Abstract:
A communications system includes a first PTT network, and a second PTT network having a plurality of interoperability gateways. A method for efficiently selecting an interoperability gateway for PTT calls terminated at the second PTT network includes receiving a PTT call request from the first PTT network, the PTT call request identifying a target subscriber unit on the second PTT network and forwarding the PTT call request to a first interoperability gateway. Notification is received if the request has succeeded, and the notification includes an identification of the target and a second interoperability gateway. This information is stored in cache for directing subsequent PTT calls to the same target to the corresponding second interoperability gateway. In one embodiment, the first PTT network is an iDEN network and the second PTT network is a HPPTT network and the PTT call is a HPPTT call.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to wireless communications systems and more particularly to systems and methods for facilitating Push-to-Talk (PTT) calls between a code division multiple access (CDMA) network and a second PTT network. 
     BACKGROUND OF THE INVENTION 
     PTT services, also known as walkie-talkie or dispatch services, provide near-instant, half-duplex communication between a PTT caller and one or more target PTT users. PTT services are typically offered as a subscription service on a wireless communications system, which may also offer interconnect, short message service (SMS), packet data and other communications services to its subscribers. Each wireless communications system facilitates communications using one or more wireless technologies such as CDMA, global system for mobile communication (GSM) or time division multiple access (TDMA). Known PTT technologies include Nextel&#39;s Direct Connect®, Qualcomm&#39;s QChat and Push-to-Talk over Cellular (PoC). 
     A PTT call spanning more than one PTT network or technology may be connected through one or more gateways that translate between signaling and media protocols of the networks. For example, a High Performance PTT (HPPTT) CDMA network (e.g., QChat) includes a plurality of interoperability gateways adapted to receive PTT call requests from an external PTT network, such as an Integrated Digital Enhanced Network (iDEN network). A PTT call from a subscriber on an iDEN network to a target subscriber on a conventional HPPTT network may be forwarded to any interoperability gateway on the HPPTT network. The gateway forwards the call to a regional HPPTT Server which manages the PTT call within the HPPTT network. 
     The target subscriber, however, may be located in a remote region, resulting in inefficient PTT call handling between the HPPTT network and the external PTT network. For example, a PTT call intended for a HPPTT subscriber roaming in New York may be routed to a HPPTT network interoperability gateway in California, and the HPPTT Server in California will be responsible for establishing the PTT call with the HPPTT target roaming in New York, resulting in a cross country PTT call. There is a need for an efficient system and method for handling PTT calls from an external system where the PTT targets are located in a HPPTT network. 
     SUMMARY OF THE INVENTION 
     The present invention is a system and method for efficiently selecting an interoperability gateway for HPPTT network terminated PTT calls. In one aspect of the present invention, a communications system includes a first PTT network and a second PTT network having a plurality of interoperability gateways. A method for efficiently selecting an interoperability gateway for PTT calls terminated at the second PTT network includes receiving a PTT call request from the first PTT network, the PTT call request identifying a target subscriber unit on the second PTT network and forwarding the PTT call request to a first interoperability gateway. Notification is received if the request has succeeded, and the notification includes an identification of the target and a second interoperability gateway. This information is stored in cache for directing the PTT calls to the same target to the corresponding second interoperability gateway. 
     A more complete understanding of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description. Reference will be made to the appended sheets of drawings, which will first be described briefly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: 
         FIG. 1  is an exemplary PTT network architecture in accordance with the present invention; 
         FIG. 2  illustrates an embodiment of a PTT Interoperability Infrastructure and a HPPTT interoperability gateway in accordance with the present invention; 
         FIG. 3  illustrates an embodiment of an operation of the PTT network architecture of  FIG. 1 ; and 
         FIG. 4  is a call flow diagram illustrating an embodiment of an operation of the PTT network architecture of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An embodiment of the present invention will now be described with reference to  FIG. 1 . A wireless communications system  10  provides communications services such as interconnect, short message service (SMS), packet data and push-to-talk, to a plurality of subscriber units  20 ,  22  and  24  using one or more wireless technologies such as code division multiple access (CDMA), global system for mobile communication (GSM), time division multiple access (TDMA) or Advanced Mobile Phone Service (AMPS). The subscriber units  20 ,  22  and  24  are devices adapted to communicate with the wireless communications system  10  such as mobile telephones, personal digital assistants, and portable computers. The subscriber units  20 ,  22 , and  24  may also include wireline devices coupled to the wireless communications network  10  through a physical connection, such as through the Internet. 
