Abstract:
A method provides information of a client making a first response to a network-initiated push-to-talk-over-cellular (PoC) group session when the network-initiated PoC group session is established. When it is determined that an event meeting session establishment criteria occurs at a server managing a session, the session management server sends an INVITE message to at least one client to participate in the session to be established. A response signal to the INVITE message is received from the at least one client. A message containing information of the client making the first response is sent to the client making a request to establish the session. Thus, a user is informed of identification information of an initial caller and information of a requested conference, thereby enabling a PoC group call to be smoothly performed.

Description:
PRIORITY  
       [0001]     This application claims the benefit under 35 U.S.C. §119(a) from an application entitled “METHOD AND SYSTEM FOR IDENTIFYING RESPONDENT CLIENT IN PUSH-TO-TALK OVER CELLULAR NETWORK” filed in the Korean Intellectual Property Office on Mar. 8, 2005 and assigned Serial No. 2005-19364, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to session establishment for push-to-talk-over-cellular (PoC) group call services, and more particularly, to a method and system for providing information of a client making a first response to a newly-established network-initiated PoC group session.  
         [0004]     2. Description of the Related Art  
         [0005]     Significant developments in mobile communications technology and the extension of mobile communications networks have resulted in the development of a vast array of services and applications for use with a cellular phone. At the same time, demand among cellular phone users for these additional services, such as a location, multimedia and a push-to-talk (PTT) service, is increasing. The PTT service supports various supplementary functions such as an instant messenger function and a status display function, as well as a group call and a voice call which are also provided by an existing radio or a trunk radio system (TRS).  
         [0006]     Currently, standardization of a push-to-talk-over-cellular (PoC) service that employs the PTT function in a mobile communication network is taking place. A unique feature of the PoC service is that a user can participate in a plurality of PoC sessions and can also use a call service while switching from the PoC sessions as desired. This feature is a requirement that is specified in the open mobile alliance (OMA), which is a forum for specifying mobile communications services.  
         [0007]      FIG. 1  is a schematic diagram illustrating a conventional PoC service system. Referring to  FIG. 1 , a PoC client  10 , as a service requester installed in a mobile station, is generally connected to a Session Initiation Protocol/Internet Protocol (SIP/IP) core network  30  that supports SIP and IP multimedia functions via an access network  20 .  
         [0008]     The PoC client  10  resides in a PoC user terminal to provide access to the PoC service. The PoC client  10  mainly serves to establish, participate in and terminate a PoC session. In addition, the PoC client  10  makes and transfers a talk burst, supports an instant personal alert and performs authentication when accessing the PoC service. Hereinafter, unless otherwise stated, the PoC client  10  is assumed to be the same as a PoC service subscriber or PoC terminal.  
         [0009]     The SIP/IP core network  30  is connected to a PoC server  60 , a GLMS (Group List and Management System)  50  and a presence server  70  in order to support the PoC service. The PoC server  60  performs a Controlling PoC Function for maintaining and managing a PoC session, or a Participating PoC Function for participating in a PoC session for a one-to-one PoC call or a one-to-two or more PoC call (group PoC call).  
         [0010]     Functional blocks of the PoC server will be explained below with reference to  FIG. 2 , a schematic diagram showing the structure of an ordinary PoC server. The PoC server performs a Controlling PoC Function (CF) of controlling overall maintenance and management of a PoC session, and a Participating PoC Function (PF) of controlling maintenance and management between each PoC session, which will be explained below with reference to Tables 1 and 2.  
                         TABLE 1                       Controlling PoC Function (CF)                                    Provides centralized PoC session handling           Provides centralized Media distribution           Provides centralized Talk Burst Arbitration           functionality including talker identification           Provides SIP session handling, such as SIP           session origination, termination, etc.           Provides policy enforcement for participation           in group sessions           Provides participant information           Collects and provides centralized media           quality information           Provides centralized charging reports           May provide transcoding between different codecs           Supports Talk Burst Control Protocol Negotiation                      
 
         [0011]     As shown in Table 1, the CF serves to maintain and manage a PoC session. The PoC server receives requests for the floor from PoC clients, arranges an order in which to give the clients the floor, and gives the clients the floor in that order. The PoC server also distributes a talk burst, for which an arbitrary PoC client makes a request, to all other PoC clients participating in a group PoC call, and provides information of the PoC clients participating in the group PoC call.  
