Abstract:
A method for performing a handoff in an ALL-IP network. The Mobile Station (MS) transmits measurement information to a new Base Transceiver System (BTS) and the Radio Network Controller (RNC) to start a handoff, the RNC requests the new BTS to set up a radio link with the MS. When the radio link is set up between the MS and the new BTS, the RNC requests to delete header information of the MS stored in an old BTS. The MS transmits a message including traffic header information to the new BTS, the new BTS transmits a binding update message to the MS. Then the MS transmits only the voice frame excluding the traffic header information to the new BTS.

Description:
PRIORITY 
     This application claims priority to an application entitled “Method for Performing Handoff in an ALL-IP Network” filed in the Korean Industrial Property Office on Dec. 29, 2001 and assigned Serial No. 2001-88405, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a handoff method in a mobile communication system supporting an ALL-IP network, and in particular, to a handoff method for removing an overhead of an ALL-IP-based voice transmission frame. 
     2. Description of the Related Art 
     Recently, in order to solve an inefficiency problem of radio resources during circuit switching in an IMT-2000 (International Mobile Telecommunication-2000) network supporting both a packet network and a circuit network, many attempts have been made to integrate next generation mobile communication networks into a packet-based IP (Internet Protocol) network, called an ALL-IP network, by major next generation mobile communication standardization organizations including 3GPP (3 rd  Generation Partnership Project) and 3GPP2 (3 rd  Generation Partnership Project 2). The ALL-IP network has an open-type data network structure, so it can effectively support future IP-based services and enable integrated, low-cost maintenance using IP. However, the ALL-IP network wastes bandwidths of a radio link due to an excessive traffic overhead occurring in the radio link. Specifically, in a voice service, the most fundamental service of the mobile communication system, an excessive overhead is generated in order to transmit a small frame. 
       FIG. 1  illustrates a structure of an ALL-IP network based on a third generation mobile communication system. Referring to  FIG. 1 , an ALL-IP network includes a mobile station (MS)  100 , base transceiver systems (BTSs)  110 - 1  and  110 - 2 , a radio network controller (RNC, also known as BSC)  120 , a core network (CN)  130 , and a public network (including PSTN (Public Switched Telephone Network) and Internet)  140 . The BTSs  110 - 1  and  110 - 2 , and the RNC  120  support a radio access service. The CN  130  connects the RNC  120  to the public network  140 . 
     In the ALL-IP network, a 40-bit overhead including an RTP (Real Time Protocol) header, a UDP (User Datagram Protocol) header and an IP header is generated on a radio link between the MS  100  and the BTS  110 . The 40-bit overhead represents an overhead excluding a channel overhead in a physical layer. Meanwhile, in order to perform routing and other various functions in the ALL-IP network, a voice frame, while it passes through an RTP stack and a UDP stack, adds an RTP header and a UDP header thereto. Further, an additional overhead may be optionally added to the voice frame. As a result, an actual size of the voice frame on a radio link becomes less than 50% of the whole traffic, causing a waste of bandwidths of the radio link. Such a phenomenon becomes more severe when a soft handoff in which the MS  100  occupies two or more radio channels occurs between the BTSs. 
       FIG. 2  illustrates a soft handoff procedure in which an MS occupies 2 channels. Referring to  FIG. 2 , a voice frame transmitted through downlink (or forward) and uplink (or reverse) radio channels between the MS  100  and the two BTSs  110 - 1  and  110 - 2  also includes a 40-bit overhead for voice channel transmission. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a handoff method for removing an overhead generated in voice traffic on a radio link between an MS and a BTS in an ALL-IP network. 
     According to an aspect of the present invention, there is provided a method for performing a handoff in an ALL-IP network including at least one MS (Mobile Station) for transmitting a signaling frame having a header and a voice frame to an old BTS (Base Transceiver System), which includes a header information storage for storing overhead information for the MS in association with a unique code of the MS, and an RNC (Radio Network Controller). The MS transmits measurement information to a new BTS and the RNC to start a handoff. Upon receipt of the measurement information, the RNC requests the new BTS to set up a radio link with the MS that has transmitted the measurement information. When the radio link is set up between the MS and the new BTS, the RNC deletes header information for the MS, stored in the header information storage of the old BTS. The MS transmits a message including traffic overhead information to the new BTS. Upon receipt of the message including the traffic header information, the new BTS transmits a binding update message to the MS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a structure of an ALL-IP network based on a third generation mobile communication system; 
         FIG. 2  illustrates a soft handoff procedure in which an MS occupies two channels; 
         FIG. 3  illustrates a BTS with a header information storage and an MS connected to the BTS according to a preferred embodiment of the present invention; 
