Patent Publication Number: US-2006018275-A1

Title: Apparatus and method for managing session information in a mobile communication service system

Description:
CLAIM OF PRIORITY  
      This application claims the benefit under  35  U.S.C. §119(a) of an application entitled APPARATUS AND METHOD FOR MANAGING SESSION INFORMATION IN MOBILE COMMUNICATION SERVICE SYSTEM filed in the Korean Intellectual Property Office on 6 Jul., 2005 and assigned Serial No. 200-52410, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to an apparatus and method for managing session information in a mobile communication service system. More particularly, the present invention relates to an apparatus and method for managing session information in a mobile communication service system, in which, in a dormant state supported to provide fast service to a subscriber in a system where mobile communication service is provided, i.e., in an always-on mode where a Point-to-Point Protocol (PPP) session established between a mobile station and a Packet Data Serving Node (PDSN) is always maintained, information on the PPP session is efficiently managed, and a wireless resource of a wireless network and a wired resource of a wired network are efficiently used.  
      2. Description of the Related Art  
      Remote Wireless communication service technology establishes a packet network on an existing mobile communication network to provide wireless communication service to a mobile station (MS) that is in motion.  
      The packet network is composed of a packet data service node that sets up an environment to provide data service, and a packet transmission network that transmits a packet to the MS.  
      The packet transmission network is generally constructed of two systems: Universal Mobile Telecommunications System (UMTS) that has been standardized by Third Generation Partnership Project (3GPP), a European oriented standardization organization; and Code Division Multiple Access 2000 (CDMA 2000) that has been standardized by 3GPP2, a North American oriented standardization organization.  
      And, the packet transmission network based on the CDMA 2000 may be composed of a Packet Control Function (PCF), and a PDSN, and is connected to a base station (BS) that wirelessly exchanges CDMA signals with the MS.  
      The PCF manages a buffering function and a state of the MS until packets transmitted from the PDSN are wirelessly transmitted.  
      Further, the PDSN processes a Network Access Server (NAS) function to establish a Point-to-Point Protocol (PPP) session with the MS.  
      Specifically, the PDSN serves to establish, maintain and terminate the PPP session for the MS and a logic link to exchange data wirelessly, to convert data transmitted from the MS into packets, and to transmit the converted packets to an IP network.  
      According to a call processing procedure for providing the wireless communication service to the MS in the wireless communication service system, the PDSN establishes the PPP session with the MS when a service request message is received from the MS, wherein the PPP session is for providing service on the basis of a Link Control Protocol (LCP), an authentication procedure, and an Internet Protocol Control Protocol (IPCP).  
      The PDSN performs an access authentication procedure based on specific information of the MS, assigns an IP address to the MS, and registers the specific information of the MS.  
      Accordingly, the PDSN provides the wireless communication service through the PPP session which is established with the MS.  
      In the wireless communication service system, it is important to minimize a time to establish the PPP session for providing the wireless communication service to the MS.  
      Thus, in order to support the PPP session in an active state, a null state and a dormant state in the wireless communication service technology, various proposals have been suggested.  
      In the dormant state, a wireless link where the MS exchanges the packets is actually released, and the PPP session established with the PDSN is maintained. In this case, when there are packets to be exchanged while the PPP session established between the MS and the PDSN is maintained, the wireless link is established. Accordingly, a time to establish the wireless link for exchanging the packets is shortened.  
      Specifically, when no traffic is generated between the MS and the PDSN for a predetermined time, the dormant state is maintained for a predetermined time without conversion into the null state. Then, when the traffic is generated, a time to convert the state of the PPP session to the active state is minimized.  
      However, after the state of the PPP session between the MS and the PDSN is transitioned to the dormant state, the transitioned dormant state is transitioned to the null state after a predetermined time.  
      Therefore, in order to shorten the time to establish the PPP session between the MS and the PDSN to the highest degree, it is necessary to find a way where the MS or PDSN always maintains the PPP session in the dormant state.  
     SUMMARY OF THE INVENTION  
      It is, therefore, an objective of the present invention to provide an apparatus and method for managing session information in a mobile communication service system, which can minimize the time to establish a Point-to-Point Protocol (PPP) session between a mobile station MS and a Packet Data Serving Node (PDSN) in the mobile communication service system when a traffic is generated, by efficiently managing information of the PPP session and always maintaining the PPP session in a dormant state.  
