User Plane Cellular IoT Optimizations

As part of cellular IoT optimizations introduced by 3GPP in Release 13, user plane optimization is the second mode of optimization. 

So why user plane optimization when control plane optimization is defined?

Control plane IoT optimizations as discussed in previous post can be used only when the data sent by the UE is of smaller size. For scenarios like firmware upgrade, infrequent large data (like sensor readings collected over a period as a file upload), the UE needs a dedicated radio bearer (DRB) and consequently S1U tunnel between the eNB and the EPC. However the existing EUTRAN Service Request procedure for transitioning from IDLE mode to CONNECTED mode and set up of DRB and S1U GTPU tunnel requires elaborate signaling which would drain the battery of UE. Hence the solution is to reduce the amount of signaling involved in Service Request procedure, by preserving the RRC context at the eNB even when the UE is not active in radio transmission. For this the network (eNB) after detecting UE radio inactivity decides to put the UE in an RRC suspended state, preserves the RRC context and S1AP context of the UE, and then sends a S1-AP UE Context Suspend Request to the MME and releases the RRC towards UE by sending a resume Identity.

Later when the UE decides to send an uplink data, instead of going through elaborate service request procedure, it sends an RRC Connection Resume request to the eNB and eNB resumes the S1AP context. There is no elaborate exchange of security context between MME and eNB and no setup of AS security context. The preserved AS and RRC context are just resumed from where they got suspended earlier.

The call flows as given in section 7.3a.3 of TS 36.300 explain the sequence of events.

3GPP has also specified mechanism for the network to change the UE mode of operation from Control Plane Optimization to User Plane Optimization. This change of mode will be useful in the following scenario

1. When UE went into IDLE state while using control plane optimization, the SGW / MME received a large downlink packet. MME pages the UE and UE sends a Control Plane Service Request to MME. In response to that the MME may trigger S1 AP UE Context Setup procedure and initiate DRB radio bearer setup and switch the UE to user plane mode.
2. This is explained in section 5.6.1.4.2 of TS 24.301