Abstract:
This new invention is a method of re-establishing MS-SGSN connections after a partial restart of SGSN, i.e., only the Gb-interface is restarted. The re-establishment is achieved without detaching the MS. With this method Ciphering is re-synchronised, PDP-contexts in unacknowledged mode are re-established, and SMS traffic over GPRS is re-established, while PDP-contexts in acknowledged mode are deactivated with order to re-activate. When the MS is moving within the coverage of a SGSN, the invention can be used to move the context closer to the used Gb-link. When the MS request better Quality of Service, the MS context can moved to more suitable SGSN resource with this invention.

Description:
TECHNICAL FIELD 
     The invention relates to controlling of the SGSN&#39;s of a GPRS network and in particular a method for partial restart of Gb-interfaces. (For an explanation of the terminology used, reference is made to the chapter “Abbreviation and Terminology”). 
     TECHNICAL BACKGROUND 
     Some of the protocol layers at the Gb-interface in SGSN contain traffic dependent information, e.g. received/sent sequence numbers. The traffic dependent protocol layers in question are LLC, described in GSM 04.64: “Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Mobile Station—Serving GPRS Support Node (MS-SGSN) Logical Link Control (LLC) layer specification”, version 6.7.0 Release 1997, February 2000, SNDCP described in GSM 04.65: “Digital cellular telecommunications system (Phase 2+); 
     General Packet Radio Service (GPRS); Mobile Station (MS)—Serving GPRS Support Node (SGSN); Subnetwork Dependent Convergence Protocol (SNDCP)”, version 6.7.0 Release 1997, February 2000, and GMM, described in GSM 04.08: “Digital cellular telecommunications system (Phase 2+); 
     Mobile radio interface layer  3  specification”, version 6.10.0 (6.a.0) Release 1997, April 2000. The lower most protocol layers at the Gb-interface (BSSGP, NS and L1bis) are stateless, i.e., no MS-related information which changes value during a session are stored in these layers. 
     If a CPU/processor board with volatile memory serving one or both of LLC/SNDCP protocol layers is restarted, the traffic dependent information is lost in SGSN. Volatile memory is typically dynamic RAM. 
     According to the GPRS standards, the MS&#39;es which are served by a restarted Gb-interface shall be thrown out of the SGSN-node, as stated in GSM 04.08, Section 13.6.2: “When an SGSN fails, it deletes all MM and PDP contexts affected by the failure”. 
     The SGSN will not notify the MS about the failure. When the MS eventually has something to send, it will detect that it has been thrown out of the SGSN, and thus must perform a new GPRS attach and PDP context activation. 
     When SGSN sends and receives LLC and SNDCP PDU&#39;es, the following must be stored; state variables, sequence numbers and frame dependent input to ciphering function. LLC and SNDCP PDU are sent/received at a high rate (throughput). After a processor board failure, several techniques exist to re-establish the communication between MS and SGSN: 
     Using a hard disk,
         i.e., storing traffic dependent information on disk. However, this type of memory has a high latency, and thus will lower the throughput (packets per second), i.e. not very feasible.       

     Using static RAM.
         I.e., storing traffic dependent information in static RAM (instead of dynamic RAM). Static RAM is more expensive than Dynamic RAM, and thus increases the production cost of a SGSN.   Throw out the affected MS&#39;es from the SGSN, and let them do new GPRS attach, as described in [4] GSM 03.60: “Digital cellular telecommunications system (Phase 2+);       

