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Content for  TS 23.060  Word version:  16.0.0

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6  Mobility Management FunctionalityWord‑p. 75

6.1  Definition of Mobility Management States

6.1.0  General |R8|

The Mobility Management (MM) activities related to a subscriber are characterised by one of three different MM states. In A/Gb mode, the MM states for a GPRS subscriber are IDLE, STANDBY, and READY. In Iu mode, the MM states for a GPRS subscriber are PMM DETACHED, PMM IDLE, and PMM CONNECTED. Each state describes a certain level of functionality and information allocated. The information sets held at the MS and the SGSN are denoted MM context.
The MM state relates only to GPRS MM activities of a subscriber. The MM state is independent of the number and state of PDP contexts for that subscriber.
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6.1.1  Mobility Management States (A/Gb mode)

6.1.1.1  IDLE (GPRS) State

In GPRS IDLE state, the subscriber is not attached to GPRS mobility management. The MS and SGSN contexts hold no valid location or routeing information for the subscriber. The subscriber-related mobility management procedures are not performed.
The MS performs PLMN selection and cell selection and re-selection.
Data transmission to and from the mobile subscriber as well as the paging of the subscriber is not possible. The GPRS MS is seen as not reachable in this case.
In order to establish MM contexts in the MS and the SGSN, the MS shall perform the GPRS Attach procedure.
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6.1.1.2  STANDBY State

In STANDBY state, the subscriber is attached to GPRS mobility management. The MS and SGSN have established MM contexts as described in clause "Information Storage".
Pages for data or signalling information transfers may be received. It is also possible to receive pages for the CS services via the SGSN. Data reception and transmission are not possible in this state.
The MS performs GPRS Routeing Area (RA) and GPRS cell selection and re-selection locally. The MS executes mobility management procedures to inform the SGSN when it has entered a new RA. The MS does not inform the SGSN on a change of cell in the same RA. Therefore, the location information in the SGSN MM context contains only the GPRS RAI for MSs in STANDBY state.
The MS may initiate activation or deactivation of PDP contexts while in STANDBY state. A PDP context shall be activated before data can be transmitted or received for this PDP context.
The SGSN may have to send data or signalling information to an MS in STANDBY state. The SGSN then sends a Paging Request in the routeing area where the MS is located if PPF is set. If PPF is cleared, then paging is not done. The MM state in the MS is changed to READY when the MS responds to the page, and in the SGSN when the page response is received. Also, the MM state in the MS is changed to READY when data or signalling information is sent from the MS and, accordingly, the MM state in the SGSN is changed to READY when data or signalling information is received from the MS.
The MS or the network may initiate the GPRS Detach procedure to move to the IDLE state. In IDLE state the MM and PDP contexts may then be deleted.
After expiry of the Active Timer, if running for the MS, the SGSN should clear the PPF flag in the SGSN. After expiry of the mobile reachable timer the SGSN should clear the PPF flag in the SGSN and start an Implicit Detach timer, with a relatively large value and if ISR is activated, at least slightly larger than the UE's GERAN/UTRAN Deactivate ISR timer. After the Implicit Detach timer expires, the S4-SGSN can perform an implicit detach in order to return the MM contexts in the SGSN to IDLE state.
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6.1.1.3  READY StateWord‑p. 76
In READY state, the SGSN MM context corresponds to the STANDBY MM context extended by location information for the subscriber on the cell level. The MS performs mobility management procedures to provide the network with the actual selected cell. GPRS cell selection and re-selection is done locally by the MS, or may optionally be controlled by the network.
An identifier of the cell, the Cell Global Identity including RAC and LAC, is included in the BSSGP header of the data packet from the MS; see TS 48.018.
The MS may send and receive PDP PDUs in this state. The network initiates no GPRS pages for an MS in READY state. Pages for other services may be done via the SGSN. The SGSN transfers downlink data to the BSS responsible for the subscriber's actual GPRS cell.
The MS may activate or deactivate PDP contexts while in READY state.
Regardless if a radio resource is allocated to the subscriber or not, the MM context remains in the READY state even when there is no data being communicated. A timer supervises the READY state. An MM context moves from READY state to STANDBY state when the READY timer expires. In order to move from READY state to IDLE state, the MS initiates the GPRS Detach procedure.
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6.1.1.4  State Transitions and FunctionsWord‑p. 77
The movement from one state to the next is dependent on the current state (IDLE, STANDBY, or READY) and the event that occurs (e.g. GPRS attach).
Reproduction of 3GPP TS 23.060, Figure 16: Functional Mobility Management State Model
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Figure 16 describes the following state transitions:
Moving from IDLE to READY:
  • GPRS Attach: The MS requests access and a logical link to an SGSN is initiated. MM contexts are established at the MS and SGSN.
Moving from STANDBY to IDLE:
  • Implicit Detach: The MM and PDP contexts in the SGSN shall return to IDLE and INACTIVE state. The MM and PDP contexts in the SGSN may be deleted. The GGSN PDP contexts shall be deleted. If ISR is not activated, the P-GW and S-GW bearer contexts shall be deleted.
  • Cancel Location: The SGSN receives a MAP Cancel Location message from the HLR, and removes the MM and PDP contexts.
Moving from STANDBY to READY:
  • PDU transmission: The MS sends an LLC PDU to the SGSN, possibly in response to a page.
  • PDU reception: The SGSN receives an LLC PDU from the MS.
Moving from READY to STANDBY:
  • READY timer expiry: The MS and the SGSN MM contexts return to STANDBY state.
  • Force to STANDBY: The SGSN indicates an immediate return to STANDBY state before the READY timer expires.
  • Abnormal RLC condition: The SGSN MM context returns to STANDBY state in case of delivery problems on the radio interface or in case of irrecoverable disruption of a radio transmission.
Moving from READY to IDLE:
  • GPRS Detach: The MS or the network requests that the MM contexts return to IDLE state and that the PDP contexts return to INACTIVE state. The SGSN may delete the MM and PDP contexts. The PDP contexts in the GGSN/P-GW and S-GW shall be deleted.
  • Cancel Location: The SGSN receives a MAP Cancel Location message from the HLR, and removes the MM and PDP contexts.
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6.1.2  Mobility Management States (Iu mode)Word‑p. 78