     In the exemplary embodiment, the wireless communications system  10  includes a 3G CDMA2000 1xEV-DO network providing High Performance PTT services, and each subscriber unit  20 ,  22  and  24  is an HPPTT-enabled device. In one embodiment, the HPPTT services utilize a Voice-over-IP (“VoIP”) PTT communications technology for 3G CDMA networks that enables near-instant, one-to-one and one-to-many half-duplex communication. A HPPTT call is formed by combining separate IP connections between each subscriber unit at a HPPTT Server, such as HPPTT servers  30 ,  32  and  34 . Each HPPTT server  30 ,  32  and  34  manages HPPTT calls originating within its respective regional network  40 ,  42  and  44 , respectively, and provides HPPTT application services to the subscriber units operating therein. 
     In operation, a HPPTT call may be originated by a subscriber unit  20  with the single press of a PTT button. HPPTT software on the subscriber unit  20  initiates a call request to the regional HPPTT server  30 , which establishes IP communications with each target subscriber unit, such as subscriber units  22  and  24 , through their respective regional HPPTT servers  32  and  34 . The originating HPPTT subscriber unit  20  receives presence information from the HPPTT Server  30  concerning the availability of each target subscriber unit  32  and  34 . If a target user is available, the call originator can begin speaking. Communications between participating subscriber units are facilitated using voice-over Internet protocol (VoIP) technologies in which voice information is converted to digital form and transmitted over IP-based data networks in data packets. 
     PTT calls may also be originated from an external network to a target HPPTT subscriber unit on the HPPTT network  10 . In one embodiment, each HPPTT region includes an interoperability gateway,  50 ,  52  and  54 , respectively (referred to herein as HPPTT Gateways), that facilitates communication between the wireless communications system  10  and a PTT interoperability infrastructure (PII)  60 . The PII  60  also communicates with at least one other PTT network  70  through an inter-working gateway  74 . The PTT network  70  may be any communications system, including wireless and wireline networks, that facilitates push-to-talk communications between at least two devices. In the exemplary embodiment, the PTT network  70  is an Integrated Dispatch Enhanced Network (iDEN) adapted to communicate with the PII  60  through the interoperability gateway  74 , which is an iDEN PTT Gateway. In alternate embodiments, other dispatch protocols and/or technologies may be supported including PTT over Cellular network (PoC). A person having ordinary skill in the art will appreciate that the illustrated embodiment is exemplary and that any number of networks and wireless and wireline devices may be inter-worked and other wireless technologies and components may be implemented. 
     The PII  60  is an inter-working architecture that provides a core infrastructure to which PTT service providers may connect to enable inter-carrier and cross-technology PTT sessions. The PII  60  is adapted to assist in translating and managing PTT sessions between a plurality of PTT networks, such as the iDEN network  70  and the HPPTT network  10 . In the exemplary embodiment, signaling messages between the HPPTT network  10  and the iDEN network  70  are based on standard Session Internet Protocol (SIP) signaling and the PII  60 , HPPTT Gateways  50 ,  52  and  54  and iDEN PTT Gateway  74  support UDP and TCP transport protocols. Media messages passing between the HPPTT network  10  and the iDEN network  70  are transported using the RTP standard over UDP. In an alternate embodiment, the PII  60  uses EVRC for media packets on HPPTT-iDEN calls. The PII  60  is adapted to handle media transcoding between networks, though it is contemplated that media transcoding may be performed by the iDEN PTT Gateway  74  or other interoperability gateways. 
     In operation, a subscriber unit  72  initiates a PTT call on the PTT network  70  to a target subscriber unit  20  on the HPPTT network  10 . The PTT network  70  determines that subscriber unit  20  is not a subscriber of the PTT network  70 , for example, by checking the domain of the target address. The PTT network  70  next forwards the initial PTT request to the PII  60  through the iDEN PTT Gateway  74 . In one embodiment, the iDEN PTT Gateway  74  translates signaling and messaging to a common protocol used by the PII  60 , such as SIP. The PII  60  determines the home network associated with target subscriber address and forwards the request to a HPPTT Gateway, such as gateway  54 , of the HPPTT network  10 . The HPPTT network  10  processes the initial request, attempts to establish PTT communications with the target subscriber unit  20 , and responds to the PII  60 . The PII  60  manages the PTT session between subscriber units  20  and  72 , performing necessary translation between the formats and protocols of the HPPTT network  10  and iDEN network  70 . 
     An embodiment of the PII  60  and the HPPTT Gateways will be described in greater detail with reference to  FIG. 2 . The PII  60  includes a PTT controller function  82  which is adapted to manage PTT sessions between the PTT network  70  and the HPPTT network  10 , including the selection of one of the HPPTT Gateways to which to forward incoming PTT calls. A name translation function  84  provides address translation between the iDEN addressing scheme (i.e., UFMIs) and the HPPTT addressing scheme (i.e., SIP URI). In one embodiment, iDEN subscriber units are adapted to initiate PTT calls to a target UFMI. HPPTT subscribers are assigned a UFMI (e.g., the HPPTT target&#39;s public telephone number) by the iDEN network for use with the PTT call. 