     General Packet Radio Service (GPRS); Service Description; Stage 2”, version 6.7.0 Release 1997, March 2000, Section 13.6.2. This will be perceived as annoying for the subscriber using the Mobile. Mobiles which do not send anything and are within the same Routing Area, will use up to 54 minutes (default periodic routing area update timer interval), ref. GSM 04.08, Section 4.7.5.1 and Table 11.3a, before GMM in the MS detects it has been thrown out. For example, MS terminating SMS-traffic over GPRS might not have been deliverable for 54 minutes. Another example is push services, such as reports from stock-marked which will not be deliverable for thrown out subscribers. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method for re-establishing the LLC-links after a partial restart of the Gb-interface that avoids the drawbacks mentioned above. 
     An object of the inventive method is to keep the MS attached after a failure of a CPU/processor board in a SGSN. 
     This is achieved by first performing a preparatory step, in which the GMM, LLC and SNDCP layers locally in SGSN are prepared. The SGSN will not inform the MS about the restart event. In the next step, the MS sends, or is brought to send, an LLC frame to the SGSN. The LLC frame will supply the missing information (cell position of the MS) to the SGSN, and the re-establishment of the LLC-links is performed. 
     However, the exact scope of the inventive solution will appear from the appended patent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  show the LLC and SNDCP protocols in their correct environment within the GPRS.  FIG. 1  shows the protocol stack used for payload (e.g., IP-packets) and  FIG. 2  shows the protocol stack involved when MS and SGSN wants to do signalling, including Short Message Service (SMS). 
         FIG. 3  is a sequence diagram showing the steps of one of the sequences constituting the invention; Sequence: Gb-interface restarted, GMM re-initiates LLC and SNDCP. 
         FIG. 4  is a sequence diagram showing the steps of Sequence: Uplink traffic after restart of Gb-interface. 
         FIG. 5  is a sequence diagram showing the steps involved in Sequence: Downlink unacknowledged traffic after restart of Gb-interface. 
         FIG. 6  is a sequence diagram of the steps of Sequence: Downlink acknowledged traffic after restart of Gb-interface. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     LLC layer has a reset mechanism that is used in this invention in a new context when the Gb-interface is restarted. The LLC reset mechanism is standardized to be used when the MS enters a new SGSN without doing a new GPRS attach, (i.e., Inter SGSN Routing Area Procedure, as described in GSM 03.60, Section 6.9.1.2.1 &amp; 6.9.1.3.2). 
     When a restart of Gb-interface in SGSN occurs, the LLC and SNDCP layers locally in SGSN are prepared, i.e., the affected MS-contexts activates a reset flag, GMM is set in mobile-reachable state, mobile-reachable timer is started, LCC layer is set in suspend state with page=true and ciphering key is set. SNDCP layer in SGSN gets its activated PDP-contexts. In the next step, the MS sends, or is brought to send, an LLC frame to the SGSN. The LLC frame will supply the missing information (the cell location of the MS) to the SGSN, and all layers will be re-established (the LLC reset mechanism). Note that the MS is connected to the same SGSN as before restart. GMM signalling, SMS traffic and PDP-context in unacknowledged mode is re-established. SGSN orders the MS to deactivate and reactivate PDP-contexts in acknowledged mode. 
