matrix 13 to first route the digitized DTMF "A" signal to the voice channel on the trunk 6a which is transmitted to the payphone 1. After a predetermined period, the microprocessor 18 then controls the matrix 13 to disconnect the digitized DTMF "A" signal from said voice channel. After a predetermined interval, the microprocessor 18 controls the matrix 13 to route the digitized DTMF "C" signal to said voice channel, for a preselected further period. It should be noted that the periods during which the "A" and "C" DTMF signals are routed to the payphone 1, and the gap interval between the them, should be long enough to allow the DTMF detector in the payphone 1 to properly detect the signals and the silence. In the exemplary embodiment, the signal periods and the gap interval are each 200ms.

As discussed above, when the cellular payphone 1 receives the activation message DTMF sequence, it enables voice communication and commences charging for the call.

As discussed above, all outgoing calls from cellular payphones which are handled by the cellular switch 5, are routed by the cellular switch 5 to the trunk 6a. It is possible, however, to program the cellular switch 5 so that some of the channels of the trunk 6a are used for outgoing calls and some for incoming calls. Because the CPSIC 10 is concerned only with calls originating from a cellular payphone, the microprocessor 18 is programmed to monitor the signalling data of only outgoing channels on the trunks 6a and 6b. As such, the microprocessor is programmed in accordance with channel assignment data used by the cellular switch

5 to assign each channel as an outgoing or an incoming channel. The microprocessor 18 can also be programmed to determine, independently of said assignment data, whether a channel is an incoming or an outgoing channel by scanning each channel and following the flow of signalling information.

A suitable commercially available device for implementing the microprocessor 18 is the Z-80 microprocessor manufactured by Zilog Inc. of Campbell, California. In order to provide the necessary performance required of the microprocessor 18, the Z-80 microprocessor can be operated with a master clock frequency of 1.79MHz.

Clocking signals for the PCM trunk interface devices 12a and 12b, the digital matrix 13, the D/A converters 16, and the selector 21 are generated by a phase locked loop (PLL) 15 which is coupled to a master clock generator 14 and to a trunk timing output of the PCM trunk interface device 12b. The PLL 15 generates the clocking signals using a 16.384MHz master clock signal from the master clock generator 14, and a trunk timing signal recovered by the PCM interface device 12b from the trunk 6b, such that the clocking signals generated are synchronized to the timing of the trunk 6b. A suitable commercially available device which can be used to implement the PLL 15 is the MT8941 integrated circuit manufactured by Mitel.

It should be noted that the apparatus and method of the present invention can be used for calls originating from the cellular payphone 1 to other cellular telephones, whether the called cellular telephones are coupled to the same cellular switch 5 or to a different cellular switch. In the case where the originating cellular payphone and the called cellular telephone are coupled to different cellular switches, the call will be carried over trunks similar to those coupling the cellular switch 5 with the PSTN 8, and the cellular switch of the called cellular telephone will act

13 like the PSTN 8, generating the same type of signalling to indicate when the call has been answered. Therefore, in this case, a CPSIC 10 placed on a trunk coupling the cellular switches will provide the same functionality as though it were placed on a trunk between the cellular switch 5 and the PSTN 8, as described above.

In the case of calls from the cellular payphone 1 to another cellular telephone coupled to the same cellular switch 5, a CPSIC 10 is placed between two trunks coupled to the switch 5. The first trunk is designated as an outgoing trunk and the second trunk is designated as an incoming trunk. Therefore, any calls from the cellular telephone 1 will be routed through the cellular switch 5, the outgoing trunk, the CPSIC 10, back through the incoming trunk to the cellular switch and finally to the called cellular telephone. In this case, the cellular switch 5 provides the necessary signalling over the incoming trunk to the CPSIC 10 to indicate when the called cellular telephone has answered.