     In the exemplary embodiment, the PII  60  communicates with the HPPTT Gateways  50 - 54  and iDEN PTT Gateway  74  using SIP messaging. During PTT call setup, the PTT controller function  62  queries a location service database  66  which stores the last known location subscriber units who participating in inter-network calls through the PII  60 . If the HPPTT target  20  is found in the location service database  66 , the PTT controller function  62  forwards the PTT call request to the HPPTT Gateway associated with the HPPTT target  20 &#39;s last known location. If the HPPTT target  20  is not found, then the PTT controller function  62  selects a HPPTT Gateway, in accordance with stored routing rules, to receive the PTT call request. 
     An embodiment of an operation of the exemplary system will now be described with reference to  FIGS. 3 and 4 . A PTT call originated on the iDEN network is directed to at least one HPPTT target subscriber unit  20 . The iDEN PTT Controller  76  on the iDEN network  70  is the serving PTT processor for the call, and the iDEN Subscriber Database  78  is adapted to direct all HPPTT targeted calls to the iDEN PTT Gateway  74 . The iDEN PTT Gateway  74  receives the page request from the iDEN network  70 , prepares a corresponding SIP INVITE request and transmits the SIP INVITE request to the PII  60  for forwarding to the HPPTT target subscriber unit  20 . In step  1 , the PII  60  receives the SIP INVITE request and replies with a provisional response to the iDEN PTT Gateway  74 . The PII  60  selects a HPPTT Gateway associated with the target HPPTT network and forwards the request to the selected gateway. In the exemplary embodiment, the PII  60  lacks current information regarding the location of the subscriber unit  20  in the HPPTT network and selects a HPPTT Gateway  54  based on stored routing rules. The SIP INVITE message is routed through the SIP network to the selected HPPTT Gateway  54 . 
     In step  2 , the HPPTT Gateway  54  transmits a HPPTT Page Request message to a regional HPPTT Server  34 . In step  3 , the HPPTT Server  34  queries a local cache for the location of the target subscriber unit  20 . If the target subscriber unit  20  is not found in the cache, the HPPTT Server  34  queries the HPPTT Subscriber Database  46  in step  4  for the location of the target subscriber unit  20 , and the HPPTT Subscriber Database  46  returns the target subscriber unit&#39;s location in step  5 . In the exemplary embodiment, the target subscriber unit  20  is located in a second HPPTT Region  40  that includes the regional HPPTT Server  30  and the HPPTT Gateway  50 . Upon receiving the location of the target subscriber unit  20 , the HPPTT Server  34  then transmits a HPPTT Page Request message to the target subscriber unit  20  through a second regional network in step  6 . 
     In step  7 , the target device returns a HPPTT Call Accept message to the HPPTT Server  34 , and the HPPTT Server  34  transmits the Call Accept message to the HPPTT Gateway  54  that received the call request. In one embodiment, the HPPTT Call Accept message is a SIP response message (e.g., 2000K) that is sent from the HPPTT Server to the HPPTT Gateway  54 , which includes a regional ID of the HPPTT Server  30  serving the target subscriber unit  20 . In step  8 , upon receipt of the Call Accept message, the HPPTT Gateway  54  queries the HPPTT Gateway-to-HPPTT Regional ID mapping table (see mapping table  56  in  FIG. 2 ) for the regional ID received in the message. The HPPTT Gateway  54  forwards the SIP 200 OK response message to the PII  60 , including the ID of the serving HPPTT Gateway  50 . The PII  60  returns an SDP answer (acknowledgement message) to the HPPTT Gateway  54 , and stores the regional ID of the target subscriber unit  20  in the location database  66  (see  FIGS. 1 &amp; 2 ). The PII  60  then facilitates the PTT call through the HPPTT Gateway  54 . 
     In step  9 , when the PII  60  receives a subsequent call request directed to the target subscriber unit  20 , the PII  60  queries the location database  66  for the target subscriber unit  20  and selects the HPPTT Gateway corresponding to the database entry—in this case HPPTT Gateway  50 . The PII  60  transmits a SIP INVITE message to the HPPTT Gateway  50  which is local to the target subscriber unit  20 . 
     With the introduction of efficient HPPTT Gateway selection, the initial PTT call to the roamed HPPTT target will be routed internally within the HPPTT network. Subsequent PTT calls to the roamed HPPTT target will be routed to the HPPTT Gateway where the HPPTT target was last located. In other words, subsequent calls are routed automatically to the appropriate HPPTT region. In one embodiment, if the target subscriber unit  20  is no longer located in the HPPTT Region  40 , then steps  2  through  8  are repeated and the target subscriber unit&#39;s current location, if known, is stored in location database  66  for use in subsequent calls. 
     Having thus described various embodiments of the present invention, it should be apparent to those skilled in the art that certain advantages of the within described system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention.