Layer3 Protocols – 4G

The RRC protocol is the signaling exchanged between the mobile and the evolved Node Base station (eNB) over the LTE-Uu radio interface.

The RRC protocol performs the following functions:

• Broadcast of system information related to the characteristics of the radio interface.
  • Control of the RRC connection. This procedure includes the paging, establishment, modification and release of the Signaling Radio Bearer (SRB) and the Data Radio Bearer (DRB). It also includes the activation of security mode over the LTE-Uu interface, the procedure which consists of putting mechanisms in place to encrypt the traffic and RRC signaling flows and to control the integrity of the RRC signaling flows.
  • Control of handover. This procedure executes the changing of cell between two eNB entities (intrasystem handover or intersystem handover).
  • Measurement reporting. The eNB entity can trigger measurements carried out by the mobile, either periodically or on demand, to prepare for handover.
  • Transport of the NAS messages exchanged between the mobile and the MME entity.

From the point of view of the eNB entity, the mobile may be in one of two operational states:

• idle mode (RRC_IDLE) or
• connected mode (RRC_CONNECTED).

In idle mode, the mobile is not known to the eNB entity. It remains in this state until the RRC connection setup procedure is completed. The setup procedure is triggered by the mobile when it wishes to transmit traffic or signaling data. In that state, the mobile used the SRB0 bearer.

In connected mode, the mobile can transmit and receive signaling and traffic data. The mobile is attributed to an identifier that is unique to the cell, the Cell Radio Network Temporary Identity (C-RNTI). In this state, the mobile uses either the SRB1 bearer for RRC messages with possible associated NAS messages, or the SRB2 bearer for RRC messages transporting solely NAS messages.

Table below summarizes the type of SRB, the mode of RLC (Radio Link Control) protocol and the channels used by the different RRC messages over the radio interface.

In RRC_CONNECTED, there is an RRC context established—that is, the parameters necessary for communication between the terminal and the radio-access network are known to both entities. The cell to which the terminal belongs is known and an identity of the terminal, the Cell Radio-Network Temporary Identifier (C-RNTI), used for signaling purposes between the terminal and the network, has been configured. RRC_CONNECTED is intended for data transfer to/from the terminal, but discontinuous reception (DRX) can be configured in order to reduce terminal power consumption. Since there is an RRC context established in the eNodeB in RRC_CONNECTED, leaving DRX and starting to receive/transmit data is relatively fast as no connection with its associated signaling is needed.

Although expressed differently in the specifications, RRC_CONNECTED can be thought of as having two substates, IN_SYNC and OUT_OF_SYNC, depending on whether the uplink is synchronized to the network or not. Since LTE uses an orthogonal FDMA/TDMA-based uplink, it is necessary to synchronize the uplink transmission from different terminals such that they arrive at the receiver at (approximately) the same time. The procedure for obtaining and maintaining uplink synchronization in short the receiver measures the arrival time of the transmissions from each actively transmitting terminal and sends timing-correction commands in the downlink. As long as the uplink is synchronized, uplink transmission of user data and L1/L2 control signaling is possible. If no uplink transmission has taken place within a configurable time window, timing alignment is not possible and the uplink is declared to be non-synchronized. In this case, the terminal needs to perform a random-access procedure to restore uplink synchronization prior to transmission of uplink data or control information.

In RRC_IDLE, there is no RRC context in the radio-access network and the terminal does not belong to a specific cell. No data transfer may take place as the terminal sleeps most of the time in order to reduce battery consumption. Uplink synchronization is not maintained and hence the only uplink transmission activity that may take place is random access to move to RRC_CONNECTED. When moving to RRC_CONNECTED the RRC context needs to be established in both the radio-access network and the terminal. Compared to leaving DRX this takes a somewhat longer time. In the downlink, terminals in RRC_IDLE periodically wake up in order to receive paging messages, if any, from the network.

Reference – André Perez, in Implementing IP and Ethernet on the 4G Mobile Network, 2017