This document describes the basic functioning of the 5G RRC layer, describes the periodic output, and explains the various configuration options that influence its behavior.

Developer documentation, such as UE connection control flow, reestablishment, or handover, are described in a separate page.

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General

The RRC layer controls the basic connection setup of UEs as well as additional procedures. It is the fundamental building block of OAI’s CU-CP, and interacts with lower layers (DU, basically MAC and RLC) through F1AP messages, and with the CU-UP through E1AP messages. More information can be found in the respective F1AP page and E1AP page.

Periodic output and interpretation

Similarly to the scheduler, the RRC periodically prints information about connected UEs and DUs into file nrRRC_stats.log in the current working directory of the executable running the RRC (typically, nr-softmodem). The output lists first all UEs that are currently connected, and then all DUs, in order.

For each UE, it prints:

UE 0 CU UE ID 1 DU UE ID 40352 RNTI 9da0 random identity c0f1ac9824000000:
    last RRC activity: 5 seconds ago
    PDU session 0 ID 10 status established
    associated DU:  DU assoc ID 8

where UE 0 is the UE index, CU UE ID and DU UE IDs are the IDs used to exchange over F1 (cf. scheduler logs). Further, it shows RNTI, when the last RRC activity happened, the status of PDU sessions and which DU is associated (through the SCTP association ID).

For each DU, it prints:

1 connected DUs
[1] DU ID 3584 (gNB-OAI-DU) assoc_id 8: nrCellID 12345678, PCI 0, SSB ARFCN 641280
    TDD: band 78 ARFCN 640008 SCS 30 (kHz) PRB 106

i.e., an index ([1]), the DU ID and it’s name, the SCTP association ID (assoc_id 8, cf. UE information), and DU specific information for the cell (cell ID, physical cell identity/PCI, the SSB frequency in ARFCN notation, the band and Point A ARFCN, subcarrier spacing/SCS, and the number of resource blocks/PRB). Only one cell per DU is supported.

As of now, it does not print information about connected CU-UPs or AMFs.

Configuration of the RRC

Split-related options (when running in a CU or CU-CP)

See F1 documentation for information about the F1 split. See E1 documentation for information about the E1 split.

RRC-specific configuration options

In the gNBs section of the gNB/CU/CU-CP configuration file is the RRC-specific configuration

cell-specific options

Note that some SIBS are configured at the CU and some at the DU; please consult the MAC configuration as well for SIB configuration.

• gNB_ID and gNB_name: ID and name of the gNB
• tracking_area_code: the current tracking area code in the range [0x0001, 0xfffd]
• plmn: the PLMN, which is a list of entries consisting of:
• mcc: mobile country code
• mnc: mobile network code
• mnc_length: length of mobile network code, allowed values: 2, 3
• snssaiList: list of NSSAI (network selection slice assistence information, “slice ID”), which itself consists in
  • sst: slice service type, in [1,255]
  • sd (default 0xffffff): slice differentiator, in [0,0xffffff], 0xffffff is a reserved value and means “no SD” Note that: SST=1, no SD is “eMBB”; SST=2, no SD is “URLLC”; SST=3, no SD is “mMTC”
• enable_sdap (default: false): enable the use of the SDAP layer. If deactivated, a transparent SDAP header is prepended to packets, but no further processing is being done.
• cu_sibs (default: []) list of SIBs to give to the DU for transmission. Currently, SIB2 is supported.

UE-specific configuration

• um_on_default_drb (default: false): use RLC UM instead of RLC AM on default bearers
• drbs (default: 0): the number of DRBs to allocate for a UE, only useful for do-ra or phy-test testing

Neighbor-gNB configuration

Refer to the handover tutorial for detailed information about gNB neighbors and handover procedures.

Required configuration parameters

To define a neighbor cell in the configuration file, the following parameters are required:

• gNB_ID - identifier of the neighbor gNB (e.g., 0xe01)
• nr_cellid - cell identifier of the neighbor cell (e.g., 11111111)
• physical_cellId - physical cell ID for radio identification (e.g., 1)
• absoluteFrequencySSB - SSB frequency in ARFCN notation (e.g., 643296)
• subcarrierSpacing - numerology index: 0=15kHz, 1=30kHz, 2=60kHz, 3=120kHz
• band - 3GPP frequency band number (e.g., 78 for 3.5GHz)
• plmn - PLMN configuration object with:
• mcc - mobile country code (3 digits, e.g., 001)
• mnc - mobile network code (2-3 digits, e.g., 01)
• mnc_length - number of digits in MNC (must be 2 or 3)
• tracking_area_code - tracking area identifier (e.g., 1)

Example configuration structure:

neighbour_list = (
  {
    nr_cellid = 12345678;
    neighbour_cell_configuration = (
      {
        gNB_ID = 0xe01;
        nr_cellid = 11111111;
        physical_cellId = 1;
        absoluteFrequencySSB = 643296;
        subcarrierSpacing = 1; # 30 kHz
        band = 78;
        plmn = { mcc = 001; mnc = 01; mnc_length = 2 };
        tracking_area_code = 1;
      }
    );
  }
);

Refer to the handover tutorial for complete examples and detailed setup instructions.