Add 5G RRC documentation

doc/RRC/rrc-usage.md

+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](./rrc-dev.md).
+
+[[_TOC_]]
+
+# 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](../F1-design.md) and [E1AP page](../E1AP/E1-design.md).
+
+# 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](../F1-design.md) for information about the F1 split.
+See [E1 documentation](../E1AP/E1-design.md) 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
+
+- `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.
+
+### 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
+
+    TBD
+
+Refer to the [handover tutorial](../handover-tutorial.md) for more information.