Retainability
Module Objective
After completing this learning element, the participant should be able to:
Identify NSA Retainability Introduction
Restate NSA Retainability gNB Release Scenarios (gNB detected)
Locate NSA Retainability gNB Release Scenarios (UE Detected)
Repeat NSA Retainability eNB Release Scenarios
Review NSA Retainability KPIs
Discuss SA Retainability (gNB and AMF Release Scenario)
Report SA Retainability (UE Detected Scenario)
Define Quiz/Exercise
Index
NSA Retainability Introduction
NSA Retainability gNB Release Scenarios (gNB detected)
NSA Retainability gNB Release Scenarios (UE Detected)
NSA Retainability eNB Release Scenarios
NSA Retainability KPIs
SA Retainability (gNB and AMF Release Scenario)
SA Retainability (UE Detected Scenario)
Quiz/Exercise
Index
NSA Retainability Introduction
NSA Retainability gNB Release Scenarios (gNB detected)
NSA Retainability gNB Release Scenarios (UE Detected)
NSA Retainability eNB Release Scenarios
NSA Retainability KPIs
SA Retainability (gNB and AMF Release Scenario)
SA Retainability (UE Detected Scenario)
Quiz/Exercise
NSA Retainability Introduction
Introduction
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Retainability is the ability to retain a connection until normally ended. It is one of the most important
metrics to assess the network performance
Connection releases can be normal or abnormal
Abnormally terminated connections that release a bad radio link can be desired in order to improve:
• End user throughput (when re-establishing a new, better connection)
• Capacity (the earlier bad radio links are release the more chances those resources will be used for
UE in better radio conditions using higher MCS (Modulation and Coding Scheme) and MIMO.
It is possible, via parameterization, to influence the release of a connection so e.g. radio link failures
are more likely to happen
When using traditional DCR (drop call ratio) to calculate retainability results can be low e.g. in case of
networks dominated by FWA (fixed wireless access) as the duration of the connection has a big
impact on the DCR. Using drops-per-hour metric (or “mean time between drops”) in those cases will
solve the problem
NSA Retainability Introduction
NSA Overview (1/2)
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•
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The eNB and the SgNB can initiate the release of the 5G Radio Bearer.
Releases can be due to ‘normal’ or ‘abnormal’ reasons
The share of 5G releases due to RLF depends on the B1 threshold value for establishing the radio
links
• If B1 has low value, SgNB additions are attempted in bad 5G RSRP areas increasing the probability
the radio link terminates due to RLF
Note: From counters point of view, a 5G connection is established when the gNB receives the SgNB
Reconfiguration Complete message form the eNB. As this message happens before the 5G RACH
procedure has been completed, a failure in the RACH procedure can be considered a dropped call
instead of a failed call establishment
NSA Retainability Introduction
NSA Overview (2/2)
SgNB initiated Release Scenarios
MeNB initiated Release Scenarios
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UE inactivity (considered normal release)
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Normal release (incl. UE inactivity)
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A2 release (5G RSRP becomes bad)
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Detach (also viewed as normal release)
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Release due to missing A3 handover
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VoLTE call started (normal release)
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SgNB-detected 5G radio link failure
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4G radio link failure
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UE-detected 5G radio link failure (communicated via eNB
via RRC connection)
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4G handover (if EN-DC handover is not supported)
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eNB reset / partial reset
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Abnormal reasons (PDCP Count Rollover or GTP-U Error)
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Various other scenarios
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gNB reset / partial reset
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Possibly other scenarios
UE initiated Release Scenarios
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Protection against overheating
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Conservation of battery power
Index
NSA Retainability Introduction
NSA Retainability gNB Release Scenarios (gNB detected)
NSA Retainability gNB Release Scenarios (UE Detected)
NSA Retainability eNB Release Scenarios
NSA Retainability KPIs
SA Retainability (gNB and AMF Release Scenario)
SA Retainability (UE Detected Scenario)
Quiz/Exercise
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to UE Inactivity (1/4)
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UE is considered inactive by SgNB if there is no 5G data
received or transmitted and if there is no data in the
buffer for a given time period defined by the inactivity
timer (nsaInactivityTimer)
•
Inactivity timer value is a compromise between a quick
release of a connection (better for PUCCH capacity and
less UE battery consumption) and keeping the
connection longer to avoid another SgNB addition
when more packets arrive (less control plane load in the
network)
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Inactivity timers are independent for UL and DL but
both must be terminated to release the connection
•
An idle connection released due to inactivity does not
impact the user experience but impacts the retainability
KPIs
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to UE Inactivity (2/4)
Example of how inactivity timer value impacts the dropped connection ratio
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Inactivity timer changed from 10 to 5 sec (highlighted period)
5G21A
Improved drop ratio
The drops per volume metric was not impacted
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to UE Inactivity (3/4)
Example of how inactivity timer value impacts the dropped connection ratio
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Inactivity timer changed from 10 to 5 sec (highlighted period)
5G21A
No change in RACH success ratio
But RACH failures per GByte is worse
Conclusion: End-user performance has probably degraded by reducing the NSA inactivity timer
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to UE Inactivity (4/4)
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Counters related to UE inactivity releases:
Counter id
Counter name
Comment
M8080C45
SGNB_TOT_REL_INIT_SGNB
Total SgNB-initiated releases, eNB perspective
M55112C00007
X2_SGNB_REL_REQUIRED_SENT
Total SgNB-initiated releases, gNB perspective
M8080C44
SGNB_NOR_REL_INIT_SGNB_INACT
Releases due to 5G inactivity, eNB perspective
M55112C02001
SGNB_RELEASE_REQ_UE_INACT
Releases due to 5G inactivity, gNB perspective
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to A2 measurement report (1/2)
Feature id
Feature name
5GC001954
Introduction of A2 based SgNB release
5GC002177
A2 based SgNB release
It is possible to release the 5G radio link when 5G RSRP becomes lower than a threshold.
