RRC

Radio Resource Control

The Radio Resource Control (RRC) protocol is used in UMTS and LTE on the Air interface. It is a layer that exists between UE and eNB and exists at the IP level. This protocol is specified by 3GPP in TS 25.331 for UMTS and in TS 36.331 for LTE. RRC messages are transported via the PDCP-Protocol.

The major functions of the RRC protocol include connection establishment and release functions, broadcast of system information, radio bearer establishment, reconfiguration and release, RRC connection mobility procedures, paging notification and release and outer loop power control. By means of the signalling functions the RRC configures the user and control planes according to the network status and allows for Radio Resource Management strategies to be implemented.

The operation of the RRC is guided by a state machine which defines certain specific states that a UE may be present in. The different states in this state machine have different amounts of radio resources associated with them and these are the resources that the UE may use when it is present in a given specific state. Since different amounts of resources are available at different states the quality of the service that the user experiences and the energy consumption of the UE are influenced by this state machine.

In order to comprehend the Radio Resource Control (RRC) state machine, we should study the architecture of the third generation mobile cellular system, the Universal Mobile Telecommunications System (UMTS). The UMTS is composed by two subsystems, the UTRAN (Umts Terrestrial Radio Access Network) and CN (core network) :\

The RNC is responsible for the packet scheduling, radio resource control and handover control.\ Unlike other wireless technologies like WIFI in which clients concurrently send packets and collisions occur (and backoff algorithms), in the UMTS there is a centralized entity that schedules resources to each client. In order to manage the UE access to radio resources, the UMTS introduces a radio resource control protocol (RRC) that attributes each UE a specific state machine (based on radio resource usage). This state machine is dependent on the 3G provider and can have different states and/or transitions. According to the reference paper -- Characterizing Radio Resource Allocation for 3G Networks -- the studied networks were composed by three states: IDLE, CELL_FACH and CELL_DCH.\ In truth other states can actually be observed in some operators before going into a disconnected mode. For example the PCH states -- URA_PCH and CELL_PCH -- are states in which the user equipment has no transport channels allocated but has to monitor the cell/UTRAN it is currently connected to in order to send CELL_UPDATES to the RCN (just in case it actually changes). This update implies temporarily changing the state to a CELL_FACH in order to be able to send data. The change from CELL_PCH, in which the UE is known at a cell level, to URA_PCH, in which the user is known at UTRAN level, may be due to an increasing number of CELL_UPDATES (e.g. travelling fast). This is useful in order to speed up incoming connections since it might take around 0.9 seconds to establish it against the 2.5s delay if the UE is on the IDLE state.\ Below is a diagram that represents the six most common states -- CELL_DCH, CELL_FACH, CELL_PCH, URA_PCH, IDLE, DISCONNECT -- and some of the possible transitions.\