6.1 Definition of Mobility Management States

23.0603GPPGeneral Packet Radio Service (GPRS)Release 17Service descriptionStage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.1.0 General

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.

NOTE: A GERAN/UTRAN MS that is also capable of E‑UTRAN access has both MM states and EPS Mobility Management (EMM) states. The EMM states and the effects on the EMM and MM states of inter‑RAT mobility between GERAN/UTRAN and E‑UTRAN are described in TS 23.401 [89].

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.

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.

6.1.1.3 READY State

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 [78].

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.

6.1.1.4 State Transitions and Functions

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).

Figure 16: Functional Mobility Management State Model

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.

6.1.2 Mobility Management States (Iu mode)

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.

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).

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.

6.1.2.4 State Transitions and Functions

Figure 17 introduces the MM states for a GPRS subscriber (PMM). The states and activations are further described below the figure.

Figure 17: PMM State Model

NOTE: In both the PMM‑IDLE and the PMM‑CONNECTED states, session management may or may not have activated a PDP context. The consequence is that in PMM‑CONNECTED state, only a signalling connection may be established. In PMM‑IDLE state, a PDP context may be established, but no corresponding connection over the Iu interface nor the radio are established.

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:

– 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.

6.1.2.4.1 Handling of Un-synchronous States in the UE and the Network

6.1.2.4.1.1 Unsynchronous PMM states in the UE and the SGSN

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.

NOTE 1: The opposite (MS in the PMM‑CONNECTED state and SGSN in the PMM‑IDLE state) shall never happen because the 3G‑SGSN may not have the RAI where the MS is really located, so downlink transfer is impossible until the periodic URA update timer expires.

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.

NOTE 2: The RNC will send a RRC CONNECTION RELEASE message with cause "Directed Signalling Connection re-establishment" when it is unable to contact the SRNC to validate the UE due to lack of Iur connection (see TS 25.331 [52]).

6.1.2.4.1.2 Unsynchronous states in the UE and the UTRAN

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.