Example benefits:
• In case of no 5G-to-5G handover support, it may be better to release a bad 5G radio link and
establish in a neighbor cell with better signal, if available.
• If 5G link is bad, there will be RLC retransmissions and this can lead to delayed TCP acks and
therefore reduced TCP throughput
• Reserving 5G for users with good 5G signal gives better 5G spectral efficiency
• Retainability KPIs grow numerically
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to A2 measurement report (2/2)
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There is no ‘best’ default A2 threshold as it is network even cell dependent e.g. factors like
DL/UL link balance need to be considered for the best value
A2 release threshold must be aligned with B1 establishment threshold, such that “ping-pong”
establishments/releases are avoided
Related counters:
Counter id
Counter name
Comments
M8080C45
SGNB_TOT_REL_INIT_SGNB
Total SgNB-initiated releases, eNB perspective
M55112C00007
X2_SGNB_REL_REQUIRED_SENT
Total SgNB-initiated releases, gNB perspective, incl. A2
releases
M55112C00502
SGNB_REL_SN_REQ_UE_LOST
Will not be incremented by A2 release. Since the
alternative to A2 release is often a dropped connection,
it can be expected that this counter will decrease when
A2 release is enabled in the network
M55112C05001
X2_SGNB_REL_REQUIRED_SENT_A2
Counts the number of A2 releases.
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to missing A3 Handover
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Event A3 is triggered when a neighbor cell becomes offset better than the primary serving cell.
Scenario: If a UE reports in an A3 event a strong cell that is not defined as a neighbor, the handover will not
happen, and the connection will continue in the bad radio link despite there is a better cell. Alarm 7115 will
be raised
The scenario can be avoided by parameter flag sgnbRelForNoHandoverEnabled
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The connection is released and it is expected that the B1 measurements will almost immediately trigger a
SgNB addition on the nearby cell
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There are no specific counter(s) for this case. The generic X2_SGNB_REL_REQUIRED_SENT is incremented
instead
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Case can be considered a normal release instead of a failure as the reason is a missing neighbor which
should be addressed
Alternative: Alarm monitoring can be used to detect missing neighbors which are then regularly added in
the configuration
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Default: sgnbRelForNoHandoverEnabled = false
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to SgNB-detected 5G Radio Link Failure (RLF) (1/7)
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SgNB can detect RLF if the PUCCH messages are not arriving as expected (either DL HARQ
feedback or CSI reports), or if the maximum number of RLC retransmissions has been reached
When SgNB detects RLF, a control timer is started
Upon timer expiration, if radio link is not recovered, the SgNB releases the UE context by sending
SgNB release message with cause value “Radio Connection with UE Lost” to the MeNB
Features:
Feature id
Feature name
5GC000478
Radio Link Failure Handling for NSA mode 3X operation
5GC001927
Enhancements to Multiple DRBs per UE (NSA mode 3x)
5GC001927 is an enhancement over the 5GC000478. With this feature, when a RLF is detected the
data transfer over the 5G leg is suspended for all SCG split bearers. Before it was only suspended
for a single SCG split bearer
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to SgNB-detected 5G Radio Link Failure (RLF) (2/7)
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There are three reasons for SgNB detected Radio Link Failure:
1. Discontinuous transmission (DTX) detection for requested DL HARQ feedback on PUCCH
2. DTX detection of CSI reports on PUCCH
3. Radio Link Control (RLC) failure when the maximum number of retransmissions is reached
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to SgNB-detected 5G Radio Link Failure (RLF) (3/7)
Cause: DTX detection for DL HARQ feedback
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SgNB counts the number of consecutive DTX detections
for the requested DL HARQ feedback reports
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RLF detection only happens for one carrier at a time
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If number of consecutive detected DTX is > 35 (default)
then SgNB-DU starts the RFL guard timer tRLFindForDU.
