RRC connection

RRC (Radio Resource Control, radio resource control layer) mainly deals with
the UE's access to the cellular network through wireless signals. It is the top
layer of the wireless network access layer, and the upper NAS is the non-access
layer, which handles the mobile network business logic. Affairs. RRC has the
following functions：

 Provide connection management, messaging and other services for the NAS
layer;
 Provide functions such as parameter configuration to the underlying protocol
entity of the wireless access network;
 Responsible for UE mobility management related measurement and control
functions.

Radio Bearer
After the UE completes random access, it indicates that the UE has obtained the
uplink resources of the eNB and can start to transmit uplink data, that is, to
establish an RB (Radio Bearer, radio bearer). RB is the premise for UE to
transmit signaling and service data between eNB transmissions, and it is also the
underlying support for UE and MME to transmit CP signaling, that is, the
support for establishing RRC connection。

RB is a general term for a series of protocol entities and configurations allocated

by eNB to UE, including PDCP protocol entity, RLC protocol entity, and a series
of resources allocated by MAC layer and PHY layer. RB transmits data on the
Uu interface between the UE and the eNB. Any data transmitted on the Uu
interface must go through the RB. In fact, RB entities at the protocol level mainly
only look at PDCP (Packet Data Convergence Protocol, packet data convergence
protocol). The upper layer (eg UE APP) hands the service data to the PDCP entity,
sends it to the PHY layer after layer-by-layer encapsulation (as well as segmentation,
mapping, and multiplexing), and the receiver (generally eNB) decapsulates layer by
layer. The PDCP entity at the end hands the business data to the upper layer. From the
perspective of the upper layer, the PDCP entity that provides the data transmission
service is the RB, and does not understand the details of PDCP.

SRB

It can be seen from the above figure that RB can transmit service data through DRB
and signaling data through SRB. Here we mainly focus on the latter. SRB (Signaling
Radio Bearers) is used as the CP signaling bearer type of the upper-layer RRC
connection for RRC message transmission. According to the different bearer
signaling, it is divided into the following three categories:

 SRB0: Carry RRC signaling before RRC connection establishment, use

CCCH (common control channel) logical channel, msg3 and 4 use SRB0, and
use TM mode at RLC (radio link control) layer;
 SRB1: Bears RRC signaling and NAS signaling before SRB2 is established,
SRB1 is established before SRB2, so DCCH (dedicated control channel)
logical channel is used, msg5 uses SRB1, and AM mode is used at RLC (radio
link control) layer;
 SRB2: Carry NAS signaling, use DCCH (dedicated control channel) logical
channel, and adopt AM (acknowledged mode) mode at RLC (radio link
control) layer. SRB2 can only be established after E-UTRAN security mode is
completed;
Process of establishing RRC connection between UE
and eNB
1. The UE sends the RRC CONNECTION REQUEST to the eNB for
transmission on SRB0, and SRB0 maps the CCCH (Common Control
Channel) at the MAC (Media Access Control) layer, so resources that do not
belong to a certain UE do not need to be created in advance. ;
2. The eNB allocates C-RNTI (cell RNTI, allocated by the master eNB for the
UE to access the new cell) to distinguish different UEs (because the eNB not
only serves one UE, the eNB will send signals to multiple UEs at the same
time, and the UE will also Received the signal sent by the eNB to other UEs,
so the UE ID needs to be distinguished between the eNB and the UE. This UE
ID is temporary, called RNTI), and RRC CONNECTION SETUP REQUEST
is sent to the UE. Transmit on SRB0;
3. After receiving the RRC CONNECTION SETUP REQUEST, the UE
establishes SRB1 at the PDCP (Packet Data Convergence Protocol) layer
according to the instructions of the eNB, and then sends RRC CONNECTION
SETUP COMPLETE on the SRB1 to the eNB to complete the establishment
of the RRC connection and sends the first uplink NAS news. SRB1 maps
DCCH (dedicated control channel) at the MAC layer, which is a resource
belonging to a certain UE and needs to be created in advance.
 RRC connection status：

1. RRC_IDLE: Idle state. The system information received by the UE at this

time includes configuration parameters of cell selection or reselection and
neighbor cell information.
2. RRC_CONNECTION: continuous state. At this time, the UE receives the
common channel configuration information.

LTE RRC connection reestablishment

tags: LTE

1 Overview
The purpose of RRC reconstruction is to restore RRC signaling connections and
reduce dropped connections. However, excessive RRC reconstruction in the source
cell will affect cell throughput and user perception. This article introduces the
definition of RRC re-establishment, triggering conditions and the handling of problem
cells with too much RRC re-establishment in the source cell and low RRC re-
establishment success rate.

2 RRC reconstruction definition and

trigger conditions
2.1 Definition of RRC reconstruction
When in the RRC connection state, if the handover fails, the wireless link fails, the
integrity protection fails, and the RRC reconfiguration fails, the RRC connection re-
establishment process will be triggered. This process aims to re-establish RRC
connection, including recovery of SRB1 operation, and safe reactivation. The UE in
the RRC_CONNECTED state has been activated and can initiate the process to
continue the RRC connection. Only when the relevant cell is a cell with a UE context,
the connection re-establishment will succeed. If E-UTRAN approves the rebuild, the
operation of SRB1 will resume, while the other RBs will remain suspended. If AS
security is not activated, the UE will not initiate this process, but directly goes to the
RRC_IDLE state.