      According to an aspect of the present invention, there is provided a mobile communication service system comprising a packet data serving node (PDSN) for transmitting an acknowledgement request message comprising information on an established point-to-point protocol (PPP) session while the session is transitioned to a dormant state, and for managing PPP session state on the basis of the response message received from each terminal; a base station(BS) for transmitting through an overhead channel the acknowledgement request message received from the PDSN and transmitting the response message received through the overhead channel to the PDSN; and at least one terminal for comparing the session information included in the acknowledgement request message received from the BS with previously stored session information and transmitting the response message to the base station through the overhead channel.  
      According to another aspect of the present invention, there is provided a mobile communication service system comprising at least one terminal for transmitting an acknowledgement request message comprising information on an established session through an overhead channel while the session is transitioned to a dormant state; a packet data serving node (PDSN) for comparing the session information included in the acknowledgement request message with previously stored session information to generate and transmit a response message; and a base station for transmitting the response message received from each terminal to the PDSN and transmitting the acknowledgement request message provided from the PDSN to each terminal through the overhead channel.  
      According to still another aspect of the present invention, there is provided a method of managing session information in a mobile communication service system having at least one terminal, a base station and a packet data serving node (PDSN). The method comprising the steps of transmitting, by the PDSN, an acknowledgement request message comprising information on a session established with each of the terminals to base station while the session is transitioned to a dormant state; transmitting, by the base station, the acknowledgement request message to each of the terminals through an overhead channel; generating, by each of the terminals, a response message as a result of comparing the session information included in the acknowledgement request message received from the PDSN with previously stored session information, and transmitting the generated response message to the base station through an overhead channel; transmitting, by the base station, the response message to the PDSN; and managing, by the PDSN, a state of the session on the basis of the response message.  
      According to yet another aspect of the present invention, there is provided a method of managing session information in a mobile communication service system having at least one terminal, a base station and a packet data serving node (PDSN). The method comprising the steps of generating, by each of the terminals, an acknowledgement request message comprising information on a session established with the PDSN when the session is transitioned to a dormant state, and transmitting the generated acknowledgement request message to the base station through an overhead channel; transmitting, by the base station, the acknowledgement request message to the PDSN; generating, by the PDSN, a response message as a result of comparing the session information included in the acknowledgement request message with previously stored session information, and transmitting the generated response message to the base station; transmitting, by the base station, the response message to each of the terminals through the overhead channel; and managing, by each of the terminals, a state of the session on the basis of the response message.  
      According to yet another aspect of the present invention, a least one terminal in a mobile communication service system, the terminal comprising: Wireless interface for receiving acknowledgement request message from PDSN through overhead channel, and for transmitting response message through overhead channel; Response processor for generating response message as a result of comparing the PPP session information included in the acknowledgement request message with previously stored PPP session information, and for transmitting response message to PDSN though wireless interface.  
      According to yet another aspect of the present invention, a method of managing session information in a mobile communication service system having at least one terminal, the method comprising the steps of: Receiving, an acknowledgement request message comprising information on a PPP session though overhead channel; Generating, a response message as a result of comparing the PPP session information included in the acknowledgement request message received with previously stored session information; Transmitting, the response message though overhead channel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, in which like reference symbols indicate the same or similar components, wherein:  
       FIG. 1  is a block diagram illustrating an overall configuration for a network connection in a mobile communication service system;  
       FIG. 2  is block diagram illustrating an interface established in a mobile communication service system;  
       FIG. 3  is an internal block diagram of a mobile station (MS) according to a first embodiment of the present invention;  
       FIG. 4  is an internal block diagram of a packet data serving node (PDSN) according to a first embodiment of the present invention;  
       FIG. 5  is a flowchart showing the procedure of exchanging messages through a traffic channel according to a first embodiment of the present invention;  
       FIG. 6  is a flowchart showing the procedure of exchanging messages through an overhead channel according to a first embodiment of the present invention;  
       FIG. 7  is a flowchart illustrating a method of managing session information in a mobile communication service system according to a first embodiment of the present invention;  
       FIG. 8  is an internal block diagram of a PDSN according to a second embodiment of the present invention;  
       FIG. 9  is an internal block diagram of an MS according to a second embodiment of the present invention;  
       FIG. 10  is a flowchart showing the procedure of exchanging messages through a traffic channel according to a second embodiment of the present invention;  
       FIG. 11  is a flowchart showing the procedure of exchanging messages through a overhead channel according to a second embodiment of the present invention; and  
       FIG. 12  is a flowchart illustrating a method of managing session information in a mobile communication service system according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. To enable a clear understanding of the present invention, related technology that is well known to those of ordinary skill in the technical field to which the present invention pertains, will not be described in detail.  
