HUAWEI BSC6000 Base Station Subsystem Signaling Analysis Guide
6 Location Update
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Location Update
About This Chapter In the GSM system, MS location information is stored in the HLR, the VLR, and t he MS. When the location information changes, relevant information in the HLR, the VLR, and the MS must be changed accordingly, accordingly, which can be realized through the location update procedure. procedure. 6.1 Location Update Procedure The location update procedure is a general procedure and is used for generic location update, periodic update, and IMSI IMSI attach. 6.2 Internal BSC BSC Signaling Procedures of Location Update This topic describes describes the internal BSC signaling procedures pro cedures of location update. 6.3 Abnormal Abnormal Location Update Cases This topic describes describes the abnormal location update cases.
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6.1 Location Update Procedure The location update procedure is a general procedure and is used for generic location update, periodic update, and IMSI IMSI attach. 6.1.1 Periodic Update Procedure Periodic update is used to periodically notify the network of t he MS availability. 6.1.2 IMSI Attach Procedure The IMSI attach procedure is the complement of the IMSI detach procedure. It is used to indicate the IMSI as active in the network. 6.1.3 Generic Location Update Procedure This topic describes the generic location update procedure.
6.1.1 Periodic Update Procedure Periodic update is used to periodically notify the network of t he MS availability. Periodic update is used to periodically notify the network of t he MS availability. The location update type information element in the Location Updating Request message indicates periodic update. The periodic location update procedure is controlled by timer T3212 in the MS. If the timer is not started, it is started each time the MS enters the Normal Service or Attempting to Update state of the MM Idle state. When the MS leaves the MM Idle state, timer T3212 continues running until it expires. The MM Idle state indicates that the MS is not activated, is in the interception state, and does not process any call procedure. Timer T3212 is stopped (is set to its initial value 0 for the next start) when: l
A Location Updating Accept or Location Updating Reject message is received.
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An Authentication Reject message is received.
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The first MM message such as the Location Updating Accepted and the CM Service Accept message is received, or ciphering mode setting is complete when the MM connection is set up, except when the most recent service state is Limited Service.
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The MS has responded to paging and has received the first correct layer 3 message except the RR message.
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Timer T3212 expires.
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The MS is deactivated, that is, the MS is switched off or the SIM is removed.
When timer T3212 T3212 expires, the following cases might occur: l
The location location update procedure is started and the ti mer shall be set to its initial value for the next start.
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If the MS MS is in other states than MM Idle, the location update procedure is delayed until the MS enters the MM Idle state.
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If the MS MS is in the No Cell Available, Limited Service, PLMN Search, or PLMN Search Normal Service state, the location update procedure is delayed until the MS leaves the service state. state.
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If the system information indicates that periodic location update shall not be used, the procedure is not started. started. The timeout value is carried carried by the Control Channel Channel Description IE in the System Information Type 3 message.
The T3212 timeout value is not changed in the No Cell Available, Limited Service, PLMN Search, or PLMN Search-Normal Search-Normal Service states. When the timeout value is changed during serving cell change and the timer is running, the MS restarts the timer with the value t modulo t1. t1 is the new T3212 timeout value, and t is the current timer value. When timer T3212 stops, the MS restarts the timer with a random value between between 1 and t1.
6.1.2 IMSI Attach Procedure The IMSI attach procedure is the complement of the IMSI detach procedure. It is used to indicate the IMSI as active in the network. The System Information Type 3 message carries an ATT flag that indicates whether the attach and detach procedures are required to be used or not. If the IMSI attach and detach procedures are required by the network, the MS will trigger the IMSI attach procedure during IMSI activation. When the MS activates the IMSI within the network coverage area or moves into the coverage area, the IMSI attach procedure is triggered only when the Update Status is Updated and the stored LAI is the same as that broadcast on the BCCH of the current serving cell. Otherwise, a generic location update procedure is triggered, which is independent of the ATT flag indication. IMSI attach is performed through the generic location update procedure. The location update type information element in the Location Updating Request message must in this case indicate IMSI attach.
6.1.3 Generic Location Update Procedure This topic describes the generic location update procedure. The generic location update procedure is as follows: l
Network requesting additional additional MS capability information information The network may initiate a classmark interrogation procedure to obtain the MS capability information such as the encryption capability and the short message transmission and reception capability.
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Network requesting for for identification The network may initiate an identification request procedure if it cannot get the IMSI from the TMSI and LAI or it requires the IMEI.
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Authentication by the network The authentication procedure may be initiated by the network upon receipt of the Location Updating Request message from the MS.
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Ciphering mode setting by the network The ciphering mode setting procedure may be initiated by the network if a new TMSI has to be allocated.
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To limit the number of location update attempts, when a location update fails, an attempt counter is used. The counter counts the number of consecutive unsuccessful location update attempts and increases by 1 when a location update fails. The attempt counter is reset when: – The MS is switched on. – A SIM is inserted. – A location update is complete. – A location update with the cause value 11 (PLMN not Allowed), 12 (Location Area not
Allowed), 13 (Roaming not Allowed in This Location Area), or 15 (No Suitable Cells in Location Area) is complete. – The service state changes from Attempting to Update. – The MS enters a new location area. – Timer T3212 expires. – The location update is triggered by the CM sublayer requests.
