Patent Publication Number: US-2010128613-A1

Title: Detection of bearer loss in an IP-based multimedia session

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to communication systems, and more particularly to bearer loss detection. 
     BACKGROUND OF THE INVENTION 
     Multimedia services over IP (Internet Protocol) are a critical service for service providers and service providers are looking at VoIP (Voice over Internet Protocol) as the first major application to use this real time service. VoIP is being built mainly using the IMS (IP Multimedia Subsystem) core network and various access networks, which include broadband connections such as cable, DSL (Digital Subscriber Line), and fiber and also various wireless access networks such as DORA, WiMAX (Worldwide Interoperability for Microwave Access), LTE (Long Term Evolution), etc. 
     Voice service in communication systems is an essential service. Therefore, all aspects of voice service have to perform extremely well. One area of interest is the area of detecting bearer loss. Detecting bearer loss during a call is a non-trivial process as this requires the many network elements both at the signaling and the bearer level to detect that there has been a loss in connectivity. 
     This is typically further complicated by the fact that the design of such networks essentially separate the signaling and the bearer network elements to a good degree. This separation makes the detection and communication of the detection even more complicated. The bearer loss or connection loss could have happened in the core network elements such as a MGW (Media GateWay) or MRF (Media Resource Function). The bearer loss or connection loss could also have happened in the access network elements such as a RNC (Radio Network Controller), GWs or even the mobile unit, also known as an access terminal, itself. 
     A first problem that is encountered is how can a communication system detect a loss of bearer in the core network. 
     A second problem that is encountered is how can a communication system detect a loss of bearer in the access network. 
     A third problem that is encountered is for various wireless access networks, the originating party of the session can go dormant on the packet access channel while the called party is reached/alerted and finally answers. This occurs because of the lack of packet data activity on the originators access connection. This issue can cause voice clipping and call setup delay. 
     One existing mechanism which can be used to help detect bearer loss is using RTCP (Real-time Transport Control Protocol). RTCP is mainly designed to determine network performance and various other aspects of the bearer connection, but loss of connection can also be gleaned from RTCP packets. Note that this kind of a protocol is mainly an end-to-end protocol and if the end devices (such as the mobile unit) are the ones that are having the issues then this protocol is not very effective. Moreover, because of the bandwidth cost associated with the effective use of RTCP, this protocol is generally not supported or used. 
     Therefore, a need exists for a way to determine when there is bearer loss in a communication system. Further, this determination needs to differentiate between true bearer loss and legitimate lack of packet data activity from one of the parties in a call. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method for detection of bearer loss in an IP-based multimedia session. If the session is setup as a VoIP call between an Access Terminal (AT) and another user B, then this session is a two-way session where traffic is expected to flow in both directions. During the process of session setup, the network elements involved in the signaling of the session establish the session. That is, the AT in conjunction with the IMS core establishes the VoIP session. As part of the VoIP session establishment the IMS core and the AT negotiate the media types, hence bandwidth, for the sessions and also the connection points, such as the IP address and ports of the bearer entities. This negotiated media information is called codec information. 
     The codec for this session is also negotiated which can tell the bearer node in the access network, such as the SGW, what packets per second count is expected. For example, the Policy and Charging Rules Function (PCRF) can tell the SGW the source and destination IP addresses of session and also that the codec is Enhanced Variable Rate CODEC (EVRC), the payload type # is 96 and the expected rate of packets is 50 packets/sec. The SGW can use this information to assess the flow of traffic in each direction. 
     The IMS core communicates the codec information to the PCRF for policy enforcement and gating purposes. The PCRF in turn sends this codec information to the Signaling Gateway (SGW) and the Packet Data Network Gateway (PGW). The codec information includes the connection information and the direction of traffic flow to and from the core network. 
     At this point the SGW knows what to expect from the core network side and also what to expect from the access network side. The SGW can now monitor the traffic flow and determine if there is a discrepancy in the flow of traffic and report this discrepancy to the PCRF. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  depicts an IMS system in accordance with an exemplary embodiment of the present invention. 
         FIG. 2  depicts a call flow of a method for detecting bearer loss in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention can be better understood with reference to  FIGS. 1 and 2 .  FIG. 1  depicts Wireless IMS system  100  in accordance with an exemplary embodiment of the present invention. Wireless IMS system  100  comprises Access Terminal (AT)  101 , Base Station (BTS)  103 , Radio Network Controller (RNC)  105 , Serving Gateway (SGW)  107 , Packet Gateway (PGW)  109 , Policy Control and Repository Function (PCRF)  111 , IMS Core  113 , IP Network  115 , Media Gateway Control Function (MGCF)  117 , and Media Gateway (MGW)  119 . 
     AT  101  is the VoIP capable mobile device in this scenario. 
     Base Station (BTS)  103  sends and receives over the air signals with AT  101 . 
     Radio Network Controller (RNC)  105  controls BTS  103  and connects to SGW  107 . 
     SGW  107  is a network element that bridges the connection from the access network and the core network via PGW  109 . Policy enforcement can be done within SGW  107 , which connection is made to PCRF  111 . 
     PGW  109  preferably works in conjunction with SGW  107  to connect to IP core network  115 . Policy enforcement can be done within PGW  109  and signals would be sent to PCRF  111 . 
     PCRF  111  is the main policy engine within network  100 . PCRF  111  is preferably a signaling-only element. In an exemplary embodiment, no bearer passes through PCRF  111 . 
