WHAT PROTOCOLS ARE INVOLVED IN THE CONTROL PLANE OF LTE ? CAN YOU EXPLAIN THEIR FUNCTIONALITY BRIEFLY ?

The question is specifically asking about the LTE Control plane protocols from the Radio Interface perspective.

Therefore, lets focus specifically on the radio interface including the control plane protocols involved as shown below.

The protocols from the RAN perspective in control plane are shown above.

The brief functionality of each of the protocol layer is shown below.

Non-Access Stratum (NAS) : NAS consists of Session Management (SM) and EPS mobility management (EMM) layers. The functions performed by NAS are:

• Mobility management for idle UEs
• UE Authentication
• EPS bearer management
• Configuration and control of Security
• Paging initiation for idle UEs

Physical Layer (PHY) : This protocol is responsible for following commands from MAC layer and perform functions such as modulation coding scheme etc. on bits which needs to be send over the interface.

Medium Access Control (MAC): The LTE scheduler resides at MAC layer of the eNodeB. MAC layer protocol at eNodeB performs multiplexing, scheduling, transport format selection, modulation scheme selection and QoS responsibilities etc. on the RAN side.

Radio Link Control (RLC) : Main functions for this protocol include concatenation and segmentation of data. In addition it has three modes of transportation based on the type of information passing through. Transparent mode, Acknowledged mode and Unacknowledged mode.

Packet Data Convergence Protocol (PDCP): It performs header compression and sequence numbering. For control plane traffic, PDCP perform both ciphering and integrity protection.

Radio Resource Control (RRC):The following control plane functions are agreed in 3GPP, to be performed by the RRC layer:

1. Broadcast of System information related to non-access stratum
2. Broadcast of System Information related to access stratum
3. Paging
4. Establishment , maintenance and release of an RRC connection between the UE and E-UTRAN such as stated in 5, 6 and 7:
5. Allocation of temporary identifiers between UE and E-UTRAN
6. Configuration of radio resources for RRC connection including Signaling Radio Bearers (SRBs)
7. Establishment, maintenance and release of point to point Radio Bearers;

Mobility functions for RRC protocol includes:

• Inter-cell handover
• UE measurement reporting control of the reporting for inter-cell and inter-RAT mobility
• UE cell selection and reselection and control of cell selection and reselection
• UE Context transfer between eNode Bs
• QoS management functions( Even though this function is spread across multiple layers , we are discussing briefly the functions associated with RRC only)
• UE measurement reporting and control of the reporting
• NAS direct message transfer to/from NAS from /to UE

On the network side, it has been agreed in 3GPP that RRC layer is to be terminated by the eNodeB