Thresholds are configurable parameters
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Radio link recovers if the number of consecutively
received non DTX for DL HARQ is > 5 before timer
expiration and the timer is stopped
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If guard timer expires and radio link hasn’t recovered,
then SgNB-DU informs SgNB-CU of the RFL with a UE
Context Modification Required message using the F1 link
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to SgNB-detected 5G Radio Link Failure (RLF) (4/7)
Cause: DTX for CSI
Channel Status Indicators (CSI) are evaluated for each UE
to detect radio link issues
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SgNB counts the number of consecutive DTX
detections for CSI reports
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If number of consecutive detected DTX is > 75 (default)
then SgNB-DU starts the RFL guard timer
tRLFindForDU. Thresholds are configurable parameters
Radio link recovers if the number of consecutively
received non CSI feedbacks is > 2 before timer
expiration and timer is stopped
If guard timer expires and radio link hasn’t recovered,
then SgNB-DU informs SgNB-CU of the RFL with a UE
Context Modification Required message using the F1 link
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•
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to SgNB-detected 5G Radio Link Failure (RLF) (5/7)
Cause: RLC Failure
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Triggered by the SgNB when the maximum
number of Radio Link Control (RLC)
Retransmissions is reached
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Max. number of retransmissions is configured with
the dlMaxRetxThreshold parameter
In case of RLC failure SgNB-DU informs SgNB-CU
via UE Context Modification Required message
Cause: RLC failure
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NSA Retainability gNB Release Scenarios (gNB detected)
Release due to SgNB-detected 5G Radio Link Failure (RLF) (6/7)
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The underlying cause for all the SgNB detected radio link failures is bad radio link quality
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It is possible to control via parameterization how quick the gNB declares an RLF:
Parameter
Object
Range
Default
Recommended
actGnbInitRlf
NRBTS
False / true
True
True
rlpDetCsiBsiThreshold
NRCELL
0..10000
75
10000
rlpDetDlHarqThreshold
NRCELL
0..10000
35
35
rlpRecDlHarqThreshold
NRCELL
0..10000
5
5
rlpRecCsiBsiThreshold
NRCELL
0..10000
2
2
tRLFindForDU
NRBTS
300 ms to 3000 ms
300 ms (0)
3000 ms (10)
tWaitingRlRecover
NRBTS
0 ms to 60000 ms
500 ms (3)
500 ms (3)
dlMaxRetxThreshold
NRBTS
1..32
8
8
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Issue: The scheduler can reduce the PUSCH allocation to 3 PRB when UL radio link quality is bad to make the PUSCH link more robust
than the PUCCH format 2 radio link used for CSI
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Risk: that SgNB unnecessarily declares RLF due to missing PUCCH messages
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Recommendation: to disable RLF due to CSI or BSI missing reports by setting a high value, e.g. rlpDetCsiBsiThreshold =10000
NSA Retainability gNB Release Scenarios (gNB detected)
Release due to SgNB-detected 5G Radio Link Failure (RLF) (7/7)
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SgNB-detected radio link failures are not visible in drive test logs but in BTS PM counters or
BTS/interface traces
Counters related to SgNB-detected Radio Link Failure
Counter id
Counter name
Comments
M8080C45
SGNB_TOT_REL_INIT_SGNB
Total SgNB-initiated releases, eNB perspective
M55112C00007
X2_SGNB_REL_REQUIRED_SENT
Total SgNB-initiated releases, gNB perspective
M55112C00502
SGNB_REL_SN_REQ_UE_LOST
Includes both SgNB-detected and UE-detected RLFs
M55116C00014
RLF_INIT_SGNB_DLHARQ_OR_CSI
M55116C00015
RLF_INIT_SGNB_MAX_RLC_RETX
Counters for radio link failures detected by gNB. These
counters will only increment if the connection does not
recover and is released. Only incremented by NSA
traffic
NSA Retainability gNB Release Scenarios (gNB detected)
Release initiated by gNB due to abnormal reasons
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Abnormal reasons are a PDCP count rollover or a GTP-U error
However, the % of connections released by ‘abnormal’ reasons in real networks is very small (<0.01%) so
they tend to be ignored
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It has been observed that when DL user plane is only sent via 5G leg*, a failed 5G RACH procedure
increments the NX2CC_UE_RELEASE_ABNORMAL counter. If that is the case, abnormal releases are a big
