Abstract:
The invention relates to a method of notifying an Application Function, AF, in a communications network of resource restrictions relating to a communications session. The network includes a Policy and Charging Rules Function, PCRF, for authorising and controlling flows of data in the session. In the method the AF sends an authorisation request to the PCRF for establishing the communication session. The authorisation request includes an indication that the AF is to be notified of resource restrictions for the data flows in the session. The PCRF notifies the AF of the resource restrictions.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a method for notifying an Application Function in a communications network of resource restrictions relating to a communications session. 
       BACKGROUND 
       [0002]    The IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IP Multimedia services provide a dynamic combination of voice, video, messaging, data, etc. within the same session. As the number of basic applications, and the media which it is possible to combine, increases, so will the number of services offered to the end users, giving rise to a new generation of personalised, rich multimedia communication services. 
         [0003]    The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly. 
         [0004]      FIG. 1  illustrates schematically how the IMS fits into the mobile network architecture in the case of a GPRS/PS access network. As shown in  FIG. 1  control of communications occurs at three layers (or planes). The lowest layer is the Connectivity Layer  1 , also referred to as the bearer plane and through which signals are directed to/from user terminals accessing the network. The GPRS network includes various GPRS Support Nodes (GSNs). A gateway GPRS support node (GGSN)  2  acts as an interface between the GPRS backbone network and other networks (radio network and the IMS network). 
         [0005]    The IMS  3  includes a core network  3   a,  which operates over the middle, Control Layer  4  and the Connectivity Layer  1 , and a Service Network  3   b.  The IMS core network  3   a  includes nodes that send/receive signals to/from the GPRS network via the GGSN  2   a  at the Connectivity Layer  1  and network nodes that include Call/Session Control Functions (CSCFs)  5 , which operate as SIP proxies within the IMS in the middle, Control Layer. The 3GPP architecture defines three types of CSCFs: the Proxy CSCF (P-CSCF) which is the first point of contact within the IMS for a SIP terminal; the Serving CSCF (S-CSCF) which provides services to the user that the user is subscribed to; and the Interrogating CSCF (I-CSCF) whose role is to identify the correct S-CSCF and to forward to that S-CSCF a request received from a SIP terminal via a P-CSCF. 
         [0006]    At the top is the Application Layer  6 , which includes the IMS service network  3   b.  Application Servers (ASs)  7  are provided for implementing IMS service functionality. Application Servers  7  provide services to end-users, and may be connected either as end-points, or “linked in” by an S-CSCF. Certain Application Servers  7  will perform actions dependent upon subscriber identities (either the called or calling subscriber, whichever is “owned” by the network controlling the Application Server  7 ). 
         [0007]    The IMS architecture makes it possible to deploy peer-to-peer applications where two or more users exchange data during a SIP session. Examples of such peer-to-peer applications include Multimedia Telephony (MMTel), Push to Talk over Cellular (PoC), streaming, real-time video sharing, file sharing, gaming etc. The transport connection(s) is (are) negotiated dynamically by means of the SIP/SDP protocol exchange between the two end points (user terminals). 
         [0008]    However, in order to support such peer-to-peer applications, there are two basic requirements: (i) a mechanism is needed to selectively control the SIP signal flows associated with the IMS session(s) of a subscriber; and (ii) a functionality is needed to control the IP flows through the dynamically negotiated transport connections in order to apply an effective charging for usage of services. One important aspect concerns the resources required for the session, which will impact on the Quality of Service (QoS) provided for the session (e.g. the data rate at which data is transferred between the end users). In the discussion below the term QoS is used to refer to those parameters of a requested or on-going session that determine the Quality of the session Service experienced by the end user. The principal bearer resource characteristic affecting QoS is the available bandwidth for the session. 