         [0012]     As shown in Table 2 below, the PF manages a PoC session between the CF and each PoC client. Particularly, the PF relays the floor between the PoC client and the CF when the PoC client makes a request for the floor or when the CF gives the floor to the PoC client. In addition, the PF relays media between the CF and the PoC client, performs transcoding between different codecs, and filters one of two concurrent PoC sessions according to the choice of a PoC user when there is simultaneous talking in the two active PoC sessions.  
                         TABLE 2                       Participating PoC Function (PF)                                    Provides PoC session handling           May provide the Media relay function between           PoC client and Controlling PoC server           May provide user media adaptation procedures           May provide the Talk Burst control message           relay function between PoC client and Controlling           PoC server           Provides SIP session handling, such as SIP session           origination, termination, etc, on behalf of the           represented PoC client.           Provides policy enforcement for incoming PoC           session (e.g. access control, incoming PoC           session barring, availability status, etc.)           May collect and provide media quality information           Provides participant charging reports           May provide filtering of media streams in           the case of simultaneous sessions           May provide transcoding between different codecs           May support Talk Burst Control Protocol Negotiation           Stores the current Answer Mode and Incoming PoC           Session Barring preferences of the PoC           client                      
 
         [0013]     In the PoC service system described above, the PoC user can input information on a group and its members to the GLMS  50  through his/her PoC terminal, and can receive information about PoC users whom he or she can call through an individual or group list transmitted from the GLMS  50 . Alternatively, the information on the group and its members may be input, modified and managed in the GLMS  50  via a reliable communication network such as the Internet or an Intranet.  
         [0014]     In order to use the PoC service, the PoC user registers his/her PoC address with the SIP/IP core network  30 . The SIP/IP core network  30  stores PoC user information at the request of the PoC user. Thus, when another PoC user tries to request a group PoC call, the PoC user registers his/her information in the SIP/IP core network  30  in advance, and requests the group PoC call to his/her SIP/IP core network  30  by using group identification information transmitted from the GLMS  50 . At this time, the SIP/IP core network  30  performs address determination and domain location determination by using information of the call requesting PoC user and then transfers a PoC call request to a home PoC server with which the call requesting PoC user is registered. In regard to the PoC call request, the PoC server prepares to establish a PoC session, obtains each user&#39;s information from the GLMS, and then transfers a PoC call request signal to a corresponding SIP/IP core network. When a PoC call request is made to users within an Intradomain, the PoC server performs both the CF and PF. The PoC server, which manages a call-requested PoC user, requests a PoC call to the PoC user after the SIP/IP core network performs the locating procedure, by using information it receives from the PoC user that is transmitted to the PoC server.  
         [0015]      FIG. 3  is a schematic diagram of CF and PF blocks of a PoC server. Referring to  FIG. 3 , PoC clients  111 ,  121 ,  131  and  141  provide access to a CF  100  through PFs  110 ,  120 ,  130  and  140  respectively, thereby establishing a PoC session. Here, when the floor is granted to a requester qualified as a talker from the CF  100 , media based on speaking of the corresponding PoC client is transmitted to each PoC client.  
         [0016]     First, the terminating side can set up its own answering modes according to the request of a PoC user. The answering modes can be generally divided into an auto answer mode and a manual answer mode.  
         [0017]     The auto answer mode refers to sending an immediate answer to the originating side in a corresponding network in place of the manual answer of a receiver when included in a PoC user list designated on the terminating side. The auto answer is sent instead of operating the terminal in the network because the PoC server has a function of storing the answer mode and the corresponding user list according to a request of the terminal to set up the answer mode. Meanwhile, the manual answer mode corresponds to when the user is not included in an auto answer user list or the answer is ambiguous, or when the receiver sets up all users to make the manual answer, and indicates that a PoC call request is transmitted to the user&#39;s terminal through a receiving network and a call is connected by approval of the PoC user.  
         [0018]     Second, the PoC system is divided into an on-demand session mode and a pre-established (or early) session mode, according to the connection set up with a PoC server within a user&#39;s home network.  