part (5% - 10%) of the total releases although they should be considered as failed connection setups rather
than drop connections
*dlDataSplitMode = “dlOverF1U”
Counter id
Counter name
Comment
M8080C45
SGNB_TOT_REL_INIT_SGNB
Total releases, eNB perspective
M55112C00007
X2_SGNB_REL_REQUIRED_SENT
Total releases, gNB perspective
M55112C01001
NX2CC_UE_RELEASE_ABNORMAL
NSA Retainability gNB Release Scenarios (gNB detected)
Release initiated by gNB due to other reasons
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A considerable number of connections is released due to other reasons that are not captured
in PM counters
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Known release causes not captured explicitly in counters:
• Releases due to missing A3 handover
• Releases caused by a failed 5G signaling procedure (if SRB3 is enabled)
• Release due to transport network
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Other reasons can be investigated with X2 trace analysis
NSA Retainability gNB Release Scenarios (gNB detected)
Release initiated by gNB due to gNB reset
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When a gNB request a reset or partial reset of the X2 all the ongoing connections are also released
The impact of these releases in real networks is very small. They can be minimized by limiting site
resets to low traffic periods
Counter id
Counter name
M8080C51
SGNB_ABN_REL_INIT_SGNB_X2PR
M8080C89
SGNB_ABN_REL_INIT_MENB_X2FR
M8080C91
SGNB_ABN_REL_INIT_SGNB_X2FR
M55110C00019
X2_UEREL_INCOMING_RESET
M55110C00020
X2_UEREL_INCOMING_RESET
M55110C00027
X2_UEREL_INC_PARTIAL_RESET
M55110C00028
X2_UEREL_OUT_PARTIAL_RESET
Comment
Releases due to X2 reset, eNB perspective
Releases due to X2 reset, gNB perspective
NSA Retainability gNB Release Scenarios (gNB detected)
Releases due to UE take-back of 5G Connection
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Some of the early UE models may overheat if 5G data transfer is used for some time
UE may take back the 5G connection either due to overheat protection or due to battery
conservation
It is not possible to identify these cases from OSS counters
The eNB will forward this message to the gNB, which will release the 5G radio link and
increment counters, see above: RLF_INITIATED_UE_T310_EXPIRY or
RLF_INITIATED_UE_MAX_RLC_RETX and SGNB_REL_SN_REQ_UE_LOST
Index
NSA Retainability Introduction
NSA Retainability gNB Release Scenarios (gNB detected)
NSA Retainability gNB Release Scenarios (UE Detected)
NSA Retainability eNB Release Scenarios
NSA Retainability KPIs
SA Retainability (gNB and AMF Release Scenario)
SA Retainability (UE Detected Scenario)
Quiz/Exercise
NSA Retainability gNB Release Scenarios (UE Detected)
Release due to UE-detected 5G Radio Link Failure
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When UE detects the 5G radio link is broken it
informs the MeNB via SCF Failure Information
message
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MeNB passes the information to SgNB
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SgNB may try to recover the situation with a
PSCell Change
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If PSCell change is successful, the radio
connection is resumed
•
If PSCell is unsuccessful, the connection is
released, and the PM counters are incremented
Note: The counters for UE detected RLF are only incremented if there is actually a drop
NSA Retainability gNB Release Scenarios (UE Detected)
Release due to UE-detected 5G Radio Link Failure
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The reasons for UE initiated radio link failure:
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Random Access procedure failure
Downlink out-of-sync (OOS)
Radio Link Control ( RLC) failure when the maximum number of retransmissions is reached
Others: SCG change failure, SCG reconfiguration failure, SRB integrity failure
Certain UE models -due to overheat protection or due to battery- send RLF message possibly
causing the RLF_INITIATED_UE_T310_EXPIRY or the RLF_INITIATED_UE_MAX_RLC_RETX counter to
be incremented
NSA Retainability gNB Release Scenarios (UE Detected)
Cause: Random Access Procedure Failure (1/2)
Process:
• After receiving Msg. 1, gNB creates a temporary UE
context and schedules a grant for Msg2: Random
Access Response
• If the RAR is not received within a time window
(raResponseWindow + 3 slots), UE retransmits the
preamble (Msg1) ramping up the power by a value
specified in the RRC Connection Reconfiguration
message
• Msg. 1 retransmissions are repeated while the
number of preamble retransmissions
(preambleTransMax) is not reached
•
If this maximum is reached then the Random Access
Procedure is started again
RAR: Random Access Response
NSA Retainability gNB Release Scenarios (UE Detected)