       FIG. 1  is a block diagram illustrating an overall configuration for a network connection in a mobile communication service system.  
      Referring to  FIG. 1 , a mobile communication service system comprises a mobile station (MS)  100 , a base station (BS)  200 , a Packet Control Function (PCF)  300 , and a Packet Data Serving Node (PDSN)  400 , and a service server  500 .  
      The MS  100  and the BS  200  is connected through a wireless link, and the BS  200 , the PCF  300  and the PDSN are connected through a wired network.  
      And, the PDSN  400  is connected with the service server  500  through an Internet Protocol (IP) network.  
      The MS  100  transmits a Session Initiation Protocol (SIP) type request message to the BS  200  according to selection of a subscriber.  
      And, the BS  200  transmits the request message, which is received from the MS  100  within a service cell, to the PDSN  400  by way of the PCF  300 .  
      The PDSN  400  establishes a Point-to-Point Protocol (PPP) session with the MS  100  on the basis of the received request message, converts the request message into an IP packet, and transmits the converted IP packet to the service server  500 .  
      The service server  500  provides mobile communication service based on the request message that is received from the PDSN  400 .  
      Here, the network comprising the BS  200 , PCF  300  and PDSN  400  is called an access network which allows the MS  100  to access the IP network.  
      And, the service provided by the service server  500  may comprise real-time multimedia service, which is provided in interaction of the mobile communication network and the IP network, such as Push To Talk (PTT) service, Push To Data (PTD) service and Image Message (IM) service.  
       FIG. 2  is block diagram illustrating an interface established in a mobile communication service system.  
      As shown in  FIG. 2 , a Radio Packet (RP) interface is established between the MS  100  and the BS  200 . A 8 /A 9  interfaces are established between the BS  200  and the PCF  300 . A 10 /A 11  interfaces are established between the PCF  300  and the PDSN  400 .  
      The A 8  and A 10  interfaces are for exchanging traffic, and the A 9  and A 11  interfaces are for signaling.  
      When the PPP session of the mobile communication service system is in an active state, this indicates that the A 8  and A 10  interfaces are established to exchange traffic. When the PPP session is in a dormant state, this indicates that the A 10  interface is established, but the A 8  interface is released.  
      In other words, in order to transition the dormant state of the PPP session to the active state, the A 8  interface as the traffic interface should be established.  
      Further, the RP interface established between the MS  100  and the BS  200  may be divided into a traffic channel and an overhead channel.  
      The traffic channel may comprise a forward fundamental channel, a reverse fundamental channel, a forward dedication channel, a reverse dedication channel and so on.  
      The overhead channel may comprise a paging channel, an access channel, a forward common control channel, an enhanced access channel and so on.  
      And, the MS  100  or PDSN  400  may manage information on the PPP session in the mobile communication service system.  
       FIG. 3  is an internal block diagram of an MS according to a first embodiment of the present invention.  
      Referring to  FIG. 3 , an MS  100  of the present invention comprises a wireless interface  120 , a central processor  140 , an input unit  110  and a session information storage  130 , wherein the central processor  140  comprises a session information manager  10 .  
      The session information manager  10  comprises a timer  11  and a message processor  12 .  
      The wireless interface  120  exchanges messages through a RP interface, i.e. a traffic or overhead channel, which is established with a BS  200  wirelessly.  
      And, the input unit  110  provides a key signal based on selection of a subscriber, and the central processor  140  generates a message based on the key signal received from the input unit  110  and then transmits the generated message to the BS  200 .  
      Here, the central processor  140  transmits a service request message in a SIP format on the basis of the received key signal.  
      Further, the central processor  140  provides mobile communication service to the subscriber on the basis of data received through the BS  200 .  
      The session information storage  130  stores information on a PPP session that is established with a PDSN  400  through the BS  200 .  
      The PPP session information stored in the session information storage  130  may comprise Link Control Protocol (LCP) information, authentication information or Internet protocol control protocol (IPCP) information.  
      Here, LCP is a control protocol of a data link layer, which processes control functions such as PPP layer initiation of the data link layer. And, IPCP processes a function of automatically allocating an IP address prior to communication from among functions of PPP.  
      The session information manager  10  of the central processor  140  generates an acknowledgement request message at predetermined regular intervals or at irregular intervals, transmits the generated request message to the PDSN  400  through the BS  200 , and manages the PPP session information stored in the session information storage  130  on the basis of a response message received from the PDSN  400 .  
      And, the timer  11  of the session information manager  10  provides a due signal when a preset time has lapsed.  
      When the due signal is received from the timer  11 , the message processor  12  generates the acknowledgement request message, transmits the generated request message to the PDSN  400 , and manages the PPP session information stored in the session information storage  130  on the basis of the response signal received from the PDSN  400 .  
      The acknowledgement request message generated by the message processor  12  may comprise any one of the LCP information, authentication information and IPCP information which are the PPP session information.  
      The acknowledgement request message generated by the message processor  12  can be transmitted through the traffic or overhead channel.  
       FIG. 4  is an internal block diagram of a PDSN according to a first embodiment of the present invention.  
      Referring to  FIG. 4 , a PDSN  400  of the present invention comprises a wired interface  410 , a packet processor  440 , a database  430  and an IP interface  420 .  
      The wired interface  410  receives a message transmitted from a MS  100  by way of a BS  200 .  
      And, the database  430  stores information on a PPP session established with the MS  100 , and the IP interface  420  transmits an IP packet, which is received from the packet processor  440 , to an IP network.  
      Here, the PPP session information stored in the database  430  is stored in the same manner as that stored in the MS  100  where the PPP session is established.  
      The packet processor  440  converts data received through the wired interface  410  into the IP packet, and transmits the converted IP packet to the IP network.  
      The packet processor  440  comprises a response processor  20 . The response processor  20  compares the PPP session information included in an acknowledgement request message received from the MS  100  with that stored in the database  430 . If they are equal to each other, the response processor  20  transmits an OK response message to the MS  100 . In contrast, if not equal, the response processor  20  transmits a Not OK (NOK) response message to the MS  100 . At this time, the response processor  20  encapsulates information on an overhead channel, which indicates that the message should be transmitted through the overhead channel, into a Generic Route Encapsulation (GRE) or IP header of the response message, and allows the response message to be transmitted through the overhead channel.  
      The MS  100  comprises a session information manager  10 . When the OK response message is received from the PDSN  400 , the session information manager  10  maintains the PPP session established with the PDSN  400 . However, when the NOK response message is received, the session information manager  10  deletes the PPP session information stored in the session information storage  130 , thereby terminating the PPP session or updating the PPP session information stored in the session information storage  130 .  
      Specifically, when the OK response message is received from the PDSN  400 , the session information manager  10  maintains the PPP session and a dormant state where the RP interface is released. However, when the NOK response message is received, the session information manager  10  is transitioned to a null state, or transmits a session information request message to the PDSN  400  to thereby update the PPP session information.  
       FIG. 5  is a flowchart showing the procedure of exchanging messages through a traffic channel according to a first embodiment of the present invention.  
      Referring to  FIG. 5 , after the PPP session established with the PDSN  400  was in a dormant state, namely after the RP interface was released and thereby only the PPP session was maintained, a predetermined time has lapsed. Then, the MS  100  generates an acknowledgement request message in which information of the PPP session is included, wherein the PPP session information is stored in the session information storage  130  at step S 1 .  
      In this case, the PPP session information may comprise LCP information, authentication information or IPCP information which is included in the acknowledgement request message. The following description will be made about an example where the LCP information is included in the acknowledgement request message. However, this description may be true of an example where any one of the others is included in the acknowledgement request message.  
      The MS  100  generates the acknowledgement request message in the dormant state at regular or irregular intervals.  