2.2 Reasons for RRC reconstruction

2.2.1 The reason why the terminal initiated the reconstruction

In the 36.331 protocol, the conditions for initiating reconstruction are described as
follows:

The UE shall only initiate the procedure when AS security has beenactivated. The UE
initiates the procedure when one of the following conditionsis met:

upon detecting radio link failure, in accordance with 5.3.11; or

upon handover failure, in accordance with 5.3.5.6; or

upon mobility from E-UTRA failure, in accordance with 5.4.3.5; or

upon integrity check failure indication from lower layers; or

upon an RRC connection reconfiguration failure, in accordance with5.3.5.5;

In a nutshell, there are three reasons for UE to initiate reconstruction:

 reconfiguration failure
 handover failure
 radio link failure

2.2.2         reconfiguration failure

The original agreement is as follows:

if the UE is unable tocomply with (part of) the configuration included in

theRRCConnectionReconfiguration message:

continue using theconfiguration used prior to the reception of

RRCConnectionReconfigurationmessage;
if security has not beenactivated:

    perform the actions uponleaving RRC_CONNECTED as specified in 5.3.12, with

release cause other;

else:

    initiate the connectionre-establishment procedure as specified in 5.3.7, upon which

the connectionreconfiguration procedure endif;

In the state that the UE is in the activated state of the security mode, if the cells in the
reconfiguration message cannot be matched / compatible after receiving the
reconfiguration message, a reconstruction with a cause value of "reconfiguration
failure" is initiated.

2.2.3         handoverfailure

if T304 expires (handover failure):

NOTE:      Following T304 expiry any dedicated preamble, if provided within

therach-ConfigDedicated, is not available for use by the UE anymore.

In the handover process, the UE will start T304 after receiving the reconfiguration
message of the handover, but if the UE cannot complete the random access in the
target cell before T304 times out, it will initiate a reconstruction with the cause value
of "handover failure"

2.2.4          radio linkfailure （ OtherFailure ）

If the UE detects that "radio link failure" is currently detected, it will initiate a
reconstruction with a cause value of "other", which usually causes the RLF to have
the following three mechanisms.

Uplink RLC retransmitted to the maximum number of times

"Indication from RLCthat the maximum number of retransmissions has been

reached", including SRB and DRB, the same mechanism as the downlink SRB and
DRB on the eNB side, after the UE RLC sends a PDU, the corresponding state needs
to be fed back to the eNB side before the PDU can Complete a normal RLC schedule.
There are two reasons for not receiving the eNB status PDU. One reason is that the
eNB side uplink has not received any RLC PDU at all, so it will not respond to the
status PDU, and the other reason is the status PDU that the eNB responds to. The
reason, did not reach the UE side.

Preamble reached the maximum number of transmissions

During the handover process, the PUCCH is not activated after the handover
completion command is lost, or the PUCCH is not activated due to Ta timeout during
service maintenance. At this time, if the UE has SR transmission, the UE cannot
receive the ENB because of downlink problems. The MAC layer confirms that the SR
retransmits to the maximum number of times
(physicalconfigDedicatedschedulingRequestConfigdsr-TransMax, configured as 64
times) and triggers MAC_RA_IND (contention-based random access), which reaches
the maximum number of times due to Preamble transmission (preambleTransMax, 10
times) during random access After reporting the RLF instruction to L3, it initiates a
rebuild request.

Delay spectrum first path search failed (UE detects downlink RLF)

The UE DSP judges the delay spectrum filter value every 200ms. If it meets a certain
threshold, it reports L3 out of synchronization and starts the T310 timer. Before the
T310 times out, if it receives N311 in-sync instructions (N311 = 5), It is considered
that the UE resumes the synchronization state (100ms + 10 * (N311 -1) = 140ms, that
is, after starting T310, the terminal reports the measurement at 100ms and judges that
the records are N311 + 1, and then the measurement is reported every 10ms),
otherwise, After T310 time-out (baseline value is 200ms, corresponding MML
parameter is configured as T310 = MS200_T310), UE will trigger the rebuilding
process (including searching for cells, L3 in synchronization state and continuously
receiving N310 out-of-sync instructions reported by L1 (baseline value 6 times, N310
= n6), it is considered to be out of synchronization (the time required for triggering is
expected to be: 200ms + 10 * (N310-1) = 250ms, both 200ms measurement report
judgments are satisfied, we have recorded N310 + 1, and then measured and reported
once every 10ms ), Synchronization, reconstruction), at the same time start the T311
timer (baseline value is 10s, the corresponding MML parameter configuration is T311
= MS10000_T311), if the timeout has not been successfully rebuilt, then enter the
IDLE state.