         [0009]    3GPP has recognised such needs and is currently defining a Policy and Charging Control (PCC) Architecture (see 3GPP TS 23.203 [1]).  FIG. 2  presents the basic outline of the PCC architecture. The Application Function (AF)  16  is an element offering applications that require dynamic policy and/or charging control of traffic plane resources. Although the application services are initiated and service characteristics are negotiated at the Application Layer  6  (e.g. by an Application Server  7 —see  FIG. 1 ), in the case of the IMS the P-CSCF plays the role of the AF  16  at the SIP signalling plane (Control Layer  4 ). A Policy and Charging Enforcement Function (PCEF)  12  in the Connectivity Layer  1  monitors service data flow and enforces network policy on the user plane traffic. The PCEF  12  also applies charging based on the monitored data flow and the charging policy applied. This information is provided to an Online Charging System  13  over the Gy interface. Within a GPRS access network, the PCEF  12  is located in the GGSN  2   a.  Within the Systems Architecture Evolution defined in 3GPP Release 8, the PCEF is located in the PDN gateway. 
         [0010]    A Policy and Charging Rules Function (PCRF)  14  resides in between the AF  16  and the PCEF  12 . The PCRF  14  is the entity that controls charging based on the monitored data flow. The PCRF  14  obtains rules relating to the charging policy to be applied for particular subscribers over the Sp interface from a Subscription Profile Repository (SPR)  18 , which includes a database of subscriber information. The PCRF  14  installs these PCC rules at the PCEF  12  over the Gx interface. These ensure that only authorized media flows associated with the requested services are allowed. In addition, the PCC rules installed at the PCEF  12  ensure that the right bandwidth, charging and priority are applied through the right bearer. 
         [0011]    Once session characteristics are negotiated between the communication peers and the session characteristics are authorized within the IMS Core Network  3   a,  the AF  16  provides to the PCRF  14  an authorization of bearer resources over the Rx interface so that the corresponding resource reservation can be authorized at the Connectivity Layer  1 . 
         [0012]    However, there could be circumstances when the corresponding resource reservation cannot be authorised at the bearer layer. One such reason, for example, could be that the access operator has established restrictions on bandwidth consumption for their users or for that particular user. Under these circumstances, the current specification in 3GPP TS 29.214 [2], states that: 
         [0013]    “The PCRF shall process the received Service Information according to the operator policy and may decide whether the request is accepted or not.” 
         [0014]    However, it is not specified what the PCRF  14  does when the request is not accepted. In the absence of any explicitly defined mechanism, it can be assumed that the PCRF  14  must be configured according to the operator&#39;s preferences. This may, or may not include feedback of information to the AF  16  regarding the reason for not accepting the request. Currently the PCRF  14  does not know if the AF  16  requires such information and will therefore perform its processing (however configured) regardless of any AF preferences. 
         [0015]    For example, it may be possible for session services of a requested peer-to-peer session that has been rejected due bandwidth restrictions to be provided at a lower bandwidth, at least for certain media types, so that the session could be allowed to proceed at a lower QoS. If information concerning the bandwidth restrictions affecting the rejected session request is made available, this could be used to initiate a modified session request. The modified request could be initiated from the AF itself, but would normally be initiated by the UE after it has been made aware of the restrictions either by the AF  16  or directly from the Connectivity Layer  1 . Therefore, it can only be the AF  16  that knows whether or not the indication of bandwidth restriction is relevant for the AF  16  in order to modify the session request or to inform the UE of the allowable bandwidth for the session. 
         [0016]    Moreover, even when the indication of bandwidth restriction is relevant for the AF  16 , it is only the AF  16  that knows in which specific format it requires the information. If the PCRF  14  is configured in a certain way, it is possible that it will not feed back the information required, or not in the correct format, for this to be used by the AF  16 . For example, the PCRF  14  might be configured to compute the available bandwidth and provide this information to the AF  16  in one particular format (e.g. total available bandwidth for the session) when the AF  16  actually requires the available bandwidth per media component. 
         [0017]    The present invention has been conceived with the foregoing in mind. 
       SUMMARY 
       [0018]    According to a first aspect of the present invention there is provided a method of notifying an Application Function, AF, in a communications network of resource restrictions relating to a communication session. The network includes a Policy and Charging Rules Function, PCRF, for authorising and controlling flows of data in the session. The AF sends an authorisation request to the PCRF for establishing the communication session. The authorisation request includes an indication that the AF is to be notified of resource restrictions for the data flows in the session. The PCRF notifies the AF of the resource restrictions. 
