Please take into consideration that both MS and SGSN determine the PMM state according to its own calculation. In other words, in certain situations, you may find the MM state for the MM context on the MS is in a certain state, while you find the MM state for the same MM context on the SGSN is in a different state.
PMM state transition
Suppose we have an MS that is in 'turn off' situation. Now, The MS is in PMM-DETATCHED state.
Moving from PMM-DETACHED to PMM-CONNECTED
- Once the MS is powered on, the MS will start making 'GPRS attach'. Hence, MS will move from PMM-DETACHED to PMM-CONNECTED.
Moving from PMM-CONNECTED to PMM-DETACHED
- If the 'GPRS attach' procedure failed for any reason, The MS will remain in PMM-DETACHED state.
- Or, if the MS is in PMM-CONNECTED , and the MS or the network made a 'GPRS detach'. In this case, the MS will transition from PMM-CONNECTED to PMM-DETACHED.
Later, we will go deeper in 'GPRS detach' procedure.
Moving from PMM-CONNECTED to PMM-IDLE
When the MS is in PMM-CONNECTED. In case the PS signaling connection between the SGSN and the MS is released, the MS will move to PMM-IDLE.
- Please take into consideration that, the PS signaling connection is formed of the 'Iu connection' between the SGSN and the RNC, in addition to the 'RRC connection' between the RNC and the MS.
- The RNC, or the SGSN may release this PS signaling connection to save resources. In addition, this PS signaling connection may break due to certain failure.
Moving from PMM-IDLE to PMM-DETACHED,
- While the MS is in PMM-IDLE state. In this case, the MS should make 'RAU procedure' whenever it is necessary. When the MS start making RAU, the PS signalling connection between the SGSN and the MS should be established. Hence, the MS will move to PMM-CONNECTED state.
- In case the 'RAU procedure' failed for any reason, the PS signaling connection between the SGSN and the MS will be deleted, and the MS will move to PMM-DETACHED state.
- In addition, if the 'implicit detach' timer expired on the SGSN, the SGSN will move locally the MM context to PMM-DETACHED.
Implicit detach timer is a timer exist on the SGSN, when it expires without receiving any signaling or data from the MS, the SGSN will make a detach for the MS. We will go deeper in this later.
Moving from PMM-IDLE to PMM-CONNECTED
- When the MS in PMM-IDLE. if the MS needs to send any signaling to the network, the PS signalling connection between the MS and the SGSN should be established. To accomplish this, the MS sends a 'Service request' to the SGSN. This will establish the signaling connection between the MS and the SGSN, and the MS will move to PMM-CONNECTED.
Later, we will go deeper into 'Service Request' procedure.