      The MS  100  and the BS  200  establish the RP interface for transmitting the acknowledgement request message at step S 2 .  
      To be specific, the MS  100  and the BS  200  are transitioned from the dormant state to an active state on the basis of a packet data termination call flow, and thus establish a traffic channel.  
      When the traffic channel is established between the MS  100  and the BS  200 , the MS  100  transmits the generated acknowledgement request message to the BS  200  at step S 3 .  
      And, the BS  200  establishes an A 8  interface in order to transmit the acknowledgement request message received from the MS  100 , and transmits the acknowledgement request message to the PCF  300  through the A 8  interface at step S 4 .  
      The PCF  300  transmits the received acknowledgement request message to the PDSN  400  through an A 10  interface at step S 5 .  
      In other words, the BS  200  exchanges the acknowledgement request message transmitted to the PDSN  400  in a traffic form, thereby transmitting the acknowledgement request message through the A 8 /A 10  interface for traffic exchange.  
      The PDSN  400  compares PPP session information included in the received acknowledgement request message with that stored in the database  430  to generate a response message at step S 6 .  
      At this time, the PDSN  400  generates an OK response message if LCP information included in the acknowledgement request message is identical to that stored in the database  430 , and generate a NOK response message if not.  
      And, the PDSN  400  transmits the generated response message to the PCF  300  through the A 10  interface at step S 7 , and the PCF  300  transmits the received response message to the BS  200  through the A 8  interface at step S 8 .  
      The BS  200  transmits the received response message to the MS  100  through the traffic channel at step S 9 .  
      The MS  100  manages the PPP session information stored in the session information storage  130  on the basis of the received response message.  
      And, the MS  100  releases the traffic channel, which is the RP interface established with the BS  200  at step S 10 .  
      The MS  100  releases the RP interface established with the BS  200  and maintains the dormant state if the response message received from the PDSN  400  is the OK response message, and terminates the PPP session established with the PDSN  400  or transmits the session information request message to the PDSN  400  to update the current PPP session information with the PPP session information received because the MS  100  stores the PPP session information different from that of the PDSN  400  if the response message is the NOK response message.  
       FIG. 6  is a flowchart showing the procedure of exchanging messages through a overhead channel according to a first embodiment of the present invention.  
      Referring to  FIG. 6 , when a predetermined time has lapsed at regular or irregular intervals in a dormant state where a PPP session is established between the MS  100  and the PDSN  400 , the MS  100  generates an acknowledgement request message in which PPP session information stored in the session information storage  130  is included at step S 20 .  
      The MS  100  transmits the acknowledgement request message to the BS  200  through an overhead channel at step S 21 .  
      And, the BS  200  transmits the acknowledgement request message received from the MS  100  to the PCF  300  through an A 9  interface at step S 22 .  
      The PCF  300  transmits the received acknowledgement request message to the PDSN  400  through an A 10  interface maintained in the dormant state at step S 23 .  
      The PDSN  400  compares the PPP session information included in the received acknowledgement request message with that stored in the database  430 , and generates a response message at step S 24 .  
      Here, the PDSN  400  generates an OK response message if LCP information included in the acknowledgement request message is identical to that stored in the database  430 , and generates a NOK response message and encapsulates information on the overhead channel into a GRE or IP header if not.  
      And, the PDSN  400  transmits the generated response message to the PCF  300  through the A 10  interface at step S 25 , and the PCF  300  transmits the response message to the BS  200  through the A 8  interface at step S 26 .  
      The BS  200  transmits the received response message to the MS through the overhead channel at step S 27 .  
      The MS  100  manages the PPP session information according to a type of the received response message.  
      The MS  100  releases the RP interface established with the BS  200  and maintains the dormant state if the response message received from the PDSN  400  is the OK response message, and terminates the PPP session established with the PDSN  400  or transmits the session information request message to the PDSN  400  to update the current PPP session information with the received PPP session information because the MS  100  stores the PPP session information different from that of the PDSN  400  if the response message is the NOK response message.  
       FIG. 7  is a flowchart illustrating a method of managing session information in a mobile communication service system according to a first embodiment of the present invention.  
