The MS initiates the 'Service Request' procedure in order to establish the 'PS signaling connection' between the MS and the SGSN. The MS do this in order to move the MM context from PMM-IDLE state to PMM-CONNECTED state.
'Service Request' description and 'PS signaling' connection
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.
In 3G, When the MS is already having an MM context exist, but this MM context is in PMM-IDLE state, the MS will not be able to send or receive data or signaling to the network.
In addition, in this case there is no 'PS signalling connection' exist between the SGSN and the MS.
If the MS needs to send data or signaling to the network, it has to move the MM context to PMM-CONNECTED state. Therefore, the MS should initiate the 'Service Request' procedure. The MS do this in order to establish the 'PS signalling connection' between the MS and the SGSN.
'Service Request' signaling
- The MS sends 'Service Request' message to the SGSN
- The SGSN performs security functions in order to authenticate the MS
- The SGSN initiates the 'RAB establishment' between the SGSN and the MS
- If the QoS changed during the 'RAB establishment', the SGSN will initiate ‘PDP context modification’ to align with other nodes regarding the new QoS.
- Now, the RAB exist between the MS and the SGSN, and the MM context moved to PMM-CONNECTED state. Therefore, the MS will be able to send data to the PDN.
The 'RAB' is the Radio Access Bearer, and it is part of the PDP context between the RNC and the SGSN. Later, we will go deeper into this in 'PDP context activation'.