     IMS Core  113  is the application core and preferably includes a P-CSCF and a S-CSCF. IMS Core  113  is preferably a signaling-only element. In an exemplary embodiment, no bearer passes through IMS Core  113 . 
     IP Network  115  is a packet switched network. 
     MGCF  117  controls MGW  119 . MGCF  117  is preferably a signaling-only element. In an exemplary embodiment, no bearer passes through MGCF  117 . 
     MGW  119  connects the IP bearer to the TDM bearer. 
       FIG. 2  depicts a call flow  200  of a method for detecting bearer loss in accordance with an exemplary embodiment of the present invention. A multimedia session is initiated by AT  101  when AT  101  sends Invite message  201  to P-CSCF. Invite message  201  preferably includes various media characteristics which include connection addresses, codec information, traffic direction, etc. In an exemplary embodiment, Invite message  201  includes sender information, recipient information, SDP information, a port, and whether the flow of traffic will be in the send or receive direction or both directions. 
     P-CSCF receives Invite message  201  and forwards this message to PCRF  111  as Indicate Session Setup message  202 . Indicate Session Setup message  202  preferably includes near end port information, application type, and traffic direction. In an exemplary embodiment, PCRF  111  establishes policy and communicates this information to SGW  107 . 
     PCRF  111  communicates the Session setup information and the bearer characteristics to SGW  107  by sending Indicate Session Setup message  203 . SGW  107  preferably uses this information to adjust the dormancy timers on the originating side so that the originating access link remains active during the call setup. 
     During call setup, IMS Core  113  and/or PCRF  111  can tell SGW  107  the state of the session. In the case of the origination side, PCRF  111  can tell the originating SGW  107  that a session establishment is being attempted. This will prompt SGW  107  to override or extend the dormancy timer on the originating side, thus avoiding clipping and other end-user issues. This ability to intelligently make a decision on the access network connection solves the problem of voice clipping and call setup delay by incorrectly recognizing a lack of packet data activity as dormancy by the originating party of the session while the called party is reached/alerted and finally answers. This occurs because of the lack of packet data activity on the originators access connection. 
     P-CSCF passes Invite message  201  to the S-CSCF as Invite message  204 . 
     S-CSCF determines the route for this call via ENUM or DNS lookup and sends this call to MGCF  117  via Invite message  205  for delivery into the PSTN. 
     Upon receiving Invite message  205 , MGCF  117  sends Get Bearer Address message  206  to MGW  119 . MGW  117  sets up the bearer connection for this call and returns the bearer address to MGCF  117 . 
     MGCF  117  receives the bearer address from MGW  119  and passes this media information to S-CSCF via Session Progress message  207 . 
     S-CSCF receives Session Progress message  207  from MGCF  117 . S-CSCF passes the media information from Session Progress message  207  to P-CSCF via Session Progress message  208 . 
     P-CSCF receives Session Progress message  208  and sends this information to PCRF  111  via Indicate message  209 . PCRF  111  preferably establishes policy and communicate this information to SGW  107 . 
     PCRF  111  communicates the Session setup information and the bearer characteristics to SGW  107  via Indicate message  210 . SGW  107  preferably uses this information to adjust the bearer loss timers on the originating side. Since SGW  107  now knows the expected traffic characteristics from the Core Network via MGW  119 , SGW  107  can keep track of the received and transmitted bearer traffic. Deviation from this traffic pattern for a configurable duration of time (bearer loss timer) will cause SGW  107  to signal to the call control layer of a possible bearer loss. 
     If SGW  107  detects that there has been no packets flowing from the core network to the access side for a specific amount of time, SGW  107  can send a message to PCRF  111  alerting PCRF  111  of a possible loss of bearer issue. The signaling network elements can act on this by checking to see if the signaling connection is still up or alerting the user of the issue. This provides the ability to detect a loss of bearer in the core network. 
     P-CSCF passes the media information from MGW  119  to AT  101  via Session Progress message  211 . 
     When the called party answers the call, MGCF  117  sends OK message  212  to S-CSCF. 
     S-CSCF sends OK message  213 , which indicates that the called party answers the call request, to the P-CSCF. 
     P-CSCF sends this information to PCRF  111  via Indicate message  214 . Indicate message  214  preferably includes far end port information and traffic direction and characteristics. PCRF preferably establishes policy and communicates this information to SGW  107 . 
     PCRF  111  can send any changes in the media characteristics to SGW  107  via Indicate message  215 . From this point on SGW  107  now knows the expected traffic characteristics from MGW  119  and also from the access network, it can keep track of the received and transmitted bearer traffic. Deviation from this traffic pattern for a configurable duration of time, for example a bearer loss timer, causes SGW  107  to signal to the call control layer of a possible bearer loss. 
     Armed with this knowledge, if SGW  107  detects that there has been no packets flowing from the access side for a predetermined amount of time or detects that the access side went dormant, then SGW  107  can send a message to PCRF  111  alerting it of a possible loss of bearer. The signaling network elements can act on this by checking to see if the signaling connection is still up or by alerting the user of the issue. Note that on the access side, if the battery of AT  101  is pulled during a session or if AT  101  runs into an RF hole then the access side puts the connection into dormancy and nothing else is done. In this scenario SGW  107  will recognize the discrepancy in the current behavior and the expected behavior and signal the signaling entities to take action. This allows the communication system to detect a loss of bearer in the access network. 
     P-CSCF sends call answer message  216  to AT  101 . 
     While this invention has been described in terms of certain examples thereof, it is not intended that it be limited to the above description, but rather only to the extent set forth in the claims that follow.