      Referring to  FIG. 7 , a PPP session established between the MS  100  and the PDSN  400  is transitioned to a dormant state, the MS  100  counts whether the dormant state is maintained for a predetermined time. When a preset predetermined time has lapsed, the MS  100  generates an acknowledgement request message in which PPP session information stored in the session information storage  130  is included at step S 30 .  
      Then, the MS  100  transmits the generated acknowledgement request message to the PDSN  400  by way of the BS  200  at step S 31 .  
      Here, the MS  100  can transmit the acknowledgement request message through a traffic or overhead channel. When the acknowledgement request message is transmitted through the traffic channel, the MS  100  preferably transmits the acknowledgement request message after establishing the traffic channel with the BS  200 .  
      The BS  200  establishes an A 8  interface to transmit the acknowledgement request message to the PCF  300  when the received acknowledgement request message is transmitted through the traffic channel in a data format.  
      The PDSN  400  compares the PPP session information included in the acknowledgement request message received from the MS  100  with that stored in the database  430  at step S 32 . If they are identical to each other, the PDSN  400  generates an OK response message at step S 33 . In contrast, if not, the PDSN  400  generates a NOK response message and encapsulates information on an overhead channel into a GRE or IP header of the response message at step S 34 .  
      Then, the PDSN  400  transmits the generated response message to the MS  100  through the overhead channel by way of the PCF  300  and BS  200  at step S 35 .  
      The MS  100  checks whether the received response message is the OK response message or the NOK response message at step S 36 . If the received response message is the OK response message, the MS  100  maintains the PPP session established with the PDSN  400  in the dormant state at step S 37 . However, if the received response message is the NOK response message, the MS  100  transitions a state of the PPP session into a null state, or transmits a session information request message to the PDSN  400  to update the current session information with session information received at step S 38 .  
       FIG. 8  is an internal block diagram of a PDSN according to a second embodiment of the present invention.  
      An example where information on a PPP session established between an MS  100  and a PDSN  400  is managed by the PDSN  400  will be described with reference to  FIG. 8 .  
      As shown in  FIG. 8 , a PDSN  400  comprises a packet processor  440  which has a session information manager  10 . The session information manager  10  comprises a timer  11  for providing a due signal after the lapse of a preset predetermined time, and a message processor  12  for generating an acknowledgement request message in which PPP session information stored in a database  430  and channel information indicating that a message should be transmitted in a signaling traffic are included when the due signal is received from the timer  11 .  
      In order words, the PDSN  400  encapsulates and transmits the overhead channel information into a GRE or IP header of the acknowledgement request message in which the PPP session information is included, and the overhead channel information may be allocated to a reserved field of the GRE or IP header.  
      Further, the PDSN  400  can generate the acknowledgement request message by causing the PPP session information and the overhead channel information to be included in a message such as an LCP echo request message.  
      And, the session information manager  10  transmits the generated acknowledgement request message to an MS  100  through the traffic or overhead channel, and manages the PPP session information stored in the database  430  on the basis of a response message received from the MS  100 .  
      In other words, the PDSN  400  shown in  FIG. 8  manages the PPP session information when the PPP session established with the MS  100  is transitioned to a dormant state.  
       FIG. 9  is an internal block diagram of a MS according to a second embodiment of the present invention.  
      Referring to  FIG. 9 , a MS  100  comprises a central processor  140  which has a response processor  20 . The response processor  20  compares PPP session information included in an acknowledgement request message received through a wireless interface  120  with that stored in a session information storage  130 , and transmits a response message through a traffic or overhead channel.  
      Thus, if the PPP session information included in the acknowledgement request message received from a PDSN  400  is identical to that stored in the session information storage  130 , the MS  100  transmits an OK response message. However, if not, the MS  100  transmits a NOK response message.  
       FIG. 10  is a flowchart showing the procedure of exchanging messages through a traffic channel according to a second embodiment of the present invention.  
      Referring to  FIG. 10 , when a PPP session established between the MS  100  and the PDSN  400  is in a dormant state, the timer  11  provides a due signal after the lapse of a predetermined time, and the message processor  12  generates an acknowledgement request message in which PPP session information stored in the database  430  is included at step S 40 .  
      Here, the message processor  12  generates the acknowledgement request message in which the PPP session information is included after a predetermined time has lapsed at regular or irregular intervals.  