         FIG. 4  illustrates a soft handoff procedure in BTSs each including a header information storage in an ALL-IP network according to a preferred embodiment of the present invention; 
         FIG. 5  illustrates a message flow between elements constituting an ALL-IP network according to a preferred embodiment of the present invention; 
         FIG. 6  is a flow chart illustrating a handoff operation by an MS according to a preferred embodiment of the present invention; and 
         FIG. 7  is a flow chart illustrating a handoff operation by a BTS according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings in which like parts or elements are denoted by like reference numerals throughout the several views. 
     In the following description, the term “old BTS” indicates a BTS  110 - 1  of a cell  115 - 1  where an MS  100  was originally located, and the term “new BTS” indicates a BTS  110 - 2  with which the MS  100  will newly communicate due to a handoff. 
     The embodiment of the present invention uses a header information storage included in a BTS in order to remove an overhead generated during a handoff in an ALL-IP network. 
       FIG. 3  illustrates a BTS with a header information storage and an MS connected to the BTS according to a preferred embodiment of the present invention. A structure and operation of transmitting a voice frame according to an embodiment of the present invention will be described with reference to  FIG. 3 . 
     Referring to  FIG. 3 , an MS  100  includes an application  302 , a signaling processor  304 , a protocol stack  310 , and an interface  318 . The application  302  converts voice input by a user into voice data and outputs a voice frame with a specific length. The signaling processor  304  receives a telephone number or an IP address and generates a header generation command for generating corresponding header information. The protocol stack  310  generates header information in response to the header generation command provided from the signaling processor  304 . A voice frame  352  and a signaling frame with an overhead such as the RTP header and the UDP header, generated respectively by the application  302  and the signaling processor  304  of the MS  100 , are transmitted to the BTS  110  through different channels. The voice traffic and the signal traffic may be transmitted through a traffic channel and a traffic control channel, respectively. 
     The BTS  110  includes a signaling processor  330 , a header information storage  300 , an interface  332 , and a frame controller (of frame handler)  334 . The interface  332  interfaces signals transmitted and received to/from at least one MS  100 . The frame controller  334  controls frames transmitted and received through the interface  332 . The header information storage  300  stores header information received from the MS  100 , in association with the corresponding MS. The BTS  110  uses a code for a reverse link in order to identify the header information of the MS  100 . That is, the BTS  110  detects a unique code for the MS  100  connected thereto and stores the detected code. Further, the BTS  110  detects header information received from the MS  100  and stores the detected header information in association with the unique code of the MS  100  that has transmitted the header information. 
     Upon receipt of a call setup request from a user, the MS  100  generates a telephone number or an IP address input by the user and provides the generated telephone number or IP address to the signaling processor  304 . Upon receiving the telephone number or IP address, the signaling processor  304  provides the protocol stack  310  with a header generation command based on the received telephone number or IP address. The protocol stack  310  generates an RTP header, a UDP header, and an IP header in response to the header generation command, and provides them to the interface  318  and a physical layer  320 . The physical layer  320  carries the header information on the signaling frame, spreads the signaling frame with a user identification code, modulates the spread signaling frame into a radio signal, and transmits the radio signal to the BTS  110  through a packet control channel. After transmission of the signaling frame, the MS  100  forms a communication path through call negotiation with the BTS  110 , encodes user&#39;s voice into voice data, and provides the voice data to the physical layer  320  in a voice frame unit. The physical layer  320  modulates the voice frame into a radio signal and transmits the radio signal to the BTS  110  through the traffic channel. 
     The BTS  110  receives the signaling frame transmitted by the MS  100 . The signaling frame is provided to the signaling processor  330  and the frame controller  334  through a physical layer  336  and the interface  332 . The signal input to the signaling processor  330  is a demodulated signal of the signaling frame received in the form of the radio signal, and the signal input to the frame controller  334  is a unique code for the MS, detected from the received signaling frame. The frame controller  334  receives the unique code for the MS and allows the signaling processor  330  to control an operation of processing the signaling frame. The signaling processor  330 , under the control of the frame controller  334 , detects headers from the received signaling frame and stores the detected headers in the header information storage  300 . 