6.1.2.1  PMM-DETACHED State

In the PMM DETACHED state there is no communication between the MS and the 3G SGSN. The MS and SGSN contexts hold no valid location or routeing information for the MS. The MS MM state machine does not react on system information related to the 3G SGSN. The MS is not reachable by a 3G SGSN, as the MS location is not known.
In order to establish MM contexts in the MS and the SGSN, the MS shall perform the GPRS Attach procedure. When the PS signalling connection is established between the MS and the 3G SGSN for performing the GPRS attach, the state changes to PMM CONNECTED in the 3G SGSN and in the MS. The PS signalling connection is made up of two parts: an RRC connection and an Iu connection.
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6.1.2.2  PMM-IDLE State

The MS location is known in the 3G SGSN with an accuracy of a routeing area. Paging is needed in order to reach the MS, e.g. for signalling. The MS and SGSN have established MM contexts as described in clause "Information Storage".
The MS shall perform a routeing area update if the RA changes. Signalling towards the HLR is needed if the 3G SGSN does not have an MM context for this MS.
The MS and 3G SGSN shall enter the PMM CONNECTED state when the PS signalling connection is established between the MS and the 3G SGSN.
After expiry of the Active Timer, if running for the MS, the SGSN should clear the PPF flag in the SGSN. After expiry of the mobile reachable timer the 3G-SGSN should clear the PPF flag in the SGSN and start an Implicit Detach timer, with a relatively large value and if ISR is activated, at least slightly larger than the UE's GERAN/UTRAN Deactivate ISR timer. After the Implicit Detach timer expires, the SGSN can perform an implicit detach in order to return the MM contexts in the SGSN to PMM-DETACHED state. The MS's MM context may be deleted. The HLR may be informed about the deletion (see clause 6.7).
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6.1.2.3  PMM-CONNECTED State

