Abstract:
A telephone instrument comprises a mechanism for providing signals for transmission, a mechanism for converting signals received by the instrument into output signals, a switching circuit for selectively establishing and terminating a connection between the instrument and a circuit-switched telephone network and for selectively establishing and terminating a connection between the instrument and a packet-switched communication network. When calling, the instrument signals over the circuit-switched network for a network address for the called instrument and the packet-switched network and establishes a dual connection over the packet-switched network if such a network address is received. When receiving a call, the instrument can terminate the call connection over the circuit-switched network and continue the call by way of the packet-switched network.

Description:
FIELD OF THE INVENTION 
     The present invention relates to telephony and to telephone instruments which have dual circuit capability. 
     BACKGROUND TO THE INVENTION 
     Most telephone networks in use at the present time are circuit switched networks, in which telephone subscribers have, at least to a local switching station, a dedicated line identified by a subscriber&#39;s number. Connection to the network is achieved by a switch closing a circuit including the subscriber line to the local switching station. Telephone calls may be conveyed from switching station to switching station by a variety of links, but connection is established for the call to the ultimate recipient by means of circuit switching controlled by the generation of pulse trains or tones representing the recipient&#39;s telephone number. Although telephone messages, whether voice calls or other types of call, such as facsimile transmissions, may be conveyed between switching stations by various time division, frequency division or even packet-switched communication links, as far as the end stations, the telephone instruments, are concerned they make connections to an analog circuit-switched network. Owing to the global nature of the existing circuit-switched networks, their use will obviously continue for many years and telephone instruments are necessarily adapted to establish connections by way of the familiar circuit-switched network. 
     In recent years there has been increasing use, first in local area networks and then on a wider scale, of packet-switched networks, of which the ‘Internet’ is the best known. The present invention is based on an appreciation that the usage of packet-switched networks will provide a potential alternative communication link between telephone subscribers. 
     The present invention envisages a new form of telephone instrument, typically a handset capable of making voice calls, which has a dual circuit capability in that it is capable of establishing a call connection over the ordinary, circuit-switched telephone network but may, if desired automatically, transfer the call connection from the circuit-switched telephone network to a packet-switched network. 
     SUMMARY OF THE INVENTION 
     An instrument according to the invention would normally possess, in common with existing instruments, some means for providing signals for transmission and some means for converting signals received by the instrument into output signals. The aforementioned means may, in the case of a telephone handset instrument, include the usual microphone and loudspeaker and the usual dialing equipment or tone generation equipment by means of which an intended recipient&#39;s number can be transmitted in the format required by the circuit-switched network to the local switching station in order to establish a call connection with an intended recipient. 
     An instrument according to the invention will, however, further comprise not only a switching circuit for selectively establishing and terminating a connection between the instrument and the circuit-switched telephone network but further means for selectively establishing and terminating a connection between the instrument and a packet-switched communication network. The switching circuit for this purpose may include appropriate multiplexing equipment interposed between the call generating and receiving circuits and the terminals or connection points for the telephone network and the packet-switched network. 
     Further, the telephone instrument preferably comprises control means including means operable to define or obtain a network address of the instrument in the packet-switched communication network, the control means being operable: 
     (i) to control the switching circuit to establish a call connection to another instrument on the telephone network; 
     (ii) in response to receipt of an acknowledgement signal including the network address of the other instrument to control the switching circuit to make the call connection to the other instrument by way of the packet-switched network. 
     The instrument may signal information including its network address over the initial call connection. Alternatively, it may obtain its network address only after receipt of the said acknowledgment from the other instrument. 
     The instrument may further comprise control means operable on receipt of a call by way of the circuit-switched network to provide an acknowledgement including the network address of the instrument, and on receipt of a subsequent message over the packet-switched network to terminate connection over the circuit-switched network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of two telephones having dual connections; 
     FIG. 2 is a schematic drawing of a telephone according to the invention coupled to both a public-switched telephone network and a packet-based network; 
     FIG. 3 illustrates the operation of call logic at a calling station; and 
     FIG. 4 illustrates the operation of call logic at a called or receiving station. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates broadly the intended operation of the invention, which assumes the continued existence of the familiar circuit-switched networks. When therefore a calling instrument (hereinafter called ‘station’) needs to communicate with a second instrument (station)  2 , station  1  will initially use a circuit-switched line  3 , since there is a presumption that at least in the foreseeable future the remote station  2  will be connected to the public-switched telephone network. If however the called station  2  is connected to a packet-switched network  4  and the calling station  1  is also connected to the packet-switched network, then the call may be completed over the packet-switched network. 
     Whether a call should be completed over the packet-switched network  4  may be a matter of selection and programming and may also be dependent upon whether the cost of a call per unit time is cheaper or more expensive over the packet-switched network  4  than it is over the ordinary telephone line  3  which proceeds through the public-switched telephone network. It will be assumed in the following that at least the calling instrument or station  1  is configured or programmed to attempt to establish a packet-switched call connection with the remote station  2  if it is feasible to do so. 