     After storing the headers from the MS  100  in the header information storage  300 , the frame controller  334  detects an MS identification code for the voice frame received from the MS  100  and searches the header information storage  300  for the header information corresponding to the detected code. Upon detecting the header information, the frame controller  334  adds the detected header information to the voice frame and transmits it to the RNC  120 . By doing so, the header for the voice frame of the MS  100  is added to the voice frame received through the header information storage  300  of the BTS  110  and transmitted to a core network. Consequently, it is possible to transmit only the pure voice frame through a voice transmission channel on the radio link between the MS  100  and the BTS  110 . 
     The signal transmission from the BTS  110  to the MS  100  is performed as follows. The BTS  110  receives an IP-based transmission frame through the RNC  120 . The received transmission frame is comprised of a voice frame and a header added to the voice frame. The BTS  110  separates the header and the voice frame from the transmission frame, and searches the header information storage  300  for a code corresponding to the header information. Upon detecting the corresponding code, the BTS  110  transmits the voice frame corresponding to the detected code to the MS  100  through the voice frame transmission channel. Here, it is not necessary to transmit the separated header. However, when it is necessary to indicate caller information, the BTS  110  can carry the header on the signaling frame and transmits the signaling frame to the MS  100  during initial destination call setup before transmitting the voice frame. 
     To be specific, upon receiving a transmission frame comprised of the header and the voice frame, the frame controller  334  determines whether the header information for the MS  100  that has transmitted the transmission frame is stored in the header information storage  300 . If the header information for the MS  100  is stored in the header information storage  300 , the frame controller  334  controls the signaling processor  330  to separate the header and the voice frame from the transmission frame. The separated header is used only for identifying a destination MS, and the voice frame is transmitted to the MS  100 . The MS  100  receives the voice frame transmitted by the BTS  110  and converts the received voice frame into voice. 
       FIG. 4  illustrates a soft handoff procedure in the BTSs each including the header information storage in the ALL-IP network. Referring to  FIG. 4 , in the ALL-IP network comprised of the BTS  110  including the header information storage  300 , the voice frame and the header are separately transmitted between the MS  100  and the BTS  110 , thereby preventing addition of the overhead to the voice frame. A soft handoff method in the ALL-IP network where an overhead is not generated will now be described with reference to  FIGS. 3 and 4 . 
     A BTS  110 - 2  within a new cell  115 - 2  to which the MS  100  is handed off temporarily, stores traffic overhead information (RTP/DUP/IP header) for the MS  100  in the header information storage  300  through signaling. The MS  100  can transmit the overhead information to the new BTS  110 - 2  during a handoff signaling process because it maintains the traffic-related overhead information needed to attempt a call with the new BTS  110 - 2 . The old BTS  110 - 1  is requested to release the overhead information for the MS  100 , stored therein, through the signaling process, and then it deletes the header information for the MS  100  from a header information storage  300 - 1 . Thus, the call with the old BTS  110 - 1  is terminated. 
       FIG. 5  illustrates a message flow between elements constituting the ALL-IP network during a handoff according to a preferred embodiment of the present invention. In  FIG. 5 , the MS  100  transmits measurement information to the old BTS  110 - 1  and the RNC  120  in step  501 . The old BTS  110 - 1  and the RNC then start a soft handoff procedure using the measurement information. The RNC  120  transmits a Radio Link Setup Request message to the new BTS  110 - 2  in step  503 . Upon receiving the Radio Link Setup Request message, the new BTS  110 - 2  transmits a Radio Link Setup Response message to the RNC  120  in step  505 . The RNC  120 , which has perceived that the radio link has been set up between the MS  100  and the new BTS  110 - 2  by receiving the Radio Link Response message from the new BTS  110 - 2 , transmits an Active Set Update message to the old BTS  110 - 1  in step  507 . Upon receiving the Active Set Update message, the old BTS  110 - 1  deletes the header information for the MS  100  stored in the header information storage  300 - 1  in step  509  and transmits the Active Set Update message to the MS  100 . The MS  100  then transmits an Active Set Update Complete message to the old BTS  110 - 1  in step  511 . Steps  501  to  511  indicated by a block  510  are a conventional handoff procedure. 
     In the handoff procedure according to the present invention, steps  513  and  515  indicated by a block  520  are added. In step  513 , the MS  100  adds the traffic header information, which is temporarily stored therein to a message, for example, the Active Set Update message transmitted to the new BTS  110 - 2  and the RNC  120 , and transmits the traffic header information added message to the new BTS  110 - 2 . The additionally defined message has a message form as illustrated in Table 1. 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 7 
                 6 
                 5 
                 4 
                 3 
                 2 
                 1 
                 0 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Existing 
                 Active Set Update Field 
               