The MS location is known in the 3G SGSN with an accuracy of a serving RNC. In the PMM CONNECTED state, the location of the MS is tracked by the serving RNC. The MS performs the routeing area update procedure when RAI in the MM system information changes.
When an MS and a 3G SGSN are in the PMM CONNECTED state, a PS signalling connection is established between the MS and the 3G SGSN.
In the 3G SGSN, PS signalling connection release or failed downlink transfer with cause "IMSI unknown in RNC" changes the state to PMM IDLE.
The MS shall enter the PMM IDLE state when its PS signalling connection to the 3G SGSN has been released or broken. This release or failure is explicitly indicated by the RNC to the MS or detected by the MS (RRC connection failure). The radio connection shall also be released if a URA update fails because of "RRC connection not established", or if the URA update timer expires while the MS is out of UTRAN (or Iu mode GERAN) coverage.
After a signalling procedure (e.g. routeing area update), the 3G SGSN may decide to release the PS signalling connection, after which the state is changed to PMM IDLE.
GPRS detach changes the state to PMM DETACHED.
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6.1.2.4  State Transitions and FunctionsWord‑p. 79
Figure 17 introduces the MM states for a GPRS subscriber (PMM). The states and activations are further described below the figure.
Reproduction of 3GPP TS 23.060, Figure 17: PMM State Model
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Moving from PMM DETACHED to PMM CONNECTED in the MS:
  • GPRS Attach: The MM context shall move to the PMM CONNECTED state when a PS signalling connection is established between the MS and the 3G SGSN for performing a GPRS attach. If the GPRS attach is accepted an MM context is created in the MS.
Moving from PMM CONNECTED to PMM DETACHED in the MS:
  • GPRS Detach: The MM context shall move to the PMM DETACHED state when the PS signalling connection is released between the MS and the 3G SGSN after the MS has performed a GPRS detach or after the network-initiated GPRS detach is performed. The MM context in the MS may be deleted.
  • RAU Reject: The MM context shall move to the PMM DETACHED state when the PS signalling connection is released between the MS and the 3G SGSN after a RAU is rejected by the 3G SGSN. The MM context may be deleted.
  • GPRS Attach Reject: The MM context shall move to the PMM DETACHED state when the PS signalling connection is released between the MS and the 3G SGSN after a GPRS attach is rejected by the 3G SGSN. The MM context may be deleted.
Moving from PMM CONNECTED to PMM IDLE in the MS:
  • PS Signalling Connection Release: The MM context shall move to the PMM IDLE state when the PS signalling connection is released.
Moving from PMM IDLE to PMM CONNECTED in the MS:
  • PS Signalling Connection Establishment: The MM context shall move to the PMM CONNECTED state when the PS signalling connection is established between the MS and the 3G SGSN.
Moving from PMM IDLE to PMM DETACHED in the MS:
  • Implicit GPRS Detach: The MM context shall locally move to the PMM DETACHED state, e.g. in the case of removal of the battery, the USIM, or the SIM from the TE.
Moving from PMM DETACHED to PMM CONNECTED in the 3G SGSN:
  • GPRS Attach: The MM context shall move to the PMM CONNECTED state when a PS signalling connection is established between the MS and 3G SGSN for performing a GPRS attach. If the GPRS attach is accepted, an MM context is created in the 3G SGSN.
Moving from PMM CONNECTED to PMM DETACHED in the 3G SGSN:
  • GPRS Detach: The MM context shall move to the PMM DETACHED state when the PS signalling connection is released between the MS and the 3G SGSN after the MS has performed a GPRS detach or after the network-initiated GPRS detach is performed. The MM context in the 3G SGSN may be deleted.
  • RAU Reject: The MM context shall move to the PMM DETACHED state when the PS signalling connection is released between the MS and the 3G SGSN after a RAU is rejected.