      And, the PDSN  400  transmits the generated acknowledgement request message to the PCF  300  through an A 10  interface in a data format at step S 41 .  
      When the acknowledgement request message is received from the PDSN  400 , the PCF  300  establishes an A 8  interface according to a packet data termination call flow, and transitions a dormant state of the PPP session to an active state at step S 42 .  
      And, the PCF  300  transmits the acknowledgement request message to the BS  200  through the A 8  interface at step S 43 .  
      When the acknowledgement request message is received from the PCF  300  through the A 8  interface as the traffic channel, the BS  200  establishes the traffic channel with the MS  100  at step S 44 , and transmits the acknowledgement request message to the MS  100  at step S 45 .  
      And, the MS  100  generates an OK response message if the PPP session information included in the received acknowledgement request message is identical to that stored in the session information storage  130 , and generates a NOK response message if not at step S 46 .  
      The MS  100  transmits the generated response message to the BS  200  through the established traffic channel at step S 47 , and then releases the traffic channel at step S 48 .  
      Here, the MS  100  transmits the response message to release the traffic channel, or waits for reception of a session information request message from the PDSN for a predetermined time. When the session information request message is received, the MS  100  transmits the PPP session information stored in the session information storage  130  to the PDSN  400  or releases the traffic channel after the lapse of a predetermined time.  
      The BS  200  transmits the received response message to the PCF  300  through an A 8  interface at step S 49 , and then the PCF  300  transmits the response message to the PDSN  400  through an A 10  interface at step S 50 .  
      The PCF  300  transmits the response message to the PDSN  400 , and then releases the established A 8  interface.  
      The PDSN  400  manages the PPP session information stored in the database  430  on the basis of the received response message.  
      Specifically, the PDSN  400  maintains the PPP session established with the MS  100  in the dormant state if the received response message is the OK response message. However, if the response message is the NOK response message, the PDSN  400  terminates the PPP session established with the MS  100 , or transmits the session information request message to the MS  100  to update the current PPP session information with the received PPP session information because the PDSN  400  stores the PPP session information different from that of the MS  100 .  
       FIG. 11  is a flowchart showing the procedure of exchanging messages through a overhead channel according to a second embodiment of the present invention.  
      Referring to  FIG. 11 , when a PPP session established between the MS  100  and the PDSN  400  is in a dormant state, the timer  11  provides a due signal after the lapse of a predetermined time, and the message processor  12  generates an acknowledgement request message in which PPP session information and overhead channel information stored in the session information storage  130  are included at step S 60 .  
      And, the PDSN  400  transmits the generated acknowledgement request message to the PCF  300  through an A 10  interface in a data format at step S 61 .  
      The PCF  300  transmits the received acknowledgement request message to the BS  200  through an A 9  interface at step S 62 .  
      At this time, the PDSN  400  comprises the overhead channel information in the acknowledgement request message such that the PCF  300  or BS  200  transmits the received acknowledgement request message of a data format to the MS  100  through the overhead channel, and the BS  200  transmits the acknowledgement request message in a mode of Short Data Burst (SDB) when the overhead channel information is included in the received acknowledgement request message.  
      Further, the PDSN  400  encapsulates the overhead channel information into a GRE or IP header of the acknowledgement request message such that the PCF  300  or BS  200  can transmit the acknowledgement request message to the MS  100  in the SDB mode, and the PCF  300  or BS  200  transmits the acknowledgement request message to the MS  100  through the overhead channel according to the SDB mode when the overhead channel information is encapsulated into the GRE or IP header of the acknowledgement request message.  
      The BS  200  transmits the acknowledgement request message, which is received from the PCF  300 , to the MS  100  through the overhead channel at step S 63 .  
      At this time, the BS  200  recognizes, from the GRE or IP header of the received data, the acknowledgement request message as a message to be transmitted through the overhead channel, and then transmits a Data Burst Message (DBM) to the MS  100 , wherein the received acknowledgement request message is included in the DBM.  
      And, the MS  100  compares PPP session information included in the received acknowledgement request message with that stored in the session information storage  130 . If they are identical to each other, the MS  100  generates an OK response message. However, if not, the MS  100  generates a NOK response message at step S 64 .  