               
                 Field 
                   
               
             
          
           
               
                 Added 
                 Type 
                 Extension Length 
               
               
                 Field 
                 Version 
                 Unused 
               
             
          
           
               
                   
                 TOS (Type Of Service) 
               
               
                   
                 Protocol 
               
               
                   
                 Source Address (32 bits) 
               
               
                   
                 Destination Address (32 bits) 
               
             
          
           
               
                   
                 Type 
                 Code 
               
             
          
           
               
                   
                 MTU of next-hop network (16 bits) 
               
             
          
           
               
                   
                 Version 
                 Unused 
                 Payload Type 
               
               
                   
                   
               
             
          
         
       
     
     A field defined within the additionally defined message is shown in Table 2. 
     
       
         
               
             
               
               
             
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 Added Field: 
               
               
                 Basic Field 
               
             
          
           
               
                   
                 Type : Active Set Update Extension (message name is changeable) 
               
               
                   
                 Extension Length : length of extension field of header 
               
             
          
           
               
                 Traffic header information 
               
             
          
           
               
                   
                 Use of a part of previously defined IP/UDP/RTP header information 
               
               
                   
                   
               
             
          
         
       
     
     Upon receipt of the traffic header information transmitted from the MS  100 , the new BTS  110 - 2  stores the traffic header information in the header information storage  300 - 2 . The new BTS  110 - 2  then transmits a Binding Update message to the MS  100  in step  515 . 
     The Binding Update message is defined as indicated in Table 3. 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 7 
                 6 
                 5 
                 4 
                 3 
                 2 
                 1 
                 0 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Message Discriminator 
               
               
                   
                 Message Type 
               
               
                   
                 Transaction ID 
               
               
                   
                   
               
             
          
         
       
     
     The field within the Binding Update message is illustrated in Table 4. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
             
             
               
                 Message Field 
               
             
          
           
               
                   
                 Message Discriminator : protocol name 
               
               
                   
                 Message Type : Binding Update 
               
               
                   
                 Transaction ID : ID for identifying each call processing 
               
               
                   
                   
               
             
          
         
       
     
     Upon receipt of the Binding Update message, the MS  100  removes the traffic header from the next generated data frame and transmits the header-removed data frame to the BTS  110 - 2 . 
       FIG. 6  illustrates a handoff operation by the MS according to a preferred embodiment of the present invention. Referring to  FIG. 6 , the MS  100  transmits and receives a frame to/from the old BTS  110 - 1  in step  600 . The frame includes only the voice frame without an overhead. The MS  100  determines whether there is a handoff request in step  602 . If a handoff request exists, the MS  100  starts to perform a handoff procedure in step  604 . The MS  100  transmits the traffic overhead information to the new BTS  110 - 2  in step  606 . The MS  100  determines whether the Binding Update message is received in step  608 . If the Binding Update message is received, the MS  100  returns to step  600 . If the Binding Update message is not received, the MS  100  transmits and receives the traffic overhead along with the voice frame in step  610 . 
       FIG. 7  illustrates a handoff operation by the BTS according to a preferred embodiment of the present invention. The BTS  110  transmits and receives a frame to/from the MS  100  in step  700 . The frame includes only the voice frame without an overhead. The BTS  110  determines whether there is a handoff request in step  702 . If a handoff request exists, the BTS  110  starts to perform a handoff procedure in step  704 . The BTS  110  determines whether the traffic overhead information is received in step  706 . If the traffic overhead information is received, the BTS  110  updates the header information storage  300  in step  710  and transmits the Binding Update message to the MS  100  in step  712 . If the traffic overhead information is not received, the BTS  110  transmits and receives the traffic overhead along with the voice frame in a conventional method in step  720 . 
     As described above, addition of an excessive overhead to a voice frame, occurring in a radio region during handoff in the ALL-IP network can be prevented. Further, the inefficiency problem of bandwidths on a radio link occurring in various radio networks can be solved. 
     While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.