  • GPRS Attach Reject: The MM context shall move to the PMM DETACHED state when a PS signalling connection is released between the MS and the 3G SGSN after a GPRS attach is rejected by the 3G SGSN.
Moving from PMM CONNECTED to PMM IDLE in the 3G SGSN:
  • PS Signalling Connection Release: The MM context shall move to the PMM IDLE state when the PS signalling connection is released.
Moving from PMM IDLE to PMM CONNECTED in the 3G SGSN:
  • PS Signalling Connection Establishment: The MM context shall move to the PMM CONNECTED state when the PS signalling connection is established.
Moving from PMM IDLE to PMM DETACHED in the 3G SGSN:
  • Implicit GPRS Detach: The MM context may locally move to the PMM DETACHED state after expiry of the Implicit Detach timer. The MM and PDP context(s) in the 3G SGSN may be deleted.
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6.1.2.4.1  Handling of Un-synchronous States in the UE and the NetworkWord‑p. 80
6.1.2.4.1.1  Unsynchronous PMM states in the UE and the SGSN |R16|
In case of RRC connection release with cause "Directed Signalling connection re-establishment" or in case of an error, the PMM state of the MS and the 3G SGSN may lose synchronisation. In this case the MS may be in the PMM IDLE state while the 3G SGSN is in the PMM CONNECTED state.
This situation is recovered by a successful MS initiated connection establishment, e.g. for a RAU or for data transfer, or by a failed downlink transfer with cause "IMSI unknown in RNC", triggering a paging procedure from the 3G SGSN.
If the SGSN in PMM-CONNECTED state receives Iu connection establishment request from the MS, the SGSN shall ensure the new Iu connection and the existing one are for the same MS, and if so the SGSN shall process the new request and release existing Iu connection and all RABs associated with it. To ensure that the new Iu connection and the existing one are for the same MS, the SGSN may perform the security functions. If the Iu connection establishment request is for signalling only and if Direct Tunnel was established for the MS, the SGSN (in Gn/Gp mode) sends Update PDP Context Request(s) to the GGSN(s) or the SGSN (in S4/S5/S8 mode) sends Update Bearer Request to the S-GW, to establish the GTP tunnels between SGSN and GGSN(s)/S-GW. If the Iu connection establishment request is for data transfer the SGSN may immediately establish a new direct tunnel and, in Gn/Gp mode, send Update PDP Context Request(s) to the GGSN(s) or, in S4/S5/S8 mode, send Update Bearer Request to the S-GW and include the RNC's Address for User Plane and, downlink TEID for data.
The UE shall also perform a RAU procedure immediately on entering PMM-IDLE state when it has received a RRC Connection Release message with cause "Directed Signalling connection re-establishment" even if the RA has not changed since the last update. The UE shall perform a subsequent Service request procedure after successful completion of the RA Update procedure to re-establish the radio access bearer when it has pending user data to send.
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6.1.2.4.1.2  Unsynchronous states in the UE and the UTRAN |R16|
In abnormal cases, the UTRAN can believe the UE is in the RRC-CONNECTED state while the UE is actually in the RRC-IDLE state.
Symptoms of this condition are that the UTRAN has an Iu interface connection to the SGSN and the UTRAN pages with the RNTI but receives no answer from the UE.
For UTRAN paging triggered by CS domain pages, the RNC should take the responsibility to recover this situation by re-paging with the Core Network Identity in the cells of that RNC which are in the Location Area indicated by the CN. A consequence of this re-paging is that it may lead to the RNC having two RRC connections for one UE but different RNTIs. To resolve this, when the RNC receives the Common ID message from the MSC, the RNC may request the release of the Iu-PS connection associated with any different RNTI previously associated with that IMSI.
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6.2  Mobility Management Timer FunctionsWord‑p. 81