Cause: Random Access Procedure Failure (2/2)
Process (cont.):
• After receiving the UL grant in the Msg2 RAR, UE sends the RACH
Msg3 with the C-RNTI and the MAC Contention Resolution Timer
starts to monitor the reception of the Contention Resolution
• After successful reception of RACH Msg3, gNB schedules the
Contention Resolution Grant for the UE using the C-RNTI
• If the reception is not OK, the RACH Msg3 is retransmitted and the
MAC Contention Resolution Timer restarted
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If max. number of RA Msg3 transmission ( maxHarqMsg3Tx) is
reached and still the CRC is not OK the UE sends again the
preamble (if preambleTransMax is not reached) otherwise, it
declares RLF
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If MAC Contention Resolution expired, the UE sends again Msg1
NSA Retainability gNB Release Scenarios (UE Detected)
Cause: Downlink out-of-sync
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The UE monitors the ‘out-of-sync- and ‘in-syncindications from Layer 1 (L1)
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Upon receiving N310 consecutive “out-of-sync”
indications, the UE starts timer T310
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T310 is stopped upon receiving N311 consecutive
‘in-sync’ indications and the connection continues
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If T310 expires, RLF is detected and
SCGFailureInformation is sent by UE to MeNB
NSA Retainability gNB Release Scenarios (UE Detected)
Cause: RLC Failure
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Triggered by the UE when the maximum
number of Radio Link Control (RLC)
Retransmissions is reached
•
Max. number of retransmissions is configured
with the ulMaxRetxThreshold parameter and
passed to the UE via
RRCConnectionReconfiguration message
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In case of RLC failure the UE reports the radio
link failure to the MeNB via
SCGFailureInformation message
NSA Retainability gNB Release Scenarios (UE Detected)
Causes: SCG and SRB
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SCG change failure: Reported if the UE cannot complete the
PSCell change
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SCG reconfiguration failure: if UE cannot comply with the
configuration included in the RRC Reconfiguration message
received over SRB3
•
SRB3 integrity failure: if the integrity check for SRB3 is not
passed
•
In all these cases, the UE initiates the SCG failure information
procedure to report the error towards the MeNB which
forwards the message to the SgNB
NSA Retainability gNB Release Scenarios (UE Detected)
Release due to UE-detected 5G RLF: PSCell Change Procedure
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The UE reports a radio link failure to the MeNB via
the SCGFailureInformation message
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The MeNB sends the information to the SgNB with a
SgNB Modification Request
Upon reception of the message the SgNB starts the
RL control timer tWaitingRlRecover
While the timer is running the UE data is switched to
the LTE leg
•
•
•
The SgNB sends in the SgNB Modification Request
Acknowledge message confirming that it is aware of
the UE detected RLF
NSA Retainability gNB Release Scenarios (UE Detected)
Release due to UE-detected 5G RLF - Parameters
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•
It is possible to control via parameterization how quickly the UE declares the radio link failure
Parameters to control UE detection of radio link failure:
Parameter
Object
Range
Default
Recommended
T310
NRBTS
0 ms to 6000 ms
2000 ms
2000 ms
N310
NRBTS
1 to 20
10
10
N311
NRBTS
1 to 10
1
1
preambleTransMax
NRCELL
3 to 200
n10 (6)
n10 (6)
ulMaxRetxThreshold
NRBTS
1 to 32
8
8
NSA Retainability gNB Release Scenarios (UE Detected)
Release due to UE-detected 5G RLF - Counters
•
UE detected radio link failures are easy to identify in drive test logs because the SCF Failure Information
message is available in the RRC signaling and the failure type within that message specifies the exact
failure cause
•
Counters related to UE detection of radio link failure:
Counter id
Counter name
Comment
M8080C45
SGNB_TOT_REL_INIT_SGNB
Total releases, eNB and gNB perspective
M55112C00007
X2_SGNB_REL_REQUIRED_SENT
Total releases, eNB and gNB perspective
M55112C00502
SGNB_REL_SN_REQ_UE_LOST
Includes both SgNB-detected and UE-detected RLFs
M55116C00008
RLF_INITIATED_UE_T310_EXPIRY
DL radio link failures, detected by UE
M55116C00010
RLF_INITIATED_UE_MAX_RLC_RETX
DL radio link failures, detected by UE
M55116C00011
RLF_INITIATED_UE_SRB_INTGRTY_F
DL radio link failures, detected by UE
M55116C00009
RLF_INITIATED_UE_RACH_FAIL
RACH failures detected by UE