     The details are shown in  FIGS. 3 ,  4 ,  5  and  6  below. Several possible combinations of sequence executions exist: 
     Combination 1: 
       FIG. 3 : “Gb-interface restarted, GMM re-initiates LLC and SNDCP” (always first), then 
       FIG. 4 : “Uplink traffic after restart of Gb-interface”. 
     Combination 2: 
       FIG. 3 : “Gb-interface restarted, GMM re-initiates LLC and SNDCP”, then 
       FIG. 5 : “Downlink unacknowledged traffic after restart of Gb-interface, then 
       FIG. 4 : “Uplink traffic after restart of Gb-interface”. 
     Combination 3: 
       FIG. 3 : “Gb-interface restarted, GMM re-initiates LLC and SNDCP”, then 
       FIG. 6 : “Downlink acknowledged traffic after restart of Gb-interface”, then 
       FIG. 4 : “Uplink traffic after restart of Gb-interface” 
     These three embodiments of the invention all include a preparatory step ( FIG. 3 ) in which the SGSN performs a preparation without the MS being informed. The connection between the MS and SGSN is ultimately re-established by the MS sending an LLC-frame towards the SGSN ( FIG. 4 ), upon which the traffic is resumed without the MS being aware of the fact that the connection has been broken. However, there can be different reasons for the MS to send the LLC-frame. One reason can simply be that the user of the terminal wants to send uplink data. Alternatively, a message/packet intended for the MS is detected by the SGSN. In order to get the current position to the MS, the MS is paged. The MS must then reply to a paging with an LLC-frame (which will indicate the position. In the following text, the individual sequences will be described in detail in reference to the corresponding  FIGS. 3-6 . 
       FIG. 3 : GB-Interface Restarted, GMM RE-Initeates LLC and SNDCP 
     A Monitor Function in SGSN detects that a CPU handling LLC or/and SNDCP layers has restarted. Affected GMM in SGSN is informed about the event and will take action to re-initiate the LLC and SNDCP layers. The re-initialisation procedure is shown in  FIG. 3 . The procedure is executed per affected MS. Each step is explained in the following list.
     1) GMM layer in SGSN is informed about restart of the local Gb-interface. Affected MS contexts are found. For each MS context; a) GMM enables a flag, called “Gb-restart-flag”, a) starts the Mobile Reachable Timer (according to agreed value with the MS) and c) sets the GMM state to standby-reachable.   2) GMM initiates the LLC layer with the service primitive LLGMM-ASSIGN.request. Ciphering key and ciphering algorithm is sent (if known before restart) as service primitive parameters.   3) GMM cannot trust any known (before restart) cell position of the MS. Therefore, LLGMM-SUSPEND.request is sent to LLC with service primitive parameter page=true. LLC layer will inform GMM if any LLC users (GMM, SMS or SNDCP) in SGSN want to send something to the MS, and GMM will Page the MS.   4) GMM re-enters any established PDP contexts (before restart) to the SNDCP layer. This is important since downlink payload might be the first event to happen (Combination 2 or 3). Both PDP context in acknowledged and unacknowledged mode are (if existing before restart) re-entered. This is performed through the SNSM-ACTIVATE.indication service primitive.   5) SNDCP confirms the re-entering of PDP-contexts with SNSM-ACTIVATION.response
 