         [0019]    It is an advantage that the AF can indicate its desire to receive available bandwidth information from the PCRF. This information may then be used to modify session requests. 
         [0020]    The communications session may be a multimedia session, the PCRF authorising and controlling flows of data for each of a plurality of media components in the multimedia session. The AF may be comprised in a Proxy Call/Session Control Function in an IMS network. 
         [0021]    In Embodiments of the invention, the PCRF notifies the AF of the resource restrictions in an authorisation response message, which indicates whether or not the requested session is allowed to proceed. The indication in the authorisation request may include a request for the AF to be informed of bandwidth restrictions in the event that the requested session is not allowed to proceed due to a requested bandwidth exceeding a bandwidth restriction. The indication may specify one of a plurality of levels at which the AF is to be notified of bandwidth restrictions, the plurality of levels including a total session bandwidth level and a per media component level. 
         [0022]    It is an advantage that the PCRF is informed of the AF&#39;s preferences for notification of resource restrictions and can take account of this in its processing so that it can provide the required information to the AF in the correct format. 
         [0023]    In embodiments of the invention the method further comprises notifying the AF of any violation to the resource restrictions that apply during an on-going session. The PCRF may send a Re-Authorisation Request message that includes notification of changes to resource restrictions to the AF. On receipt of the Re-Authorisation Request the AF may determine whether or not the session can progress with the notified restrictions, and if it cannot progress may request the termination of the session. This may involve the AF sending a Session Termination Request to the PCRF. 
         [0024]    It is an advantage that the decision about terminating or continuing the session can be made by the AF, rather than having the decision made for it by the PCRF. 
         [0025]    In embodiments of the invention the indication that the AF is to be notified of resource restrictions comprises a Specific-Action Attribute Value Pair, AVP, in the authorisation request message. The authorisation request message may further comprise an Available-Bandwidth-Info AVP indicating the form in which the AF is to be notified of resource restrictions. The notification of resource restrictions may comprise a Supported-Service-Info AVP providing the information of the resources that would be available for the AF session in accordance with the form indicated in the Available-Bandwidth-Info AVP. 
         [0026]    According to a second aspect of the present invention there is provided an Application Function, AF, in a communications network for providing application sessions between user terminals communicating over the network. The AF comprises means for sending a session authorisation request to a Policy and Charging Rules Function, PCRF, in the network. The session authorisation request includes an indication that the AF is to be notified of resource restrictions for data flows in the session. The AF also comprises means for receiving a notification of the resource restrictions. 
         [0027]    The Application Function may be configured to provide multimedia application sessions. The Application Function may be comprised in a Proxy Call/Session Control Function in an IMS network. 
         [0028]    In embodiments of the Application Function the indication in the session authorisation request specifies one of a plurality of levels at which the AF is to be notified of bandwidth restrictions, the plurality of levels including a total session bandwidth level and a per media component level. 
         [0029]    In embodiments of the Application Function the notification of resource restrictions is included in an authorisation response message which indicates whether or not the requested session will receive the required resources as requested. 
         [0030]    The Application Function may be configured to provide information relating to the notified resource restrictions to the one or more of the user terminals participating in the session. 
         [0031]    Embodiments of the Application Function may be configured to modify the session authorisation request to comply with said resource restrictions. The Application Function may be configured to inform at least one of the user terminals of the modification to the session authorisation request. The Application Function may be configured to send the modified session authorisation request to the PCRF. 
         [0032]    According to a third aspect of the present invention there is provided a Policy and Charging Rules Function, PCRF, in a communications network configured to authorise and control flows of data for a session between user terminals communicating over the network. The PCRF comprises means for receiving a session authorisation request from an Application Function in the network. The session authorisation request includes an indication that the Application Function is to be notified of resource restrictions for data flows in the session. The PCRF also comprises means for sending a notification of resource restrictions to the Application Function. 
         [0033]    The PCRF may be configured to authorise and control flows of data for each of a plurality of media components in a multimedia session. 