M55116C00012
RLF_INITIATED_UE_SCG_CHGE_FAIL
RACH failures detected by UE
Index
NSA Retainability Introduction
NSA Retainability gNB Release Scenarios (gNB detected)
NSA Retainability gNB Release Scenarios (UE Detected)
NSA Retainability eNB Release Scenarios
NSA Retainability KPIs
SA Retainability (gNB and AMF Release Scenario)
SA Retainability (UE Detected Scenario)
Quiz/Exercise
NSA Retainability eNB Release Scenarios
Release initiated by eNB due to normal release
eNB normal releases cover
• normally ended calls
•
calls where the 4G and 5G inactivity counters are triggered
•
•
calls released when a VoLTE call starts
the UE detach procedure is also considered a normal release
Counter id
Counter name
Comment
M8080C47
SGNB_TOT_REL_INIT_MENB
Total releases, eNB perspective
M55112C00005
X2_SGNB_REL_REQ_RECEIVED
Total releases, gNB perspective
M8080C46
SGNB_NOR_REL_INIT_MENB_NAS
Normal eNB release
M8080C75
SGNB_REL_INIT_MENB_VOLTE
Releases due to VoLTE setup
M8080C137
SGNB_REL_INIT_MENB_UE_TRIGG
CB006908: UE triggered EN-DC take-back
NSA Retainability eNB Release Scenarios
Release initiated by eNB due to 4G RLF and due to 4G Handover
Due to 4G Radio Link Failure:
• When the eNB releases a 4G connection due to 4G Radio link failure, it also releases the 5G radio link
connection
•
•
There are no counters in the 5G BTS to differentiate between normal and abnormal eNB-initiated releases
Normal LTE Optimization procedures should be used to reduce these releases
Due to 4G handover:
• It is possible that inter-eNB handover of EN-DC connections is not allowed or because some EN-DC
neighbors have not been properly defined
•
In these scenarios whenever the eNB executes an inter-MeNB handover it also instructs the gNB to release
the connection
•
There are no specific counters in the 5G BTS to differentiate between normal and abnormal eNB-initiated
releases
NSA Retainability eNB Release Scenarios
Release initiated by eNB due to gNB/eNB reset
•
When an eNB or the gNB request a reset or partial reset of the X2 connection all the ongoing connections
are also released
•
The impact of these releases in real networks is very small
•
They can be minimized by limiting site resets to low traffic periods
Counter id
Counter name
Comment
M8080C51
SGNB_ABN_REL_INIT_SGNB_X2PR
Releases due to X2 reset, eNB perspective
M8080C89
SGNB_ABN_REL_INIT_MENB_X2FR
Releases due to X2 reset, eNB perspective
M8080C91
SGNB_ABN_REL_INIT_SGNB_X2FR
Releases due to X2 reset, eNB perspective
M55110C00019
X2_UEREL_INCOMING_RESET
Releases due to X2 reset, gNB perspective
M55110C00020
X2_UEREL_INCOMING_RESET
Releases due to X2 reset, gNB perspective
M55110C00027
X2_UEREL_INC_PARTIAL_RESET
Releases due to X2 reset, gNB perspective
M55110C00028
X2_UEREL_OUT_PARTIAL_RESET
Releases due to X2 reset, gNB perspective
Index
NSA Retainability Introduction
NSA Retainability gNB Release Scenarios (gNB detected)
NSA Retainability gNB Release Scenarios (UE Detected)
NSA Retainability eNB Release Scenarios
NSA Retainability KPIs
SA Retainability (gNB and AMF Release Scenario)
SA Retainability (UE Detected Scenario)
Quiz/Exercise
NSA Retainability KPIs
Introduction
•
Available counters do not allow for good, robust retainability KPI formulas
•
Two issues:
•
Difficulty in separating accessibility and retainability issues: From SgNB perspective, a connection is
considered established before the 5G RACH procedure has been completed making more difficult the
split between accessibility issues and retainability ones. Subtracting failed RACH events in the KPI
formulas cannot be done accurately
•
Missing dedicated counters for release reasons: Several release reasons do not have dedicated
counters. They increment generic release counters instead. It should be decided if those reasons should
be regarded as ‘normal’ or ‘abnormal’ releases
NSA Retainability KPIs
Retainability Metrics
•
Retainability has traditionally been calculated as either:
• Dropped Call Ratio (DCR): Number of dropped connections relative to all connections
•
Call Completion Ratio (CCR): Number of normally released connections relative to all connections
•
The connection duration has a big impact on the DCR as the longer the connection the more likely to drop
•
The connection duration depends on multiple factors:
•
•
Traffic pattern: FWA use cases have longer connection durations than smartphone use cases.
Inactivity timer settings: if it is long due to a requirement to limit the core network load the DCR will likely
increase
Solution:
To consider the number of drops relative to the connection duration (NR_1040a: drops per hours)
NSA Retainability KPIs
Main KPIs
•
Retainability KPIs need to be looked from the SgNB and the eNB perspective, same as the release scenarios.