The attempt counter is used to decide whether a location update needs to be initiated after timer T3211 expires. l
Location update not accepted by the network If the location update is not accepted, the network sends a Location Updating Reject message to the MS. The MS receiving a Location Updating Reject message stops timer T3210, stores the reject cause, starts timer T3240, enters state Location Updating Rejected, and awaits the release of the RR connection triggered by the network. It deletes the Equivalent PLMNs list when the reject cause is not #12, #14, or #15. Upon the release of the RR connection, the MS takes the following actions depending on the stored reject cause: – # 2 (IMSI Unknown in HLR) – # 3 (Illegal MS) – # 6 (Illegal ME)
If the reject value is #2, #3, or #6, the MS sets the update state to Roaming Not Allowed, deletes any TMSI, stored LAI, and ciphering key sequence number, and considers the SIM as invalid until it is switched off or the SIM is removed. – # 11 (PLMN not Allowed) – # 12 (Location Area not Allowed) – # 13 (Roaming not Allowed in This Location Area) – # 15 (No Suitable Cells in Location Area)
If the reject value is #11, #12, #13, or #15, the MS deletes any LAI, TMSI, and ciphering key sequence number stored in the SIM, resets the attempt counter, and sets the update state to Roaming Not Allowed. The MS stores the LAI and the PLMN identity in a suitable forbidden list, for example, in the Forbidden PLMN List for cause #11, in the list of Forbidden Location Areas for Regional Provision of Service for cause #12, in the list of Forbidden Location Areas for Roaming for cause #13 (The MS performs a PLMN selection instead of a cell selection when it returns to the MM Idle state), and in the list of Forbidden Location Areas for Roaming for cause #15 (The MS searches a suitable cell in a location area of the same PLMN). Other reject values are considered as abnormal cases. 6-4
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Release of the RR connection after location update When the location update procedure is complete, the MS (except when it has a follow-on application request and has received the proceed indication from the network) sets timer T3240 and enters the Wait For Network Command state, expecting the release of the RR connection. The network may keep the RR connection for a network-initiated MM connection, or may allow a mobile-initiated MM connection. Any release of the RR connection is initiated by the network. If the RR connection is not released within a given time controlled by t imer T3240, the MS aborts the RR connection. Then the MS enters the MM Idle state. When the MS enters the Normal Service or Attempting to Update state, either timer T3212 or timer T3211 is started.
6.2 Internal BSC Signaling Procedures of Location Update This topic describes the internal BSC signaling procedures of location update. Upon receiving the Channel Required message from the BTS, the BSC assigns a signaling channel. During random random access, upon receiving the Establish Indication message m essage from the BTS, the BSC sends the MSC a Location Updating Request message that carries the target CGI based on the MCC, MNC, LAC, and CI parameters in the Cell Attributes window. NOTE
You can find the periodic location update time in the Periodic Location Update Time Limist by choosing Cell Attributes > Idle Mode. The periodic location update time is delivered to the MS in the System Information Type 3 message.
6.3 Abnormal Location Update Cases This topic describes the abnormal location update cases. 6.3.1 Location Update Not Started or Aborted on the MS Side This topic describes the fault, probable causes, and handling suggestions. 6.3.2 Location Update Failure on the Network Side This topic describes the fault, probable causes, and handling suggestions.
6.3.1 Location Update Not Started or Aborted on the MS Side This topic describes the fault, probable causes, and handling suggestions.
Fault The location update procedure is not started or is aborted.
Probable Causes The probable causes are as follows: l
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The location update procedure is not started. The MS stays in the current serving cell and starts the normal cell reselection procedure. It starts the location update procedure as soon as it gains access or at cell change. l
Receiving an Immediate Assignment Reject message during random access (in A/Gb mode only) The location update procedure is not started. The MS stays in the chosen serving cell and starts the normal cell selection procedure. The waiting timer T3122 is reset when a cell change occurs. The location update procedure is started as soon as timer T3122 expires.
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Random access failure (in A/Gb mode only) Timer T3213 is started when random access fails. When it expires, the procedure is attempted again if still necessary. If two successive random access attempts fail, the location update procedure attempt is aborted.
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RR connection failure The location update procedure is aborted.
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T3212 expiry The location update procedure and the RR connection are aborted.
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RR release before the normal end of procedure The location update procedure is aborted.
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RR connection establishment failure (in Iu mode only)
Handling Suggestions Refer to 4.1 Immediate Assignment Procedure. Based on the cause values carried by the Location Updating Rejected message, you can also refer to the following suggestions: l
Cause values #2, #3, #6, #11, #12, and #13 Check whether the subscription information in the HLR and the subscriber data in the VLR is correct.
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Cause value #15 Check whether the the CGI and the LAC are consistent and correct on the MSC and the BSC.
6.3.2 Location Update Failure on the Network Side This topic describes the fault, probable causes, and handling suggestions.
Fault Location update fails, and the network sends a Location Updating Reject message.
Probable Causes The probable causes are as follows: 1.
RR conn conneectio ction n fa failur iluree If an RR connection fails during a common procedure integrated with the location update procedure, the behavior behavior of the network is the same as that in a common common procedure. If an RR connection fails when a common procedure does not exist, the location update procedure towards towards the MS is aborted.
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If the Location Updating Request message is received with a protocol error, the network returns a Location Updating Reject message with one of the following reject causes: l
#03 (Illegal MS)
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#97 (Message Type Non-Existent or not Implemented)
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#98 (Message Type not Compatible with the Protocol State)
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#111 (Protocol Error, Unspecified)
Upon sending the response, the network starts the channel release procedure. 3.
Network failur lure If the MSC, VLR, or HLR is faulty, the network sends a Location Updating Reject message with the cause value #17 (Network Failure). Generally, if the CGI or the LAC are not consistent on the MSC and t he BSC, the cause value may also be #17.
Handling Suggestions Refer to 4.1 Immediate Assignment Procedure. If the cause value is #17, refer to the following suggestions: 1.
Check whether whether the the CGI and and the LAC are are consisten consistentt on the the MSC and the the BSC, BSC, especiall especially y whether the both the LAC and the CI are decimal or hexadecimal.
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Chec Check k the the hard hardwa ware re of of the the MSC, MSC, VLR VLR,, or HLR HLR..
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