the concrete embodiments of the present invention will be described in detail with reference to the drawings . referring to fig2 , the following description explains the flow chart of the packet data session release process of the umb access gateway according to one embodiment of the present invention . as shown in fig2 , the session release process comprises the following steps , s 202 , when releasing the packet data session , an agw sends a pmip registration revocation message to a data attachment point of the umb access network ; s 204 , after receiving the pmip registration revocation message , the data attachment point sends a pmip revocation ack message to the agw to confirm the reception of the pmip registration revocation message ; s 206 , the data attachment point sends an ip tunnel notification message to the srnc to notify the srnc that the packet data session has been released ; s 208 , after receiving the ip tunnel notification message , the srnc sends an ip tunnel notification ack message to confirm the reception of the ip tunnel notification message , triggers the umb session release , and sends a session release message to the route set members of the at and the data attachment point ; s 210 , after receiving the session release message , the route set members and the data attachment point send session release ack messages respectively to the srnc to confirm that they have received the session release message ; s 212 , the forward / reverse ebs of the route set members sends an accounting request message to the agw ; and s 214 , the agw returns an accounting response message to the forward / reverse ebs after receiving the accounting request message . wherein , in step s 206 , if the data attachment point is also a member in the route set of the at , the data attachment point sends an ip tunnel notification release message to the other route set members of the at , and receives ip tunnel notification ack messages returned from the other route set members of the at . the non - data - attachment - point of the route set members which has one reverse pmip tunnel to the agw , automatically releases the reverse pmip tunnel after receiving the session release message . wherein , the agw starts a first timer with a specific time length , while sending a pmip registration revocation message to the data attachment point , so that if the agw does not receive a pmip registration revocation ack message within the specific time length , resend the pmip registration revocation message to the data attachment point ; the data attachment point starts a second timer with a specific time length , while sending an ip tunnel notification message to the srnc and / or forward / reverse ebs , so as to resend the ip tunnel notification message to the srnc and / or forward / reverse ebs in case that the data attachment point does not receive an ip tunnel notification response message from the srnc and / or forward / reverse ebs within the specific time length ; the forward / reverse ebs starts a third timer with a specific time length , while sending an accounting request message to the agw , so as to resend the accounting request message to the agw in case that the forward / reverse ebs does not receive an accounting response message from the agw within the specific time length . referring to fig3 , the following description explains the flow chart of the umb session release process resulted from the packet data session release when there are multiple pmip tunnels , which contains the process of fig2 . wherein , apart from a reverse pmip tunnel between the dap and the agw , there is also one reverse pmip tunnel between the ebs1 ( which may be a reverse link serving ebs ( rlse ), or a non - rlse ) and the agw , wherein , it is assumed that the ebs is the forward reverse serving ebs or forward serving ebs in this embodiment . as shown in fig3 , the session release process comprises the following steps : s 302 , there is a primary pmip tunnel between the dap and the agw , i . e ., the forward and reverse pmip tunnels , and data are being transmitted between the at and the agw . s 304 , there is a reverse pmip tunnel between ebs1 / flse / rlse and the agw for transmitting reverse data . s 306 , the agw sends a pmip registration revocation message to the dap and sets a timer trrv - pmip . s 308 , after receiving the pmip - registration revocation message , the dap sends a pmip - revocation ack message to the agw which stops the timer trrv - pmip . s 310 , the dap sends an ip tunneling ( ipt )- notification message to the srnc to indicate that the packet data session has been released , and sets a timer tpdsr - ipt . s 312 , after receiving the ipt - notification message , the srnc sends an ipt - notification ack message to the dap , which stops the timer tpdsr - ipt after receiving the ack message . s 314 , if the dap is in the route set of the at , the dap sends the ipt - notification message to the srnc to indicate that the packet data session has been released , and sets the timer tpdsr - ipt . s 316 , after receiving the ipt - notification message , the srnc sends an ipt - notification ack message to the dap which stops the timer tpdsr - ipt after receiving the ack message . s 318 , after receiving the ipt - notification message , the srnc may decide to initiate the umb session release . after releasing the umb session between the at and the srnc , the srnc sends an ias - session release message to all access network route instance ( anri ) in its route set and the dap . s 320 , the srnc sends the ias - session release message to ebs1 / flse / rlse , and sets a timer tsr - ias . s 322 , after receiving the ias - session release message , ebs1 / flse / rlse sends the ias - session release ack messages to the srnc , which stops the timer tsr - ias after receiving the confirmation message . s 324 , the srnc sends the ias - session release message to the dap , and sets a timer tsr - ias . s 326 , after receiving the ias - session release message , the dap sends an ias - session release ack message to the srnc , which stops the timer tsr - ias after receiving the ack message . s 328 , if the dap