•
From SgNB perspective the main retainability KPIs are:
•
•
•
NR_5032c: 5G SgNB triggered abnormal release ratio
NR_5036e: 5G NSA Number of UE releases due to radio link failure
•
NR_1040a: 5G SgNB triggered abnormal releases per connected hour
From eNB perspective the main retainability KPIs are:
•
•
LTE_6952e: E-UTRAN Normal Release Ratio per EN-DC UE Context
LTE_6953e: E-UTRAN Abnormal Release Ratio per EN-DC UE Context
NSA Retainability KPIs
SgNB perspective retainability KPIs - NR_5032c
NR_5032c: 5G SgNB triggered abnormal release ratio
SGNB_REL_SN_REQ_UE_LOST - RLF_INITIATED_UE_RACH_FAIL - RLF_INITIATED_UE_SCG_CHGE_FAIL + NX2CC_UE_RELEASE_ABNORMAL +
X2_UEREL_OUTGOING_RESET + X2_UEREL_OUT_PARTIAL_RESET - X2_SGNB_REL_CONF_T + X2_UEREL_INCOMING_RESET +
X2_UEREL_INC_PARTIAL_RESET
------------------------------------------------------------------------------------------------------------------------------------------X2_SGNB_REL_REQUIRED_SENT + X2_SGNB_REL_REQ_RECEIVED - RLF_INITIATED_UE_RACH_FAIL - RLF_INITIATED_UE_SCG_CHGE_FAIL +
X2_UEREL_OUTGOING_RESET + X2_UEREL_OUT_PARTIAL_RESET - X2_SGNB_REL_CONF_T + X2_UEREL_INCOMING_RESET +
X2_UEREL_INC_PARTIAL_RESET
•
The formula removes the RACH failures from both numerator and denominator. This means that accessibility problems
generally are not interfering with the KPI value. However, the RACH procedure is not only used for the initial access – also the
handover procedures and beam recovery procedures may increment the RACH counters which are subtracted. This means that
RACH failures which happen during the connection are removed, even though these may result in a dropped connection
•
All abnormal releases (NX2CC_UE_RELEASE_ABNORMAL) are considered drops
•
All MeNB-initiated releases are regarded as normal releases
•
All SgNB-initiated releases without a specific counter (releases due to missing A3 handover, SRB3 failures and possibly others)
are regarded as normal releases
•
A2 releases are regarded as normal releases
•
“Reset” releases can also be caused by accessibility issues
NSA Retainability KPIs
SgNB perspective retainability KPIs - NR_5036e
•
This KPI shows the number of UEs released due to radio link failure for NSA users
NR_5036e: 5G NSA Number of UE radio link failures =
RLF_INITIATED_UE_T310_EXPIRY + RLF_INITIATED_UE_RACH_FAIL + RLF_INITIATED_UE_MAX_RLC_RETX
+ RLF_INITIATED_UE_SRB_INTGRTY_F + RLF_INITIATED_UE_SCG_CHGE_FAIL +
RLF_INITIATED_UE_SCG_RECNF_F + RLF_INIT_SGNB_DLHARQ_OR_CSI +
RLF_INIT_SGNB_MAX_RLC_RETX
NSA Retainability KPIs
SgNB perspective retainability KPIs - NR_1040a
NR_1040a:
• More suitable for FWA use cases due to longer connection durations
NSA Retainability KPIs
eNB perspective retainability KPIs - LTE_6952e
LTE_6952e: E-UTRAN Normal Release Ratio per EN-DC UE Context =
SGNB_NOR_REL_INIT_SGNB_INACT + SGNB_NOR_REL_INIT_MENB_NAS + SGNB_REL_INIT_MENB_VOLTE + SGNB_REL_INIT_MENB_UE_TRIGG
-------------------------------------------------------------------------------------------------XX SGNB_TOT_REL_INIT_SGNB + SGNB_TOT_REL_INIT_MENB + SGNB_ABN_REL_INIT_SGNB_X2PR + SGNB_ABN_REL_INIT_SGNB_X2FR
+SGNB_ABN_REL_INIT_MENB_X2FR - SGNB_REL_TMENB_HO_X2_CNC -SGNB_REL_TMENB_HO_X2_PR_F
•
•
Only “Inactivity” is regarded as a normal 5G release
A2 releases and releases due to missing A3 handover are regarded as abnormal releases
•
Failed 5G RACH procedures are counted as abnormal releases
NSA Retainability KPIs
eNB perspective retainability KPIs - LTE_6953e
LTE_6953e: E-UTRAN Abnormal Release Ratio per EN-DC UE Context =
SGNB_TOT_REL_INIT_SGNB + SGNB_TOT_REL_INIT_MENB - SGNB_NOR_REL_INIT_SGNB_INACT - SGNB_NOR_REL_INIT_MENB_NAS SGNB_REL_INIT_MENB_VOLTE -SGNB_REL_INIT_MENB_UE_TRIGG + SGNB_ABN_REL_INIT_SGNB_X2PR + SGNB_ABN_REL_INIT_SGNB_X2FR +
SGNB_ABN_REL_INIT_MENB_X2FR - SGNB_REL_TMENB_HO_X2_CNC - SGNB_REL_TMENB_HO_X2_PR_F
-------------------------------------------------------------------------------------------------SGNB_TOT_REL_INIT_SGNB + SGNB_TOT_REL_INIT_MENB + SGNB_ABN_REL_INIT_SGNB_X2PR + SGNB_ABN_REL_INIT_SGNB_X2FR +
SGNB_ABN_REL_INIT_MENB_X2FR - SGNB_REL_TMENB_HO_X2_CNC -SGNB_REL_TMENB_HO_X2_PR_F
•
•
Only “Inactivity” is regarded as a normal 5G release
A2 releases and releases due to missing A3 handover are regarded as abnormal releases
•
Failed 5G RACH procedures are counted as abnormal releases
NSA Retainability KPIs
eNB perspective retainability KPIs - Example
Example of MeNB release reasons. Note
that for the MeNB-initiated releases
(M8080C47), very few ((2688 + 4335) /
380809 = 2%) are normal releases. This
is most likely due to many missing 4G –
5G neighbors
Index
NSA Retainability Introduction
NSA Retainability gNB Release Scenarios (gNB detected)
NSA Retainability gNB Release Scenarios (UE Detected)
NSA Retainability eNB Release Scenarios
NSA Retainability KPIs
SA Retainability (gNB and AMF Release Scenario)
SA Retainability (UE Detected Scenario)
Quiz/Exercise
SA Retainability (gNB and AMF Release Scenario)
Introduction
•
In SA deployments, the 5G radio connection can be released either by the gNB or by the AMF.