 FIG. 4 : Uplink Traffic After Restart of GB-Interface
   

       FIG. 3  has been executed; i.e., GMM, LLC and SNDCP layers are ready to handle traffic for a particular MS. In  FIG. 4 , the first event after a Gb-restart is traffic sent from the MS (which has not detected that the Gb-interface has been restarted). Each step in  FIG. 4  is explained in the following list.
     1) The MS sends a LLC frame containing either signalling traffic, SMS or payload.   2) LLC informs GMM about the event “something is received from the MS and his position is Cell-Id” with the service primitive LLGMM-TRIGGER.indication. GMM will store the cell-position and enter GMM state READY.   3) If the MS has ciphered the LLC-frame, the LLC layer in SGSN will not be able to reproduce the plain text, i.e., the Check Sum (CRC) is faulty. The received frame is discarded and LLGMM-STATUS.indication is sent to GMM about the event.   4) Since GMM knows the position of the MS, LLC layer is set into another suspend state with the service primitive LLGMM-SUSPEND.request (page=false).   5) GMM in SGSN orders LLC layer to inform the MS about need of re-initialisation of the LLC and SNDCP layer. This is done with the service primitive LLGMM-RESET.request.   6) GMM in SGSN will start to deactivate PDP-context in acknowledged mode. SGSN orders GGSN to deactivate the PDP-context. When GGSN responds, deactivated message is sent towards the MS with request of reactivation.   7) Since LLC layer has received LLGMM-RESET.request, the LLC layer will calculate a new random value of the IOV-UI and sends the LLC-XID message with parameters reset &amp; IOV-UI to the MS. SNDCP layer in SGSN will receive service primitive LL-RESET.indication. SNDCP will reset all sequence counters and compression parameters will be set to default values. i.e., any compression is ended.   8) The MS responds to the XID-message with a new XID-message   9) When LLC layer in SGSN receives this XID response, GMM is informed about the event with service primitive LLGMM-IOV.confirmation and/or LLGMM-RESET.confirmation. GMM sets the Gb-restart flag to false.   10) GMM sends LLGMM-RESUME.request, i.e., LLC-layer is told to start to transmit any buffered messages towards the MS.   
     Will the MS accept the XID message in  FIG. 4 , arrow  7  “LLC-PDU containing XID msg. (reset, IOV-UI)”? The Reset part will be accepted, but the IOV-UI part might be rejected. 
     According to GSM 04.64, Section 8.5.3.1, the MS shall only accept the new IOV-UI only before “enabling of ciphering”. What is the definition of “enabling of ciphering”? According to [1], the MS enables the ciphering when the Kc and Cipher Algorithm is set in LLGMM-assign.request received from its local GMM. 
     According to GSM 03.60, Section 6.8.1; “In the routing area update case, if ciphering was used before the routing area update, and if the Authentication procedure is omitted, then the SGSN shall resume ciphering with the same algorithm when a ciphered Routing Area Update Accept message is sent, and the MS shall resume ciphering when a ciphered Routing Area Update Accept message is received”. The term “resume ciphering” is interpreted to be the same as GMM sending a LL-UNITDATA.request with cipher-bit set to true. In order to receive a ciphered Routing Area Update Accept message, the Kc and Cipher Algorithm must be set by the MS. In other words, the MS has “enabled the ciphering” during the whole RA update sequence. 
     Is IOV-UI re-negotiated during Inter-SGSN RA update? Yes, quote from GSM 04.64, Section 7.2.1.2, “LLGMM-RESET-REQ shall be used to order LLC in the SGSN to perform an XID negotiation of Reset and IOV-UI. The LLC layer shall randomly select the value of IOV-UI.” 
     Conclusion: It is most likely that the MS will accept the re-negotiation of IOV-UI after enabling the ciphering. 
     Why reactivate PDP-context in acknowledged mode activated? 
     In  FIG. 4 , arrow  6 , the GMM in SGSN initiates deactivation of PDP-context in acknowledged mode with request of re-activation, why? 
     The LLC reset mechanism ( FIG. 3 , arrow  7 ) is standardized to be used when the MS enters a new SGSN without doing a new GPRS attach, (i.e., Inter SGSN Routing Area Procedure, GSM 03.60, Section 6.9.1.2.1 &amp; 6.9.1.3.2). When an MS changes SGSN, the new-SGSN does not have the required state information to overtake where the old SGSN lost communication to the MS. Therefore, it is required to reset the LLC-links between new-SGSN and the MS. The LLC-XID-reset message is used for this. According to GSM 04.64 Section 8.5.3.1 and GSM 04.65, Section 5.1.2.1, unacknowledged LLC-links are reset, ciphering is re-synchronised, and unacknowledged PDP-contexts are reset. PDP-contexts in acknowledged mode is halted until received-SNDCP-PDU-numbers are interchanged between the SNDCP-peers (via GMM), i.e., this question must be resolved, “how many SNDCP PDUs are received successfully by remote end before changing SGSN?” After interchanging received-PDU-numbers, the MS and SGSN know where to continue. 
     In this invention, the GMM in the MS has NOT detected change of SGSN and this interchange of received-SNDCP-PDU-numbers will not take place. In order to get out of this deadlock, SGSN must order the MS to deactivate the PDP-context in acknowledged mode with request of re-activate. 
       FIG. 5 : Downlink Unacknowledged Traffic After Restart of GB-Interface 
       FIG. 3  has been executed, i.e., GMM, LLC and SNDCP layers are ready to handle traffic for a particular MS. In  FIG. 5 , the first event after a Gb-restart is traffic bound to the MS, i.e., one of the LLC-users has something to send towards the MS. Each step in  FIG. 5  is explained in the following list.
     1) One of the LLC-users in SGSN (GMM, SMS or SNDCP) has something to send to the MS, i.e., LLC-layer in SGSN receives service primitive LL-UNITDATA.request.   2) Since the LLC-layer in SGSN is in suspend state with page-flag set to true, the service primitive LLGMM-PAGE.indication is sent to GMM in SGSN (position of the MS is not known).   3) GMM in SGSN request BSSGP layer to page the MS (GMM-PAGING.request). The BSS will page the MS at the radio network. When the MS is paged, it must send an LLC-frame as an answer. This LLC-frame will be detected in  FIG. 4 .
 