         [0034]    In embodiments of the PCRF the authorisation request from the Application Function includes an indication specifying one of a plurality of levels at which the Application Function is to be notified of resource restrictions, the plurality of levels including a total session bandwidth level and a per media component level, and the means for sending a notification of bandwidth restrictions includes means for sending a notification at the specified level. 
         [0035]    The PCRF may be configured to store in a memory said indication specifying the level at which the Application Function is to be notified of resource restrictions. 
         [0036]    The PCRF may be configured to notify said Application Function of subsequent violations to resource restrictions during an on-going session in accordance with said stored indication. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    Embodiments of the invention are described below with reference to the drawings, in which: 
           [0038]      FIG. 1  is a schematic illustration of a GPRS/PS access network showing how the IMS fits into the mobile network architecture. 
           [0039]      FIG. 2  is a schematic illustration of the network entity architecture a Policy and Charging Control (PCC) system. 
           [0040]      FIG. 3  is an illustration of the signal flows between a UE and PCC entities in accordance with embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]      FIG. 3  illustrates the signal flows between the PCC nodes for a user terminal (UE)  10 , which has registered with the IMS and has attempted to initiate a peer-to-peer multimedia application session with another UE (not shown). The application, which requires dynamic policy and/or charging control of resources is offered by an AF  16 . When the session initiation request is first received from the UE  10  by the IMS core network, the request is forwarded to the AF  16 . At step  301  in  FIG. 3  the AF  16  then sends an authorisation request (AAR) to the PCRF  14  over the Rx interface (see  FIG. 2 ). In accordance with the established function of the PCC, the authorisation request will specify the required QoS resources for the session. At step  302 , the PCRF  14  checks to see if the requested QoS resources can be provided in view of the restrictions imposed by the network. These may be network-wide restrictions, or restrictions imposed on the particular subscriber. At step  303 , the PCRF  14  sends an Authorisation Request Response (AAA) back to the AF  16 . If the QoS resources requested do not exceed any restrictions, then the AAA will simply allow the session to proceed. 
         [0042]    The present invention is concerned with what happens when the requested QoS resources cannot be provided due to restrictions in the Connectivity Layer. The basic concept of the invention involves providing the AF  16  with means for explicitly requesting the notification of restrictions when the authorization of QoS resources is rejected by the PCRF  14 . The principal bearer resource characteristic affecting QoS is the available bandwidth for the session, and the discussion below focuses on bandwidth restrictions. However, it will be appreciated that there may be other resources that could affect QoS, such as certain media characteristics, and for which the principles of the invention could also be applied. 
         [0043]    In an embodiment shown in  FIG. 3 , if the AF  16  requires notification of bandwidth limitations it provides an explicit indication to the PCRF  14  within the initial AAR. This indication also includes the AF preferences regarding the notification of bandwidth restrictions (for the total session bandwidth or for the bandwidth per media component). Accordingly, if the service information provided in the AAR is rejected (e.g. the subscriber&#39;s guaranteed bandwidth QoS is exceeded), the PCRF  14  provides the bandwidth available for the rejected session request, if requested according to the AF preferences expressed in the initial AAR. 
         [0044]    As shown in  FIG. 3 , the mechanism involves the introduction of additional information elements within the AAR sent to the PCRF  14  over the Rx interface at step  301  of  FIG. 3 . The additional information includes an explicit request [ NOTIFICATION _ BANDWIDTH _ RESTRICTION ] to be informed of bandwidth restrictions in the case the processing of the AAR command is unsuccessful due to this reason. The additional information also includes a preference for the format in which the bandwidth restrictions should be provided to the AF  16 . This may be for the whole AF session [ TOTAL   —   BW ] or for each media component [ BW   —   PER   —   MEDIA   —   COMP ]. This additional information will be discussed in more detail hereafter. 
         [0045]    This additional information, when processed by the PCRF  14 , causes it to determine the bandwidth restrictions and to inform the AF  16  accordingly if required. This is done at step  303  in  FIG. 3  by including the information in the AAA response. 