         [0019]     The pre-established session mode is designed so that the PoC user sets up a session between a PoC client and a PoC server belonging to a PoC user&#39;s home network in advance by PoC user&#39;s request. The pre-established session enables the PoC user to negotiate media parameters to be used with the PoC server in advance, and thus perform rapid call setup without renegotiating the media parameters to be used in the future between the PoC server and client. In order to set up the pre-established session, the PoC client provides supported media parameters to a Session Description Protocol Multipurpose Internet Mail Extensions (SDP MIME) body through a method of SIP INVITE, and responds to the media parameters provided from the PoC server. The PoC client sends, to the PoC user, identification information of the pre-established session for a response message received from the PoC server, together with a conference Uniform Resource Identifier (URI). When using the pre-established session, it is possible to pre-negotiate such parameters as an IP address, a port number, a codec to be used and a talk burst control protocol.  
         [0020]     The on-demand session mode refers to a state in which the PoC user does not set up the pre-established session, and indicates that the PoC user performs a PoC call connecting procedure after receiving an INVITE message of another PoC user.  
         [0021]     Meanwhile, the PoC specification that is being standardized in the OMA has the following features in addition to the aforementioned fundamental functions of the communication system.  
         [0022]     First, the PoC service supports the multilateral conference function of a half-duplex communication mode, as well as various types according to an objective of the PoC group. Specifically, a PoC conference is divided into an ad-hoc PoC group, a pre-arranged PoC group and a chat PoC group according to the feature of a participating group. First, the ad-hoc and pre-arranged PoC groups involve session establishment of dial-out to request for session setup to a conference server and forwarding the session establishment request from a server (conference server) acting as a focus to each client of interest. Next, the chat PoC group involves session establishment of dial-in to a conference server because each client is aware of information of the session of interest.  
         [0023]     Next, the setup of the answer mode according to the call request in the PoC system can be stored both in the PoC server as an element on the network and in the PoC client as a user side terminal. In particular, when being set up for the home network managing the PoC client, the answer mode is realized by the PoC server acting as the participating PoC function (PF) in the home network to which the PoC client belongs. In this manner, in the case of setting up the answer mode of the network side, the PF automatically answers the call request of the network with a session progress message as soon as the PoC call is requested by another PoC server. Thereby, the call request procedure can be easily performed as compared with the procedure where a session setup message is transmitted to the PoC client and then the PoC client answers the session setup message. Accordingly, an initial time to grant the floor can be saved.  
         [0024]     Meanwhile, a PoC session control network including the CF sends an INVITE request message to a terminating side network, and then receives a response message. SIP messages with which the terminating side network responds may comprise provisional response messages of 1XX, successful response messages of 2XX, or error response messages of 4XX, 5XX and 6XX according to setup of the terminating side network and PF. In the case of an AUTO-ANSWER response, the CF can receive a signal SIP  183  Session Progress, thus establishing a connection between the PoC server and client in the IMS network of a call requester. A call acceptance signal of the receiver is sent as the SIP  183  Session Process or SIP  200  OK response, and transmitted to the PoC client A via the PoC servers of the CF and PF. After receiving the  200  OK response or  183  Session Progress signal from the terminating PoC server, the CF determines that the PoC call is connected and then sends a signal Floor Granted that grants a talk burst floor, to the PoC client A. Granting talk burst authority according to the response ( 200  OK or  183  Session Progress) can be divided into confirmed and unconfirmed types. In the case of receiving an unconfirmed response, the CF requires a buffering function.  
         [0025]     Meanwhile, after receiving a response signal with respect to an INVITE request signal, a PoC client A of the originating side receives a Floor Granted signal sending a talk burst transmission enable signal (ring back tone) using Real-Time Transfer Protocol (RTP) Control Protocol (RTCP). The Floor Granted signal is generated from the CF having authority to arbitrate the talk burst, and sent to the PoC client via the PF managing the corresponding PoC client. Here, the Floor Granted signal can be sent without passing through the SIP/IP core network since it uses a bearer&#39;s route instead of an SIP. The PoC user who finally confirms the ring back tone sends a media stream (e.g. voice) using RTP.  
         [0026]     The conventional session establishment process of a PoC client using a pre-established session will be described below with reference to  FIG. 4 .  