 FIG. 6 : Downlink Acknowledged Traffic After Restart of GB-Interface
   
       FIG. 3  has been executed, i.e., GMM, LLC and SNDCP layers are ready to handle traffic for a particular MS. In  FIG. 6 , the first event after a Gb-restart is acknowledged traffic bound to the MS, i.e., SNDCP in SGSN has received a downlink packet that shall be sent towards the MS in acknowledged mode. Each step in  FIG. 5  is explained in the following list.
     1) SNDCP must re-establish the LLC link in acknowledged mode, i.e., LL-ESTABLISH.request is sent to its local LLC-layer.   2) Since the LLC-layer in SGSN is in suspend state with page-flag set to true, the service primitive LLGMM-PAGE.indication is sent to GMM in SGSN (position of the MS is not known).   3) GMM in SGSN request BSSGP layer in SGSN to page the MS (GMM-PAGING.request). The BSS will page the MS at the radio network. When the MS is paged, it must send an LLC-frame as an answer. This LLC-frame will be detected in  FIG. 4 . (The attempt to re-establish the LLC-link in acknowledged mode will be terminated by  FIG. 4 , and the MS will eventually take the initiative to re-active the PDP-context)   
     This invention is not tied to a specific ETSI release of the standards. The references are made to Release 97 of the GPRS standards, but this invention is also applicable to later releases where LLC and SNDCP layers exist at the Gb-interface. 
     Another field of application of the invention is when a MS is moving around within the SGSN coverage, and it is desired to move the context closer to the used Gb-link LLC and SNDCP are performing CPU consuming activities like ciphering, compression and SNDCP-segmentation. When a MS is moving around within the SGSN coverage, the MS might change physical Gb-link between BSS and SGSN. In order to prevent too much internal transfer within the SGSN, it might be desirable to change the CPU handling of the LLC and SNDCP layers for a specific MS closer to the current used Gb-link. 
     The invention is also applicable when an MS requires better Quality of Service 
     Quality of Service (QoS) is negotiated when a PDP-context is activated. When the MS performs GPRS attach, the required QoS in the sub-sequent PDP-context activation is not known. With this invention it is possible to move the MS internally to more suitable HW if required during PDP-context activation. 
     The invention is also applicable for generations of Mobile systems after GPRS 
     GPRS is a 2.5 th  generation of Mobile standards. This patent invention is also relevant for generations of Mobile systems after GPRS where the MS will not detect that equipment is restarted somewhere between base station and Packet Data Network (Gi interface in  FIG. 1 ). The restarted equipment (which is serving the MS) has one or several of these characteristics: 
     Logical link control with sequence numbers per logical link. These sequence numbers are updated after each sent or received link frame. 
     One of the input to the Ciphering Algorithm is dependent of previous number of sent/received frames/packets on the logical link. 
     Segmentation of network protocol data unit (N-PDU) into logical link control protocol data units (LL-PDU), and re-assembly of LL-PDU into a N-PDU. 
     Compression/decompression of user data. 
     Compression/decompression of protocol control information. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 Abbreviation and Terminology 
               
             
          
           
               
                 Abbreviation/Terminology 
                 Description 
               
               
                   
