6.2 Mobility Management Timer Functions

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

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.

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.

6.2.3 Mobile Reachable Timer Function

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 [89] 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 [119].

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).