There are a few Mobility Management timers exist on the MS and the SGSN. Using those timers the nodes will be able to properly manage the states of the MM context of a certain MS. 

Mobility Management timers

Ready timer (2G only)

Once the MS is in the READY state, a timer on the MS and the SGSN will start counting. In case there is no data or signaling sent between the MS and the SGSN till this timer expires, the MM context will move to the STANDBY state.

Note: the average time for this timer in practical scenarios is around 30 Sec.

Mobile Reachable timer

When the MS is in the STANDBY state (2G), or PMM-IDLE (3G). If down link data from the PDN to the MS came to the SGSN, the SGSN should send paging request to all the sites exist under the RA of the MS. Then,the mobile will move to READY state (2G), or PMM-CONNECTED state (3G).

• Once the MS enters STANDBY state (2G), or PMM-IDLE (3G), the SGSN starts the reachable timer. this timer will reset every time the SGSN receives a data or signaling from the MS. As long as the reachable timer not expired, the SGSN will know the RA of the MS. Therefore, the SGSN will be able to send paging request to the MS.

• once the reachable timer expires without receiving any PDU from the MS, the SGSN will mark this MS as unreachable. Therefore, the SGSN will not be able to page the MS.

The SGSN marks an MS as reachable or not reachable through a flag on the SGSN , which is the 'PPF flag'.

• If the PPF flag is 1, the MS is reachable.
• if the PPF flag is 0, the MS is unreachable.

Note: in practical scenarios, Mobile reachable timer may be between 30 and 60 minutes.

Implicit Detach Timer

The 'implicit detach' timer starts on the SGSN after the reachable timer expires. After the 'implicit detach' timer expires, the SGSN can perform an 'implicit detach' in order to return the MM contexts in the SGSN to IDLE (2G), or to PMM-DETACHED (3G).

Purge timer

After the MS moves to the IDLE (2G), or PMM-DETACHED (3G), The SGSN may not immediately delete the 'MM context' of this MS. The SGSN may keep the MM context, and will trigger the 'Purge timer'. After this timer expires, the SGSN will execute the purge procedure.

The Purge procedure allows the SGSN to inform the HLR that it has deleted the MM and PDP contexts of an MS.

Note: in practical scenarios, this timer may be around 24 hours.

Periodic RAU timer

The 'Periodic RAU Timer' determines the amount of time the MS should regularly make a periodic RAU with the SGSN, even if the RA is not changed.

The 'RAU timer' value should be less than the 'reachable timer' value. this will make the MS make 'periodic RAU' before the reachable timer expire. Therefore, the MS will always be reachable, unless a problem occurs in the MS, or the network.

Please know that, the MS is 3G PMM-CONNECTED state is not making a periodic RAU. The MS in 3G makes periodic raw when it is in PMM-IDLE state. 

Note: in many practical scenarios, RAU timer is configured to be between 30 and 60 minutes.