         [0027]     In  FIG. 4 , the PoC client A  111  of an originating side forwards terminating side group information (a URI list including address information of terminating side clients) and various pieces of information for session establishment (PoC group identifier, pre-established session URI, session type URI parameter and SIP request method) through a SIP REFER message (S 11  and S 12 ), and receives a response to the SIP REFER message (S 13  and S 14 ). Then, a PoC server A  100  receiving the SIP REFER message sends an INVITE message for a session request to a corresponding address. When receiving a first response to the INVITE message, the PoC server A  100  notifies that the session is connected (S 15 ), forwards a Talk Burst Control Protocol (TBCP) message for granting the floor to the originating side PoC client A  111  (S 17 ), and sends media (S 18 ).  
         [0028]     A protocol used for the TBCP message in practice uses RTCP application (RTCP APP). The session connection and the floor are given when the terminating side makes the first response in order to swiftly granting the floor in the beginning. The foregoing procedure connects the session with the PoC user who makes the first response, sends the media, and then receives information of the users responding to the session request through a NOTIFY message (S 21  and S 22 ) and acknowledges the same through a  200  OK message (S 23  and S 24 ). The TBCP message forwards PoC group information requested by the originating side client in the form of a PoC session identifier and a PoC group identity, but does not contain information of the terminating side client who made the first response to the session request, thereby making it impossible to know to whom the user of the PoC client obtaining the floor first grants the floor. Further, a time delay results between grant and release of the floor for mutual identification of users who are initially connected, and thus constitutes inefficient use of resources.  
         [0029]     In addition, the terminating side receiving the request for session connection fails to get information on a list of participants in an ad-hoc group, making it difficult to determine an attribute of the session in which the terminating side participates.  
       SUMMARY OF THE INVENTION  
       [0030]     It is an object of the present invention to provide a method for providing identifiers such as information on a display name or address of a first respondent of a terminating side when the floor is initially granted to a session establisher in a PoC group session setup procedure.  
         [0031]     It is another object of the present invention to enable a PoC server to provide a terminating side client with information on additional users invited to a group when a specific subscription session is an ad-hoc group.  
         [0032]     In order to accomplish these objectives, the present invention is directed to showing that the identification information can be forwarded using an SIP protocol by defining the message format and corresponding fields of a TBCP protocol forwarded through the pre-established session and taking into account a more ordinary PoC group session even when a PoC server and conference server of the pre-established session are not compatible with each other.  
         [0033]     According to the present invention, there is provided a method for providing information of a client making a first response in a push-to-talk-over-cellular (PoC) network, including sending, by the session management server, an INVITE message to each of a plurality of clients to participate in the session to be established when it is determined that an event meeting session establishment criteria occurs at a server managing a session, receiving a first response signal to the INVITE message from one of the plurality of clients, and sending a message including information of the one of the plurality of clients making the first response to at least another of the plurality of clients, to request establishment of the session.  
         [0034]     According to the present invention, there is provided a system for providing information of a client making a first response in a push-to-talk-over-cellular (PoC) network, including an initiating client for requesting that a session be established, at least one participating client for participating in the session, and a session management server for managing the session between the initiating client and the at least one participating client, by sending an INVITE message to the initiating client and the at least one participating client to participate in the session to be established when an event meeting the session establishment criteria is met, and sending a first response message including information on a first one of the initiating client and the at least one participating client to respond to the INVITE message to at least one other client that has been sent the INVITE message.  
         [0035]     According to the present invention, there is provided a session management server for providing information of a client making a first response in a push-to-talk-over-cellular (PoC) network, wherein, when session establishment information is received from a terminal and an event meeting session establishment criteria received from the terminal is met, the session management server sends an INVITE message to clients to participate in a session to be established and sends a message including information of a first one of the clients making a response to the INVITE message. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description, taken in conjunction with the accompanying drawings, in which:  
         [0037]      FIG. 1  is a schematic diagram illustrating a conventional PoC service system;  
         [0038]      FIG. 2  is a schematic diagram showing the structure of a conventional PoC server;  
         [0039]      FIG. 3  is a schematic diagram of CF and PF blocks of a conventional PoC server;  
         [0040]      FIG. 4  shows a signaling flow for the session establishment process of a PoC client using a conventional pre-established session;  
         [0041]      FIG. 5  shows a signaling flow for a process of identifying a subscription session and a respondent when an ad-hoc PoC group call is made using a pre-established session in accordance with a first embodiment of the present invention;  
         [0042]      FIG. 6  shows a signaling flow for a process of identifying a subscription session and a respondent when a PoC group call is made using a pre-established session in accordance with a second embodiment of the present invention;  
         [0043]      FIG. 7  shows an RTCP payload format for implementing the present invention;  
         [0044]      FIG. 8  shows an INVITE message for identifying a subscription session and a respondent in order to implement the present invention; and  
         [0045]      FIG. 9  shows an SIP INVITE message for forwarding a list of receivers in a network-initiated ad-hoc session. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0046]     Hereinafter, exemplary embodiments of the present invention will be described in full detail with reference to the accompanying drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for the sake of clarity and conciseness.  