6.2.1  READY Timer Function (A/Gb mode)

The READY timer function maintains the READY timer in the MS and SGSN. The READY timer controls the time an MS remains in READY state in the MS and the SGSN. The READY timer shall be reset and begin running in the MS when an LLC PDU is transmitted, and in the SGSN when an LLC PDU is correctly received. When the READY timer expires, the MS and SGSN MM contexts shall return to STANDBY state.
The length of the READY timer shall be the same in the MS and SGSN. The initial length of the READY timer shall be defined by a default value. The SGSN, and only the SGSN, may change the length of the READY timer by transmitting a new value in the Attach Accept or Routeing Area Update Accept messages.
If the READY timer length is set to zero, the MS shall immediately be forced into STANDBY state. If the timer length is set to all 1s (binary), the READY timer function shall be deactivated, i.e. the timer no longer runs and the MS remains in READY state.
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6.2.2  Periodic RA Update Timer Function

The Periodic RA Update Timer function monitors the periodic RA update procedure in the MS. The length of the periodic RA update timer is sent in the Routeing Area Update Accept or Attach Accept message. The periodic RA update timer is unique within an RA. Upon expiry of the periodic RA update timer, the MS shall start a periodic routeing area update procedure.
The SGSN may allocate long periodic RAU timer value to the MS as per clause 5.3.13.5.
If the MS is in GERAN/UTRAN coverage but out of GERAN/UTRAN PS coverage when the periodic RA update timer expires, then, if the MS is IMSI-attached to a network in network operation mode I, the periodic location update procedure (or other appropriate location update procedure) shall be started immediately. In addition, and irrespective of whether or not the MS was IMSI-attached, regardless of the network operation mode, the periodic RA update procedure (or other appropriate update procedure) shall be started as soon as the MS returns to GERAN/UTRAN PS coverage.
If the MS is out of GERAN/UTRAN PS coverage or camps on E-UTRAN when the periodic RA update timer expires then:
  • if the MS is both IMSI- and GPRS-attached and returns to GERAN/UTRAN coverage in a cell that supports packet-domain services in network operation mode I, then the combined RA / LA update procedure with IMSI attach requested shall be started as soon as the MS returns to GERAN/UTRAN coverage;
  • if the MS is both IMSI- and GPRS-attached and returns to GERAN/UTRAN coverage in a cell that supports packet-domain services in network operation mode II, or if a GPRS only-attached MS returns to GERAN/UTRAN coverage in a cell that supports packet-domain services, then the periodic RA update procedure shall be started as soon as the MS returns to GERAN/UTRAN coverage; or
  • if the MS returns to GERAN/UTRAN coverage in a cell that does not support packet-domain services, and if the MS is IMSI-attached, then the periodic location update procedure (or other appropriate location update procedure) shall be started as soon as the MS returns to GERAN/UTRAN coverage in that cell. In addition, and irrespective of whether or not the MS was IMSI-attached, the periodic RA update procedure (or other appropriate update procedure) shall be started as soon as the MS returns to GERAN/UTRAN PS coverage.
If the MS lost GERAN/UTRAN PS coverage or camped on E-UTRAN but the periodic RA update timer did not expire while not camping on a GERAN/UTRAN PS cell, then the MS shall not perform the periodic RA update procedure because of the MS's return to GERAN/UTRAN PS coverage.
If the MS lost GERAN/UTRAN coverage or camped on E-UTRAN but the periodic RA update timer did not expire while not camping on a GERAN/UTRAN cell, the MS shall not perform the periodic RA update procedure because of the MS's return to GERAN/UTRAN coverage.
If the MS's periodic RAU timer expires and ISR is activated the MS shall start the GERAN/UTRAN Deactivate ISR timer. After the GERAN/UTRAN Deactivate ISR timer expires the MS shall deactivate ISR by setting its TIN to "GUTI".
The GERAN/UTRAN Deactivate ISR timer is stopped when the MS performs a successful RAU.
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6.2.3  Mobile Reachable Timer FunctionWord‑p. 82
The Mobile Reachable Timer function monitors the periodic RA update procedure in the SGSN. If the SGSN supports PSM the SGSN uses the function also to know when an MS is not reachable due to entering PSM. The SGSN starts the mobile reachable timer with a value slightly longer than the periodic RA update timer used by the MS. If the SGSN has allocated an Active Time to the UE, then the SGSN starts the Active Timer with the value of Active Time.
The Active Timer and the mobile reachable timer are stopped when the READY state or PMM CONNECTED state is entered. The Active Timer and the mobile reachable timer are reset and started when the state returns to STANDBY or PMM IDLE.
If the mobile reachable timer expires or the Active Timer, the SGSN can deduce that the UE is not reachable and shall clear PPF. Typically, in GPRS, this causes the SGSN to stop sending GPRS paging or CS paging messages to the MS, but other features (e.g. MSC/VLR-based call forwarding) may happen immediately. PPF is set when the next activity from the MS is detected. The MM and PDP contexts shall be kept in the SGSN.
When an MS first registers in an SGSN, then PPF is set.
The PPF in the SGSN is specific to GERAN/UTRAN access.
TS 23.401 specifies a separate PPF for E UTRAN. If the SGSN is combined with an MME, the SGSN's PPF shall have no impact on whether or not the MME performs paging in E UTRAN.
If the SGSN is requested to monitor Reachability for Data and the MS enters READY state or PMM CONNECTED, the SGSN sends a Monitoring Report message to the address that was indicated in the related Monitoring Request as described in TS 23.682.
When the SGSN applies General NAS level Mobility Management Congestion Control to a MS, the SGSN may need to adjust the mobile reachable timer and/or implicit detach timer (as clause 5.3.6.2.4).
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