Abstract:
Conveying Push-To-Talk and Push-To-Signal (PTT/PTS) information over an IP network through signaling bits defined in and propagated with the Real-Time Protocol (RTP) stream allows the PTT/PTS information to remain synchronized with the voice packets of the stream. The maintenance of synchronization of PTT/PTS signals and voice packets eliminates problems arising from failure of synchronization between the two classes of packets, notably the “shoot/don&#39;t shoot” problem. The signaling bits employed were formerly used to convey legacy network information, but are no longer in active use in the Voice-over-IP (VoIP) realm, since VoIP provides alternative pathways for their information.

Description:
RELATED APPLICATIONS  
       [0001]     This patent claims the benefit of the filing date of U.S. Ser. No. 60/838,282 filed Aug. 17, 2006 whose entire contents are incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention concerns the signaling of telephone call information in an Internet-protocol network, and more specifically the signaling of PTT/PTS (Push-To-Talk/Push-To-Signal) telephone call information in an Internet-protocol network.  
       BACKGROUND OF THE INVENTION  
       [0003]     PTT/PTS is used in many government and military networks to control muting of a telephone handset&#39;s transmitter and to signal connected equipment such as radios that their transmitters are to be keyed (i.e., that their transmit functions are to be enabled) When a connection is established between a telephone having a PTT/PTS capability and a radio requiring transmit keying, the radio&#39;s transmit function will be turned on when the PTT button is depressed on the telephone, and turned off again when the PTT button is released. PTT reports the state of the phone&#39;s transmitter, and PTS requests keying of a remote transmitter. In practice in general usage, PTT and PTS often coincide.  
         [0004]     In most existing communications networks such as TDM (Time Division Multiplex), transmission delays are minimal, and the PTT/PTS information can be carried in parallel with the voice using either CAS (Channel-Associated Signaling) or through a separate message-based interface such as ISDN PRI (Integrated Services Digital Network Primary Rate Interface). Since delays are small and predictable, this method of conveying the PTT/PTS information is satisfactory.  
         [0005]     However, IP networks generally have higher delay (latency) and greater variability in latency than many conventional legacy networks. Unless steps are taken to prevent it, such networks are prone to what is commonly known as the “shoot/don&#39;t shoot” problem. Specifically, a PTT/PTS phone user depresses the PTT button on the handset and speaks the words “don&#39;t shoot”, but the distant radio&#39;s transmitter is not keyed until the word “shoot” is received, so that only the word “shoot” is broadcast from the radio. This can happen if the PTT/PTS information experiences a greater delay through the IP network than the voice data does. Elimination of this problem in a Voice-over-Internet (VoIP) telephone system would constitute a significant advantage for such a system.  
       SUMMARY  
       [0006]     The present invention conveys Push-To-Talk and Push-To-Signal (PTT/PTS) information over an IP network through the use of signaling bits formerly used to convey legacy network information. These signaling bits are no longer in active use in the Voice-over-IP (VoIP) realm, since their information is now carried through the VoIP network by other means. The signaling bits used are defined in and propagated with the Real-Time Protocol (RTP) stream, so that the PTT/PTS information remains synchronized with the voice packets of the stream. The maintenance of synchronization of PTT/PTS signals and voice packets eliminates problems arising from failure of synchronization between the two classes of packets, notably the “shoot/don&#39;t shoot” problem. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  shows the configuration of a PTT/PTS. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]     The Internet Engineering Task Force (IETF) RFC 2833 standard, incorporated herein by reference, defines a method for sending “telephone events” over the Real-Time Protocol (RTP) data stream. These are time-tagged in a manner similar to that of the voice packets, and are used, for example, for reliable transmission of Dual-Tone-Multi-Frequency (DTMF) signals end-to-end over an Internet Protocol (IP) network during an established call. Although the RFC 2833 standard does not include a method for sending PTT/PTS signals specifically, it does define sending four independent signaling bits termed the “ABCD bits”. The original purpose of the ABCD bits was to relay the ABCD supervisory bits of legacy T1 and E1 transmission systems, but in the evolving IP networks, these signals are no longer needed since in most cases call setup and teardown are handled by special messages in the call management protocol. For example, call answer and disconnect are handled in the IP&#39;s Session Initiation Protocol (SIP) by the “200 OK” and “BYE” methods respectively.  
         [0009]     The fact that the ABCD bits are passed end-to-end in the RTP stream via the RFC 2833 mechanism means that they suffer exactly the same latency (delays) as the voice packets. This latency synchronization is important, because the “shoot/don&#39;t shoot” problem is due to the differential latency between the PTT/PTS information and the voice information. Since the two latencies are equal for both voice packets and the ABCD bits, the differential latency is zero, and consequently ABCD bits remain in the same real-time relationship to the voice packets at both ends of the call. If the PTT/PTS information were passed via the usual call control messages, additional PTT/PTS processing delays would be expected, and both the packet routing and packet priority would be expected to differ from those of the RTP stream. In that event, the differential latency would not be zero, and the required synchronization would not be obtained.  
         [0010]     The present invention therefore uses the now-available ABCD bits to transmit a call&#39;s PTT and PTS signals with exact synchronization with the voice packets also in the RTP stream. In a basic embodiment of the present invention, one of the ABCD bits is used to transmit the PTT signal, and another ABCD bit is used to transmit the PTS signal. Since there are four bits available (A, B, C, and D each provide one bit), and only two signals to be sent (PTT and PTS), an acknowledgement embodiment of the invention provides for use of the remaining two bits for acknowledgement signals returned to the source of the PTT/PTS signals.  
         [0011]     See  FIG. 1 . The present invention comprises one or more communication terminals  10 , one or more communication terminals  11 , and an IP routing network  40  connected to each terminal  10 ,  111  via a link  35 . Each communication terminal  10  may comprise an IP radio  20 , an IP telephone  30 , or any other type of IP device capable of PTT/PTS signaling. In  FIG. 1 , IP telephone  30  is shown with a PTT/PTS switch  32  provided on its handset. Communication terminal  11  may comprise a media gateway  50  and a radio  60  having keying input (i.e., input controlled by a microphone key or switch, not shown here).  
         [0012]     A call between IP phone  30  and communication terminal  11  illustrates the working of the invention. When activated during the call, PTT/PTS switch  32  of IPtelephone  30  generates a PTT bit activation in a Real-Time Protocol (RTP) packet stream directed to the radio  60  of communication terminal  11 . RTP is not involved in the setting up or tearing down of the link. That is done by another protocol (SIP) that is not relevant to this invention. Each RTP packet stream has a number of legacy bits, termed ABCD bits. These bits were included in the RFC 2833 standard to control T1 and E1 line channel-associated signaling transmissions over the Internet. Since equipment using push to talk and push to signal features generates no T1 and E1 line channel-associated signaling, the equipment  10 ,  11  and  20  uses the legacy ABCD bits to provide push to talk and push to signal control bits.  
         [0013]     The radio  60  of communication terminal  11  detects the PTT bit activation generated by PTT/PTS switch  32 , and initiates one or more actions appropriate for PTT signaling as a result. In an acknowledgement embodiment of the invention, radio  60  generates a PTT-acknowledgement bit activation in a return RTP packet stream directed to IP phone  30 . This series of actions comprises a PTT communication sequence.  
         [0014]     In like manner, when activated during the call, PTT/PTS switch  32  of IP telephone  30  generates a PTS bit activation in a Real-Time Protocol (RTP) packet stream directed to the radio  60  of communication terminal  11 . The radio  60  of communication terminal  11  detects the PTS bit activation generated by PTT/PTS switch  32 , and initiates one or more actions appropriate for PTS signaling as a result. In an acknowledgement embodiment of the invention, radio  60  generates a PTS-acknowledgement bit activation in a return RTP packet stream directed to IP phone  30 . This series of actions comprises a PTS communication sequence.  
         [0015]     The following assignment of bits to functions in the present invention is provided for illustrative purposes, and should not be taken in any limiting sense. In the illustrative embodiment, bit A in the outgoing RTP stream reflects the PTT status of the local device (the source of the outgoing stream. Optionally, on receipt of hit A from a remote device, a local device may acknowledge receipt by setting bit B in its own outgoing stream returning to the remote device. Similarly, bit C in the outgoing RTP stream reflects the PTS status of the local device. Optionally, on receipt of bit C from a remote device, a local device may acknowledge receipt by setting bit D in its own outgoing stream returning to the remote device.  
         [0016]     The following table summarizes the assignment of ABCD bits to functions described in the previous paragraph:  
         [0017]     Bit Usage  
         [0018]     A Source device&#39;s PTT signal  
         [0019]     B Receiving device&#39;s receipt status of PTT signal from source device  
         [0020]     C Source device&#39;s PTS signal  
         [0021]     D Receiving device&#39;s receipt status of PTS signal from source device  
         [0022]     Through the use of the ABCD bits, the invention synchronizes the PTT/PTS signaling of a call with the voice packets of the same call in the RTP packet stream, thereby eliminating communication problems created by the loss of said synchronization.  
         [0023]     While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof to adapt to particular situations without departing from the scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope and spirit of the appended claims.