3 RRC connection reconstruction

process
Successful reconstruction process

 
 

Failed reconstruction process

1.1, UE sends RRC Connection Reestablishment Request message. According to

different scenarios, the reconstruction reason carried in the message is different from
the cell information:

 The cause of reconfiguration triggered by reconfiguration failure is

"reconfigurationFailure", where C-RNTI and physCellId are the cell information
 The cause of the re-triggering triggered by handover failure is "handoverFailure",
where C-RNTI and physCellId are the information of the source cell
 The cause of the reconstruction triggered by the failure of the wireless link is
"otherFailure", where C-RNTI and physCellId are the cell information

1.2. The eNodeB performs security parameter authentication. If the UE's security
parameter authentication information is consistent with the eNodeB, the UE
authentication is passed. After UE identity authentication, the eNodeB releases the
original resources and re-enters and allocates resources;

1.3. The eNodeB sends an RRC Connection Reestablishment message to the UE on

the CCCH channel. The message carries information about the newly allocated
resources. The UE receives the RRC Connection Reestablishment message,
reconfigures the wireless resources according to the message instructions, and
activates encryption and integrity protection;
2. The UE sends an RRC Connection Reestablishment Complete message to the
eNodeB.

4 RRC connection reconstruction

problem location and processing
RRC reconstruction problem location flowchart:

4.1 Fault alarm

Alarms can be observed on the U2000 side, and for each alarm, there are relevant
handling suggestions, which can be read through the U2000's online help. As shown
below:
Alarms can also be queried via MML commands: LST ALMAF (Query Current
Alarm), LST ALMLOG (Query Alarm Log)

If the following alarm occurs, you need to eliminate the alarm, and then perform
subsequent optimization analysis:

Alarm, Alarm
NE level Alarm name
level number
important 26538 Radio unit clock abnormal alarm
important 29243 Warning of the decline in cell service capacity
important 26529 Radio frequency unit standing wave alarm
important 26540 RF unit AC power failure alarm
important 26235 Radio unit maintenance link abnormal alarm
important 26322 BBU IR transceiver abnormal alarm
important 29248 Radio unit service unavailable alarm
important 26260 System clock is unavailable
important 26508 Abnormal RF interface IR interface alarm
Alarm of the deterioration of the optical interface
ENODEBimportant 26506
performance of the RF unit
important 29205 X2 interface configuration update failure alarm
important 26324 BBU IR interface abnormal alarm
Base station control plane transmission interruption
important 29207
alarm
Abnormal send and receive alarm of the RF unit
important 26503
optical module
important 26242 Configuration file damage alarm
important 26222 Abnormal transmission optical interface alarm
important 26245 Inconsistent configuration data alarm

1.1 Missing neighbors

The terminal is rebuilt to other stations. If the reconstruction is to be successful, the
following requirements are required for the two stations before and after the
reconstruction

 There is an X2 link between the two sites and it is a Huawei base station
  There is a two-way neighbor relationship between the two cells before and
after reconstruction

Missing neighbors will cause the target cell to have no UE context. The UE initiates
RRC connection re-establishment in the target cell, and the re-establishment will
inevitably be rejected, causing RRC re-establishment failure

Neighboring cell miss-matching needs to be optimized by combining working

parameters, electronic maps, and information, and adding necessary neighboring cells.
External cell verification is required for mismatched neighboring cells, and external
cell data needs to be extracted for comparison and verification with existing network
cell data

1.2 Interference check

The interference is usually divided into uplink interference and downlink interference,
internal interference and external interference. Interference is easy to cause the radio
link to fail RLF. When the uplink RLC reaches the maximum number of times, it will
trigger the RRC connection re-establishment of this cell because of other failure

External interference generally refers to interference caused by interference sources

other than the current network standard. External interference sources cause
interference to the TD-LTE frequency band due to illegal or improper use. It is
embodied as co-channel interference. Common external interferences include:
military area communication system, bank ATM machine signal interference device,
school and social test center signal shielding device, can be checked by frequency
sweep

LTE intra-network interference, due to the same frequency networking adopted by

LTE, there will inevitably be co-frequency interference. Unreasonable PCI planning
may also cause MOD3 interference

Generally, for the downlink, when the RSRP of the serving cell is higher than
-90dBm, but the SINR is lower than -6dbm, it can basically be considered as a
problem of downlink interference (when the neighboring cell is wrong / missing or
the handover is not timely, service may also occur The RSRP signal of the cell is very
good, but the SINR is very poor). The downlink interference is usually the guide
frequency pollution, which means that there are more than 3 cells in the coverage area
that meet the handover conditions. Due to the signal fluctuations, frequent cell
reselectors often ping-pong. , May cause RRC reconstruction

Uplink interference can be determined by querying PRB interference noise statistics.

The average value of interference noise detected on each PRB in the system uplink
(milliwatt decibel)> -110, it is determined that there is uplink interference, as shown
in the figure:

 
Generally, when there is no interference, the uplink and downlink are balanced, and
when there is interference in the downlink, it will be reflected in the downlink is
limited, the uplink is not limited, and when there is uplink interference, the uplink is
limited but the downlink is not limited .