Functionality of RRC layer in LTE?

Question

What is the functionality of RRC layer in LTE and how it is different from UMTS.

Answer 1

RRC is one of the important layer in LTE towards Air interface (Uu).I would like to explain here RRC w.r.t to eNodeB. RRC controls all the lower layers like phy, mac, rlc, pdcp. RRC communicates to lower layers for cell specific configiuration/reconfiguration and UE specific configuration/reconfiguration. RRC performs various operation which could not be covered here. But I would like to share some common operation(s).

When UE wants to attach to core network to get a service. It has to make rrc connection with eNodeB. This type RRC mainly comes in picture and creates SRB1 for the UE. This dedicated signalling radio bearer is created so that UE can send it's NAS message to core network. NAS message is carried by RRC Connection Setup Complete. And this received NAS message is forward to MME via Initial UE message which is a S1-AP message.

Once MME finishes its job to handle this NAS message. It sends Initial context Setup Request to eNodeB. EnodeB RRC performs AS security using security mode command and security mode complete message. Enodes RRC generates ciphering and integrity keys from the Kenb received from MME. After AS security activation, RRC establishes SRB2 and DRB's at lower layers and information the same configuration to UE via rrc connection reconfiguration to UE. UE does the same job at its own end and acknowledges the same to eNodeB via RRC connection reconfiguration complete message.
I have just covered rrc functionality while handling attach procedure.

Answer 2

The RRC protocol covers a number of functional areas:

• System information handles the broadcasting of SI, which includes NAS common information. Some of the system information is applicable only for UEs in RRC_IDLE while other SI is also applicable for UEs in RRC_CONNECTED.

• RRC connection control covers all procedures related to the establishment, modification and release of an RRC connection, including paging, initial security activation, establishment of Signalling Radio Bearers (SRBs) and of radio bearers carrying user data (Data Radio Bearers, DRBs), handover within LTE (including transfer of UE RRC context information4), configuration of the lower protocol layers,5 access class barring and radio link failure.

• Network controlled inter-RAT mobility includes handover, cell change orders and redirection upon connection release, security activation and transfer of UE RRC context information.

• Measurement configuration and reporting for intra-frequency, inter-frequency and inter-RAT mobility, includes configuration and activation of measurement gaps.

• Miscellaneous functions including, for example, transfer of dedicated NAS information and transfer of UE radio access capability information.

Answer 3

RRC protocol layer exists in UE & eNodeb, It is part of LTE air interface control plane. The main services and functions of the RRC sublayer include:

• Broadcast of System Information related to the non-access stratum (NAS);
• Broadcast of System Information related to the access stratum (AS);
• Paging;
• Establishment, maintenance and release of an RRC connection between the UE and E-UTRAN
• Security functions including key management;
• Establishment, configuration, maintenance and release of point to point Radio Bearers;
• Mobility functions
• QoS management functions;
• UE measurement reporting and control of the reporting;
• NAS direct message transfer to/from NAS from/to UE.
• Protocol specification

3GPP TS 36.331 (http://www.3gpp.org/ftp/Specs/html-info/36331.htm) - Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC)

Credit: lteworld.org