     FIG. 2 illustrates a calling station  10  which includes a telephone instrument  1  comprising a familiar handset  11  and a keypad  12 . The instrument  1  is connected to a voice codec  13  which takes analog signals from the handset, digitizes them and passes the digitized data to a central processing unit (CPU)  14 . The CPU may be of the digital signal processor type. It is employed to process signals both from the handset  1  via the voice codec  13  and also a packet interface  15  and to couple signals from a PSTN interface  16 . The packet interface may be any one of a variety of packet interfaces, such as an ASDL interface or an Ethernet interface. The packet interface, in this example, transmits Ethernet packets in which the message data is constituted by voice data obtained by way of the voice codec  13  and CPU  14  from the instrument  1 . 
     FIGS. 3 and 4 illustrate the operation of a station  10  when operating as a calling station and a called or receiving station respectively. 
     In the system broadly shown in FIG. 1, and on the assumption that at least telephone  1  is disposed in a station  10  having a facility for communicating by way of the packet-switched network  4 , the operation is as follows. 
     User A at station  10  including instrument  1  calls a user B, having telephone  2  by means of obtaining the keypad information entered by the keypad  12  and calling via the public-switched telephone network, as shown at stages  30  and  31  of FIG.  3 . If the call did not connect (stage  32 ), a fresh attempt is made to initiate the call (stage  30 ). In any case, the CPU will collect the digits from the keypad and signal them down the PSTN interface  16  to the public-switched telephone network. 
     Once a circuit has been established by way of the public-switched network, users A and B will be able to talk over the circuit as in a standard telephone call. Once the call is established, the originating station will attempt to signal the receiving station to establish whether the receiving station has a packet-switched network that could be used for continuing the call. A basic signaling method that will work on all types of public-switched telephone networks is in-band with the voice signal. One method is to employ DTMF signals to send information from A to B, namely a sequence of tones along with the user&#39;s voice data which indicates to the receiving station that the sending station  10  is equipped with a secondary packet interface  14 . This is shown by stage  33  in FIG.  3 . 
     If the receiving station is not equipped with the logic to respond to the DTMF tones, the sending station will continue to communicate over the PSTN network, as shown by stages  35  until the call is completed (stage  36 ). If however the receiving station is equipped with the appropriate logic, it can respond to the sending station by indicating the network address on the packet network (stage  34 ). This address might be fixed and unchanging or might be assigned dynamically in some way. One known method to assign addresses dynamically is to have the station communicate to a centralized entity that will use a protocol to communicate to calling station a free address. This communication of the address (however allotted) of the receiving station B is communicated over the packet-switched network to the packet interface  15  and the CPU  14 . This is shown by stage  34  in FIG.  3 . 
     Once the called station has responded to the calling station with its packet network address the calling station can attempt to obtain a connection over the packet network. Using the packet network address obtained from the called station, the calling station will attempt to connect over the packet network. If the calling station can establish a connection then digitized voice data may be sent from instrument  1  by voice codec  13  and CPU  14  over the packet network. Optionally, the called station can verify the quality of the packet link to see whether it is adequate for both voice calls. The quality check may determine latency, jitter, reliability or some other attribute of the link. 
     Thus as shown in FIG. 3, there is establishment of the packet connection at stage  37  and the sending of voice data over the packet link (stage  38 ) until the call is completed. 
     With two links established, one over the circuit-switched network and one over the packet-based network, the called station can decide which route to choose. A variety of methods may be used to perform the change over from voice on the public-switched telephone network to voice data on the packet network. For example, the called station may determine when it is appropriate to switch over from the packet-switched telephone network to the packet network. The called station will analyse the voice information arriving on the packet-switched telephone network interface and wait for silence in the voice call. At this point the voice information that is arriving on the packet interface can be sent to the handset without the user being aware of the switch-over. At this point the packet-switched telephone network call can be terminated and the remainder of the call connection can continue over the packet network. 
     Thus, as shown in FIG. 4, the station  4  logic operates as follows. 
     Stage  40  indicates the rest state where the station is waiting for a call. Stage  41  indicates the answering of the call over the public-switched telephone network. Stage  42  performed in the CPU  14  requires a determination of whether the caller requested the network address of the station. If no such request was made, the call over the public-switched telephone network is continued until the call is terminated (stage  43 ). 
     If the caller requested the network address for the called instrument, the address is returned (stage  44 ) over the public-switched telephone network using DTMF tones or other mechanism. 
     If the call arrived on the packet interface from the calling IP address (stage  43 ) the call is connected over the packet network (stage  46 ). The connection quality of the packet is verified (stage  47 ) and there is a further determination to see whether the quality is acceptable for the packet voice path (stage  48 ). If the call did not arrive from the packet interface from the calling IP address or the voice quality is not acceptable for the packet voice path, the call continues over the public-switched telephone network until hang-up. If however the call did arrive on the packet-based network and the voice quality is acceptable for the packet voice path, the called station analyses the public-switched telephone network and packet data for silence (stage  49 ) and then terminates the call over the public-switched telephone network by operation of the CPU until the call is complete, when the packet connection is dropped (stage  50 ).