         [0047]     First, the present invention is constructed of a subordinate network (SIP/IP core network, radio access network, etc.) connected at both ends with a conventional PoC system, PoC terminal and PoC server. The PoC terminal comprises a PoC client for using PoC services, and the PoC server operates as CF or PF according to the owner of a PoC group session. The details of the conventional PoC system have been described in connection with the conventional art and thus ARE omitted here.  
         [0048]      FIG. 5  shows a procedure of forwarding information on a group and respondent client through a pre-established session when a network-initiated ad-hoc PoC session is established by forwarding information on the group to a terminating side client, and thus receiving a first response to the request when a request to establish the network-initiated ad-hoc PoC session is made to a PoC client with whom a pre-established session has been set up.  
         [0049]     In  FIG. 5 , it is assumed that subscription information on PoC session establishment is set for a PoC server, and then a PoC client sets up a pre-established session. The subscription information on PoC session establishment can be set up by performing service setting on a SIP PUBLISH message. Information set for the server comprises an event initiating session connection, and in the case of a group call, the identity of a corresponding group or a URI list of members of an ad-hoc group. The PoC server sends an INVITE message to corresponding members by triggering a preset event (S 101 ). At this time, the INVITE message comprises a group identity and the requester identifier of a network-initiated session.  
         [0050]     The group identity is forwarded as a PoC group identity taking the URI format of a pre-arranged group or chat group, and comprises an URI list of corresponding members in the case of the ad-hoc group. A format of the INVITE message (S 101 ) in which the URI list of the ad-hoc group members is included will be described below with reference to  FIG. 9 .  
         [0051]     Meanwhile, when an auto answer mode is set for the PF of a terminating side, the PF sends a provisional response  183  Session Progress (S 102 ), and selectively receives an acknowledge (ACK) signal that acknowledges the provisional response (S 103 ). Further, the PF sends the ad-hoc group member information received in step S 101  to a corresponding terminating side client (S 104 ).  
         [0052]     After receiving the provisional response (S 102 ), a CF forwards a Talk Burst Control Protocol (TBCP) message informing that the session is connected to a PoC client A  1110  connected by the pre-established session (S 105 ). At this time, the TBCP message comprises the PoC group identity (to confirm whether it is the same as the session identifier generated when the pre-established session is set up) indicating that the session is connected, and the identifier of the terminating side client who responds first. A detailed Real-time Transport Protocol Control Protocol Application (RTCP APP) format of the TBCP message will be described with reference to  FIG. 7 .  
         [0053]     The PoC client A  1110  performs procedures of granting the floor and sending a media after identifying the respondent client. Then, the CF buffers the media (S 108 ), and then receives a final response ( 200  OK) (S 110 ). Thereafter, the media is forwarded to a corresponding PoC client B  1210 .  
         [0054]      FIG. 6  shows a plan for forwarding the same identification information when a PoC server is not the same as a PF setting up a pre-established session in the process of  FIG. 5 . Steps S 201 , S 202 , S 203  and S 204  of  FIG. 6  are the same as steps S 101 , S 102 , S 103  and S 104 , respectively, of  FIG. 5 . However, since a server hosting a conference is not the same as a PF connected through the pre-established session, there is a need for a process (S 205 ) of sending an INVITE message for a session request between two servers. At this time, the INVITE message includes information on an established group and a client sending a response. A format of the INVITE message will be described with reference to  FIG. 8 .  
         [0055]     Meanwhile, after receiving the SIP INVITE message in step S 205 , a PF A  1100  sends a TBCP Connect message through a pre-established session (S 207 ). Here, the TBCP Connect message includes information on an established PoC group and identification information on a respondent client, as in  FIG. 5 . Thereafter, processes of granting the floor, sending a media, and buffering the media are the same as those of  FIG. 5 .  
