The MM context exist on both the MS and the SGSN. One of the information elements exist inside the MM context, is the 'MM state' the MS currently exist in.
Below figure shows the states the MS can exist in, and the events that triggers the transition from one state to another.
MM state transition
First, the MS is in 'switch off' situation. Therefore, the MS will be in IDLE state.
Moving from IDLE to READY
- Now you turned on the MS. Then , the MS will start making 'GPRS attach' procedure with the network. Hence, the MS will move from IDLE state to READY state.
Moving from READY to STANDBY
There is a timer exist on both the MS and the SGSN, which is the 'READY timer'. Once the MS enters to READY state, the timer starts counting. Every time a data or signaling is received from MS, this timer will reset.
The READY timer value is the same on the MS and on the SGSN. Only the SGSN is able to change the value of the ready timer. The SGSN sends the value of this timer to the MS during the 'GPRS attach' procedure.
Later, we will go deeper in Timers used in Mobility Management.
- If the 'READY timer' expired without sending any data or signalling from the MS to the SGSN, the MS will move from the READY state to STANDBY state.
- Also, if the SGSN detected abnormal conditions on the link between the BSC and the MS, the the MM context will move to STANDBY state on the SGSN.
Moving from STANDBY to READY
- In case the MS is in STANDBY state, when the MS send signaling to the SGSN, the MM context will move from STANDBY state to READY state.
Moving from READY to IDLE
- Now, if the MS made a 'GPRS detach' with the network, because the MS is turned off, or for any other reason. In this case, the MS will move from READY state to IDLE state.
Later, we will learn the signaling of 'GPRS detach' procedure.
Moving from STANDBY to IDLE
- The SGSN can detach the MS by an 'implicit detach', which means a detach procedure on the SGSN, without coordination with the MS'. in this case, the state of the MS on the SGSN will move from STANDBY to IDLE.
- Also, the SGSN may receive 'Cancel location' message from the HLR, instructing the SGSN to detach the MS. In this case, the state of the MS on the SGSN will move from STANDBY to IDLE.