               
               
                 Ack 
                 Acknowledged mode, same as 
               
               
                   
                 Asynchronous Balanced Mode (ABM) 
               
               
                   
                 in LLC 
               
               
                 BSS 
                 Base Station System 
               
               
                 BSSGP 
                 BSS GPRS Protocol 
               
               
                 Cell-Id 
                 Identifies uniquely a radio cell 
               
               
                   
                 within a BSS 
               
               
                 CPU 
                 Central Processing Unit 
               
               
                 CRC 
                 Cyclic Redundancy Check, same as 
               
               
                   
                 Frame Check Sequence (FCS). 
               
               
                 DL 
                 Downlink (direction: MS &lt;-SGSN&lt;- 
               
               
                   
                 GGSN) 
               
               
                 Gb 
                 Interface between BSS and SGSN 
               
               
                 GMM 
                 GPRS Mobility Management &amp; Session 
               
               
                   
                 Management 
               
               
                 GGSN 
                 Gateway GPRS Switching Node 
               
               
                 Gi 
                 Interface between GGSN and IP- 
               
               
                   
                 network 
               
               
                 Gn 
                 Interface between SGSN and GGSN 
               
               
                 GPRS 
                 General Packet Radio Service 
               
               
                 GTP 
                 GPRS Tunnelling Protocol 
               
               
                 IOV-UI 
                 Input Output Vector - 
               
               
                   
                 unacknowledged mode 
               
               
                 IP 
                 Internet Protocol 
               
               
                 Kc 
                 Ciphering Key 
               
               
                 LLC 
                 Logical Link Control 
               
               
                 LLGMM 
                 Interface offered by LLC to the 
               
               
                   
                 GMM (LLC user) 
               
               
                 L1 
                 Layer One (in OSI model) 
               
               
                 L1bis 
                 Frame Relay in the GPRS Release 
               
               
                   
                 97. 
               
               
                 L2 
                 Layer Two (in OSI model) 
               
               
                 MAC 
                 Medium Access Control 
               
               
                 MS 
                 Mobile Station 
               
               
                 Page/Paging 
                 The procedure to locate the 
               
               
                   
                 position of the MS. 
               
               
                 PDP 
                 Packet Data Protocol, e.g., IPv4 
               
               
                 PDU 
                 Protocol Data Unit 
               
               
                 RLC 
                 Radio Link Control 
               
               
                 SAPI 
                 Service Access Point Identifier 
               
               
                 Service primitive 
                 “signals” which are passed 
               
               
                   
                 vertically between two protocol 
               
               
                   
                 layers 
               
               
                 SDU 
                 Service Data Unit 
               
               
                 SGSN 
                 Serving GPRS Switching Node 
               
               
                 SGSN upper layers 
                 This includes; GMM, SMS, LLC and 
               
               
                   
                 SNDCP 
               
               
                 SNDCP 
                 Subnetwork Dependent Convergence 
               
               
                   
                 Protocol 
               
               
                 SM 
                 Session Management 
               
               
                 SMS 
                 Short Message Service 
               
               
                 TCP 
                 Transmission Control Protocol 
               
               
                 TE 
                 Terminal, connected to the Mobile 
               
               
                   
                 Station 
               
               
                 TLLI 
                 Temporary logical link Identity 
               
               
                 UDP 
                 User Datagram Protocol 
               
               
                 UL 
                 Uplink (direction: BSS -&gt;SGSN) 
               
               
                 Um 
                 Air Interface between MS and BSS 
               
               
                 Unack 
                 Unacknowledged mode, same as 
               
               
                   
                 Asynchronous Disconnected Mode 
               
               
                   
                 (ADM) in LLC 
               
               
                 XID 
                 eXchange Identification, 
               
               
                   
                 mechanisms in LLC and SNDCP layer 
               
               
                   
                 to negotiate protocol parameters 
               
               
                   
                 between peers.