The network inanities 'Paging procedure' towards to the MS, in order to move the MM context from STANDBY to READY in 2G, or from PMM-IDLE to PMM-CONNECTED in 3G.
Paging procedure in 3G
When the MS is in PMM-IDLE state, there is no PS signalling connection exist between the MS and the SGSN.
The PS signaling connection is the Iu connection between the RNC and the SGSN, in addition to the RRC connection between the RNC and the MS.
If the network is having down link data that should be transferred to the MS, the network should initiate 'Paging procedure' towards the MS.
The network do this in order to establish the PS signalling between the MS and the SGSN, which will move MS 'MM context' from PMM-IDLE to PMM-CONNECTED state.
While the MS is in PMM-IDLE state, the network already knows in which RA the MS exists in. In addition, the MS is having a PDP context already established, but no RABs exist between the MS and the SGSN.
The 'PDP' is the data session between the MS and the GGSN, and RAB is part of this PDP. Later we will go deeper in 'PDP context activation' procedure.
'Paging' signaling in 3G
- The SGSN receives down link data, which it should send to the MS that exist in PMM-IDLE state.
- The SGSN sends 'paging request' to the RNC. This will include the IMSI of the MS, the P-TMSI of the MS, and the RA the MS exists in.
- The RNC will forward the paging request to the cells exist under this RA.
- When the MS receives the 'Paging', it will initiate a 'Service request' towards the network.
Paging procedure in 2G
When the MS 'MM context' is in STANDBY state in 2G, the network knows it exist under which RA.
Therefore, when there is down link data need to be delivered from the network to the MS, the network should initiate 'Paging' procedure towards this MS.
This will move the MS from ‘STANDBY’ state to ‘READY’ state, then the network will be able to deliver the down link data.
'Paging' signaling in 2G
- The SGSN receives down link data, that it should send to the MS that exist in STANDBY state.
- The SGSN sends 'paging request' to the BSC. This will include the IMSI of the MS, the P-TMSI of the MS, and the RA the MS exist in.
- The BSC will forward the paging request to the cells exist under the RA.
- The MS will reply with any frame, which will be interpreted by the BSC as a paging response.
- The BSC, will forward the MS frame to the SGSN, including the cell identity the MS is currently using. Now the MS moved to the ‘READY’ state, and the SGSN will be able to send the data to it.