has previously created an air link record for the at , then the dap sends an accounting requesting ( interim ) message to the agw , and starts a timer tact . this step could be performed after step s 310 . s 330 , the agw sends an accounting response message to the dap which stops the timer tact after receiving the accounting response message . s 332 , ebs1 / flse / rlse sends an accounting request message to the agw , and starts the timer tact . this step could be performed after step s 316 . s 334 , the agw sends an accounting response message to the ebs1 / flse / rlse , which stops the timer tact after receiving the response message . referring to fig4 , the following description explains the flow chart of packet data session release process of the umb access gateway according to another embodiment of the present invention . as shown in fig4 , the session release process comprises the following steps : s 402 , when releasing the packet data session , an agw sends a pmip registration revocation message to a data attachment point of the umb access network and an ebs which has one reverse pmip tunnel to the agw ; s 404 , after receiving the pmip registration revocation message , the data attachment point and the ebs which has one reverse pmip tunnel to the agw delete the pmip tunnels between them and the agw respectively , and send a pmip registration revocation ack message to the agw respectively so as to confirm the reception of the pmip registration request message ; and s 406 , the data attachment point sends an ip tunnel notification message to the srnc , so as to notify the srnc that the packet data session has been released . wherein , after receiving the ip tunnel notification message and then acquiring that the packet data session has been released , the srnc may release the umb session or not . wherein , the agw starts a first timer with a specific time length , while sending a pmip registration revocation message to the data attachment point and the ebs which has one reverse pmip tunnel to the agw , so as to resend the pmip registration revocation message to the data attachment point or the ebs which has one reverse pmip tunnel to the agw in case that the agw does not receive the pmip registration revocation ack message within the specific time length . the data attachment point starts a second timer with a specific time length , while sending an ip tunnel notification message to the srnc , so as to resend the ip tunnel notification message to the srnc in case that the data attachment point does not receive the response message from the srnc within the specific time length . referring to fig5 , the following description explains the flow chart of the umb session releasing process initiated by the agw resulted from packet data session releasing when there are multiple pmip tunnels , which contains the process of fig4 . as shown in fig5 , the session releasing process comprises the following steps : s 502 , there is forward reverse pmip tunnel between the dap and the agw , and data are being transmitted between the at and the agw . s 504 , there is a reverse pmip tunnel between ebs1 and the agw for transmitting reverse data . s 506 , the agw sends a pmip - registration revocation message to the dap , and sets a timer trrv - pmip . s 508 , after receiving the pmip - registration revocation message , the dap sends the pmip - revocation ack message to the agw which stops the timer trrv - pmip . s 510 , the agw sends the pmip - registration revocation message to ebs1 , and sets the timer trrv - pmip . s 512 , after receiving the pmip - registration revocation message , ebs1 sends a pmip - registration revocation ack message to the agw which stops the timer trrv - pmip . s 514 , the process assumes that the dap is not in the route set of the at . the dap sends an ipt - notification message to the srnc to indicate that the packet data session has been released , and sets a timer tpdsr - ipt . s 516 , after receiving the ipt - notification message , the srnc sends an ipt - notification ack message to the dap , which stops the timer tpdsr - ipt after receiving the ack message . s 518 , after receiving the ipt - notification message , the srnc may decide to initiate umb session release . after releasing the session between the at and the srnc , the srnc sends an ias - session release message to all access network route instance ( anri ) in its route set . s 520 , the srnc sends the ias - session release message to ebs1 / flse / rlse , and sets a timer tsr - ias . s 522 , after receiving the ias - session release message , ebs1 / flse / rlse sends an ias - session release ack message to the srnc , which stops the timer tsr - ias after receiving the ack message . s 524 , if the dap has previously created an air link record for the at , the dap sends an accounting request ( interim ) message to the agw , and starts a timer tact . this step can be performed at any time after step s 514 . s 526 , the agw sends an accounting response message to the dap , which stops the timer tact after receiving the response message . s 528 , ebs1 / flse / rlse sends an accounting request message to the agw , and starts the timer tact . this step can be performed any time after step s 520 . s 530 , the agw sends an accounting response message to the ebs1 / flse / rlse which stops the timer tact after receiving the response message . based on the above description , based on the fact that multiple pmip tunnels are not under consideration in the umb session release process resulted from packet data session release in the current umb access network technology , the present invention puts forward a session release process for the case of multiple pmip tunnels , so as to improve the session release process of the umb access network technology . the above description is to illustrate embodiments of the present invention , not to limit the present invention . various alterations and changes to the present invention are apparent to those skilled in the art . the scope defined in claims shall comprise any modification , equivalent substitution and improvement within the spirit and principle of the present invention .