         [0056]     The procedures of  FIGS. 5 and 6  apply to a particular case wherein the pre-established session exists on the side of the PoC client A, and the auto answer mode is set up on the terminating side (PoC client B). Thus, the principle of the present invention can be equally applied to a modified establishment procedure according to various modes (answer mode, pre-established session mode, on-demand mode, etc.) of the PoC service.  
         [0057]      FIG. 7  shows a format and fields of a TBCP Connect message forwarded through a pre-established session in the processes of  FIGS. 5 and 6 . The TBCP message is composed of an RTCP packet according to the OMA standard for PoC Release  1 . The RTCP packet requires RTCP transmission-related general information such as version information, packet type, sub-type information, packet length etc., and such information as the address and name of a PoC user and the talk burst connection. In the present invention, the RTCP packet comprises identification information of a respondent client and the PoC group of a network-initiated session.  
         [0058]      FIG. 8  shows an SIP INVITE message for forwarding identification information of a respondent client.  
         [0059]     In the INVITE message, general headers and contents are not described. Rather, the following description regards the header and contents of particular importance in t the present invention.  
         [0060]     First, a CF notifies that a URI included in a header field concerns the address of a conference server managing a request PoC group call by adding an “isfocus” tag to a “Contact” header of the INVITE message. Next, the portion composed of MIME contents comprises an SDP offer part for general media negotiation, and an XML part for information transmission of a corresponding conference. To this end, the header part has the field value of a content-type “multipart/mixed.” To forward information of a target PoC client who responds to the conference first, a field “conference-info” registered with IANA indicating a subscription conference can be used. At this time, general information on the subscription conference can be obtained through a field value “conference-description,” and detailed information on the PoC client accessing the conference can be obtained through a field value “users” up to a PoC address of the corresponding client, machine address and state used for access, a method of participating in the conference, and an attribute according to each media type.  
         [0061]     Identification information of a respondent and a group session when a network-initiated session is initially established can be forwarded through the “conference-info” field. Further, the two pieces of identification information can be reprocessed into a TBCP message using the “conference-info” field at the PF, and forwarded to the target client.  
         [0062]      FIG. 9  shows an SIP INVITE message for forwarding a receiver list to the target client in a network-initiated ad-hoc session. (e.g., steps S 201  to S 204  in  FIG. 6 ) Similar to  FIG. 8 , a CF first notifies that a URI included in a header field concerns the address of a conference server managing a request PoC group call by adding an “isfocus” tag to a “Contact” header of the INVITE message. Next, the portion of the message composed of MIME contents comprises an SDP offer part for general media negotiation, and an XML part for information transmission of a corresponding conference. In order to forward information on members of an ad-hoc group in the XML part, a field “resource-lists” registered with IANA can be used. Thereby, information on the address of a PoC client requested to connect to an ad-hoc session is forwarded to each terminating PoC client through the value of a field “list.” Parameters included in the “list” field can be included with the SIP URI and user name (display name) mapped with each other, but only information of the user name may be sent if necessary.  
         [0063]     Only two pieces of identification information of a group and a respondent client can be forwarded through the INVITE message. The other additional information of  FIGS. 8 and 9  can be omitted.  
         [0064]     In the procedure of establishing the session for the group call using the conventional PoC technology, the initial session is connected by the terminating PoC client receiving the first successful SIP response signal, and thus the floor is granted to the originating PoC client. At this time, no identification information of the initial respondent is sent, so that a proper call is made until the user granted the floor identifies his/her target. In addition, in the case of the network-initiated PoC group session, the user subscribing to the session needs to confirm which group call the SIP session request made to the user indicates.  
         [0065]     According to the present invention, in the PoC group session initiated from the network by such necessity, when the pre-established session is set up by the subscriber to the session, the user is notified of identification information of an initial caller and information of a requested conference, hence the PoC group call is smoothly performed. In addition, when subscription to the ad-hoc group is made, information on a list of corresponding call recipients is forwarded to the terminating PoC client. Thus, the receiver to whom a request to participate in the group session is made can selectively decide about session participation based on the member information.  
         [0066]     While the present invention has been described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in from and detail may be made therein without departing from the scope of the present invention as defined by the following claims.