      The MS  100  transmits the generated response message to the BS  200  through the overhead channel at step S 65 , and the BS  200  transmits the received response message to the PCF  300  through the A 9  interface at step S 66 .  
      At this time, the MS  100  transmits the DBM in which the response message is included to the BS  200  through the overhead channel.  
      The PCF  300  transmits the response message, which is received from the BS  200 , to the PDSN  400  through the A 10  interface at step S 67 , and the PDSN  400  manages the PPP session information stored in the database  430  on the basis of the received response message.  
      Specifically, the PDSN  400  maintains the PPP session established with the MS  100  in the dormant state if the response message received from the MS  100  is the OK response message. However, if the response message is the NOK response message, the PDSN  400  terminates the PPP session established with the MS  100 , or transmits the session information request message to the MS  100  to update the current PPP session information with the received PPP session information because the PDSN  400  stores the PPP session information different from that of the MS  100 .  
       FIG. 12  is a flowchart illustrating a method of managing session information in a mobile communication service system according to a second embodiment of the present invention.  
      Referring to  FIG. 12 , when a preset predetermined time has lapsed after a PPP session established with the MS  100  was transitioned to a dormant state, the PDSN  400  generates an acknowledgement request message in which PPP session information and overhead channel information stored in the database  430  are included at step S 70 .  
      And, the PDSN  400  transmits the generated acknowledgement request message to the MS  100  at step S 71 .  
      At this time, the PDSN  400  may transmit the acknowledgement request message to the MS  100  through a traffic or overhead channel. When the acknowledgement request message is transmitted to the MS  100  through the traffic channel, the PCF  300  establishes an A 8  interface for transmitting the acknowledgement request message received from the PDSN  400 , and the BS  200  establishes the traffic channel with the MS  100 .  
      And, the PCF  300  transmits the acknowledgement request message through the established A 8  interface, and then the BS  200  transmits the acknowledgement request message, which is received from the PCF  300 , to the MS  100 .  
      Further, the PDSN  400  transmits the generated acknowledgement request message to the PCF  300  in a data format.  
      In contrast, when transmitting the acknowledgement request message through the overhead channel, the PDSN  400  encapsulates the overhead channel information into a GRE or IP header of the acknowledgement request message, and then transmits the acknowledgement request message to the PCF  300  through the A 10  interface in the data format.  
      And, when the overhead channel information is encapsulated into the GRE or IP header of the received acknowledgement request message, the PCF  300  or BS  200  transmits the acknowledgement request message to the MS  100  through the overhead channel.  
      The MS  100  compares the PPP session information included in the acknowledgement request message received from the PDSN  400  with that stored in the session information storage  130  at step S 72 . If they are identical to each other, the MS  100  generates an OK response message at step S 73 . However, if not, the MS  100  generates a NOK response message at step S 74 .  
      Then, the MS  100  transmits the generated response message to the BS  200  through the overhead channel, and then the BS  200  transmits the received response message to the PDSN  400  by way of the PCF  300  at step S 75 .  
      The PDSN  400  checks whether or not the received response message is the OK response message at step S 76 . If the response message is the OK response message, the PDSN  400  maintains the PPP session established with the MS  100  in the dormant state at step S 77 . If the response message is the NOK response message, the PDSN  400  transitions a state of the PPP session into a null state, or transmits the session information request message to the MS  100  to update the current PPP session information with the received PPP session information at step S 78 .  
      As set forth above, according to the embodiments of the present invention, when a predetermined time has lapsed in the state where the PPP session established between the MS and the PDSN in the mobile communication service system was transitioned to the dormant state, the PPP session is not transitioned to the null state, but it is still maintained in the dormant state when information on the established PPP session is checked to be identical, and thereby it is possible to maximize a time when the PPP session established between the MS and the PDSN is maintained in the dormant state.  
      Therefore, it is possible to maximize the efficiency of wired and wireless resources in the mobile communication service system, and to minimize a start time of mobile communication service provided by request of the subscriber.  
      Although exemplary embodiments of the present invention have been described, it will be understood by those skilled in the art that the present invention should not be limited to the described exemplary embodiments. Various changes and modifications can be made within the spirit and scope of the present invention as defined by the following claims.