         [0046]    A further refinement is shown in the lower part of  FIG. 3 . Here, there is an on-going session involving the UE  10  where the PCEF  12  is monitoring the data traffic. Also, the PCRF  14  has stored the additional information including the AF preferences indicated by the AF  16  in the initial AAR at step  301  in a memory for the lifetime of the AF session. At step  310  a bearer plane event (e.g. loss or release of a bearer previously handling media IP flows for one particular user) takes place. This is notified to the PCRF  14  over the Gx interface from the Connectivity Layer. This means that the required QoS resources according to the service information provided in the initial AAR from the AF  16 , which was originally accepted, can no longer be guaranteed. Since, in this case the PCRF  14  had stored the AF preferences indicated by the AF  16  in the initial AAR for the lifetime of the AF session, then, according to this embodiment, the PCRF can provide updated information about any changes to the available bandwidth for the AF session to the AF  16  if the AF had requested it and in accordance with the AF preferences. 
         [0047]    In this case, the notification of available bandwidth to the AF  16  is included within a Re-Authorisation Request (RAR) sent from the PCRF  14  to the AF  16  at step  311 . The up-dated available bandwidth is sent with the RAR in accordance with the stored AF preferences (either specifying the total available session bandwidth, or specifying the available bandwidth per media component). At step  312  the AF  16  responds by sending a Re-Authorisation Response (RAA) to the PCRF  14 . This RAA may include a modified request for QoS resources that complies with the restrictions so that the AF session can continue (although with modifications to the Authorized QoS resources). If at this point the AF determines that the AF session cannot progress with the notified restrictions, the AF  16  additionally requests the termination of the Rx session. In that case, as shown at step  313 , the AF  16  sends a Session Termination Request to the PCRF  14 . At step  314 , the PCRF responds by confirming that session is terminated. 
         [0048]    In the embodiments shown in  FIG. 3  the request and further notification of bandwidth restrictions are handled within the current framework for notifications over the Rx interface, and are included as a new trigger within the existing Specific-Action Attribute Value Pair (AVP). A new value for the Specific-Action AVP is then defined as follows. 
         [0049]    NOTIFICATION_OF_BANDWIDTH_RESTRICTIONS 
         [0050]    In the AAR, this value indicates that the AF requests the PCRF to provide a notification in the event that bandwidth limitations are experienced. (The AAR will also include the AF preferences for the form in which the bandwidth restriction information is to be provided by the PCRF, which will be included within a new “Available-Bandwidth-Info” AVP—see below). 
         [0051]    Within the AAA, this value is used when the PCRF experiences bandwidth limitations during initial provisioning of session information or during subsequent modification of session characteristics (e.g. addition of a new media type). 
         [0052]    Within the RAR, this value is used when the PCRF experiences bandwidth limitations due to bearer plane events and not all the service data flows within the AF session are affected. 
         [0053]    If this value is not included within the Specific-Action AVP within the AAR request, the AF will not be informed of the bandwidth that would be available for the requested AF session. In that case an AF session establishment or modification may be rejected by the PCRF, but the PCRF will not send information about bandwidth limitations to the AF. Also, if the value is not included within the Specific-Action AVP of the initial AAR, the PCRF will not notify the AF of subsequent bandwidth restrictions experienced. 
         [0054]    Otherwise, if this value is included within the Specific-Action AVP within the AAR request the AAR will also include an “Available-Bandwidth-Info” AVP. This expresses the AF preferences for the form in which it requires the information about bandwidth restrictions. 
         [0055]    When the AAR includes the “Available-Bandwidth-Info” AVP, the PCRF will also include within an AAA or RAR an Available-Service-Info AVP (as defined in 3GPP Change Request C3-070716. The Supported-Service-Info AVP provides the information of the bandwidth that would be available for the AF session following the preferences expressed by the AF. 
         [0056]    The “Available-Bandwidth-Info” AVP may be defined of type Enumerated, with the following values: 
         [0057]    Total-Bandwidth (1) 
         [0058]    This value is used when the AF requires to be notified of the total available bandwidth for the whole AF session (e.g. when an AA-Request cannot be accepted by the PCRF due to bandwidth restrictions). 
         [0059]    Per-Media-Component (2) 
         [0060]    This value is used when the AF requires to be notified of the available bandwidth for the different media components within the AF session (e.g. when an AA-Request cannot be accepted by the PCRF due to bandwidth restrictions).