Chapter 8 GPRS Mobility Management Flow
8.4 GPRS Location Management Function
The location management function provides the following:
Solution of selecting the cell and PLMN
Solution of obtaining the RA of the MS that is in STANDBY and READY status
Solution of obtaining the cell identifier of the MS that is in READY status
The MS periodically compares the CGI and RAI in the MM context with those in the message received from the system, thus generating the cell updating and RA updating requests. In addition, the MS periodically initiates RA updating request. The location management procedure falls into the following:
Cell updating procedure
Routing area updating procedure
Combined RA/LA updating procedure
8.4.1 Cell Updating Procedure
1) IDLE mode cell updating procedure when MS is in READY status
When an MS that is in READY status moves from one cell to another in the same RA, the MS initiates the cell updating procedure. The cell updating procedure is described as follows:
The MS sends a free-type upstream LLC frame that contains the MS ID to the SGSN to initiate the cell updating procedure.
After receiving the LLC frame, the BSS adds the CGI (RAC+LAC) of the new cell to the header of the BSSGP frame and then sends the frame to the SGSN.
After receiving the BSSGP frame, the SGSN saves the CGI of the new cell to the MM context of the MS. The services sent to the MS are directly transmitted to this cell.
2) Transmission mode cell updating procedure when MS is in READY status During packet data transmission, if the MS finds another more suitable adjacent cell through signal measurement or cell selection parameters broadcasted on the PBCCH/BCCH, it stops receiving the system messages from the previous cell but starts to receiving the system messages from the new cell. Then the MS enters this new cell and sends a CELL UPDATE message to the SGSN. This message is transparently transmitted to the PCU. When the SGSN receives the CELL UPDATE message and identifies that the MS is receiving the downstream packets, it sends a PURGE message (containing the BVCIs of both the previous and new cells and the TLLI of the MS) to the PCU to notify the PCU that the MS moves from one cell to another.
The PCU finds the previous cell through the BVCI of the previous cell. After that, the PCU exports or transfers the TLLI-related LLC frames that are not transmitted or not confirmed from the previous cell to the transmission queue of the new cell. After that, the PCU reallocates resources for the MS in the new cell. The new TBF stream is set up for the MS in the new cell and then the data transmission is started. Note: If the cell updating procedure is implemented in different BSSs, the PCU removes the TLLI- related LLC frames from the previous cell.
If the data transmission is implemented in LLC confirm mode, the LLC-PDUs removed by the PCU will be retransmitted. If the cell reselection procedure is implemented in LLC non-confirm mode, the LLC-PDUs removed by the PCU will be discarded.
8.4.2 Routing Area Updating Procedure
The RA updating procedure falls into the following: Intra-SGSN RA updating, inter- SGSN RA updating, intra-SGSN combined RA/LA updating, and inter-SGSN
combined RA/LA updating.
When the IMSI-attach and GPRS-attach MS moves to an RA that works in network operation mode I, the combined RA/LA updating occurs. The RA that works in other network operation modes does not support the paging coordination, so it is meaningless for the MS to initiate the combined RA/LA updating procedure.
In addition, different types of MSs support different updating procedures. The MS of class A initiates only the RA updating procedure rather than combined RA/LA updating procedure during the CS service. The MS of class B does not initiate any updating procedure during the CS service. The MS of class C never initiates the combined RA/LA updating procedure.
8.4.3 Periodical RA/LA Updating Procedure
All GPRS-attach MSs (except the MS of class B during the CS service) must initiate periodical RA update procedure. The periodical RA updating procedure is equal to the intra-SGSN RA updating procedure (except for updating type).
The IMSI-attach but GPRS-detach MS must initiate the periodical LA updating procedure.
For the IMSI-attach and GPRS-attach MS, the updating procedure is determined by the network operation mode:
In mode I: periodical RA updating procedure only
In mode II or III: respectively the periodical RA updating procedure and periodical LA updating procedure
8.4.4 User Data Management Procedure
If the user subscription data (QoS file or VPLMN address) in the HLR is modified or removed, the HLR can implement the inserting user data procedure or removing user data procedure to notify the SGSN.
In addition, through the inserting user data procedure the HLR can notify the SGSN to insert one or more PDP contexts or modify one or more existing PDP contexts.
8.4.5 MS Class Mark Processing Function
The GPRS adopts a different way from the GSM for processing the MS class mark.
When an MS is attached to the GPRS, its class mark contained in the MM message is sent to the network. The class mark is stored in the network until the MS changes to GPRS-detach status. This saves radio resources by avoiding the MS class mark being transmitted on the radio interface.
The MS class mark falls into the following: radio access class mark and SGSN class mark. The radio access class mark indicates the MS radio access capability such as frequency band, multiple slots, and power level. In addition, it indicates some other information required by the BSS to implement the radio resource management. The SGSN class mark indicates other capabilities irrelevant to the radio access, such as encryption.
The SGSN regards the radio class mark as an information field and provides it to the BSS in each downstream BSSGP PDU.
The SGSN stores the SGSN class mark and transmits it to the new SGSN.
To improve the efficiency, the initial access stage is advanced in the specifications for
the BSS to directly obtain the simple radio access class mark from the MS.
In this case, the BSS does not need to obtain the entire radio access class mark from the SGSN, thus quickly implementing transmission triggering for the MS. The simplified class mark can be contained in the initial random access message or in the first upstream radio block.