•
gNB release scenarios:
•
UE inactivity (this is viewed as normal release)
•
gNB-detected 5G radio link failure
•
UE-detected 5G radio link failure
•
Redirection (either due to A2 or due VoLTE call)
•
Possibly other scenarios
AMF release scenarios:
•
•
Detach
•
Completion of the signaling procedure
•
Inactivity
•
Some of the cases where the UE has detected RLF, do an RRC Re-establishment, and the removal of the old context is done. This context removal is
sometimes initiated by the AMF.
•
Successful inter-RAT handover
•
Release due to CN-detected mobility
•
Double N2 connections
•
Possibly other scenarios
SA Retainability (gNB and AMF Release Scenario)
Cause: UE inactivity (1/3)
•
The gNB treats the UE as inactive if there is no 5G data received or transmitted and if there is no data in the
buffer for a given period
•
When inactivity timer expires in the gNB, it sends the NGAP: UE Context Release Request to the AMF
•
AMF acknowledges this message with a NGAP: UE Context Release Command and proceeds to release the
connection.
•
Inactivity timer’s value is a compromise between:
• releasing the connection quickly -> using less resources from admission control perspective (mainly
PUCCH capacity), generating less uplink interference (if proactive UL scheduling is enabled) and using less
UE battery power
• keeping the connection longer and easing the load on the core network by avoiding the need to do a
connection establishment when more packets arrive
SA Retainability (gNB and AMF Release Scenario)
Cause: UE inactivity (2/3)
•
In 23R1, CB008120: Additional standard slices support allows to set different SA inactivity timers for each
slice and each DRB
•
SA inactivity timer value also impacts the value of many of the drop ratio formulas, similar to the NSA
case.
•
For end-user perspective it is irrelevant if the release of the idle connection is due to inactivity or to radio
link failure
Parameter
Object
Range
Default
Recommended
srbInactivityTimerSA
NRBTS
0..600 seconds
60 seconds
60 seconds
drbInactivityTimerSA
NRBTS
1..7200 seconds
10 seconds
10 seconds
drbInactivityTimerSA
NRBTS.NRDRB
1..604800 seconds
10 seconds
10 seconds
drbInactivityTimerSA
NRBTS.SNSSAI
1..604800 seconds
10 seconds
10 seconds
SA Retainability (gNB and AMF Release Scenario)
Cause: UE inactivity (3/3)
•
Counters related to 5G SA inactivity releases:
Counter id
Counter name
Comment
M55125C00501
NNGCC_UE_CTXT_REL_CMD_RECD
All UE context releases (gNB and AMF initiated)
M55125C08503
NG_FLOW_REL
All QoS flow releases (gNB and AMF initiated)
M55125C01001
NNGCC_UE_CTXT_REL_REQ_SENT
All gNB-initiated UE context releases
M55125C00503
NNGCC_UE_CTXT_REL_REQ_INACTIVE
Context releases due to inactivity
M55125C09004
NG_FLOW_REL_GNB_UE_INACTIVITY
QoS flow releases initiated by the gNB due to inactivity
M55125C08504
NG_FLOW_REL_NORMAL
All normal QoS flow releases, incl. inactivity
SA Retainability (gNB and AMF Release Scenario)
Cause: gNB detected RLF
Feature id
Feature name
5GC000731
Radio Link Failure (RLF) detection and handling at the gNB (SA mode)
Feature 5GC000731 introduces the radio link failure detection for RRC Connected 5G UEs operating
in SA mode
• The feature reuses the mechanisms of the counterpart feature for NSA mode (5GC000478) with
some adjustments needed for SA
• gNB continuously monitors the 5G radio link and detects RLF based on:
• DTX detection for requested DL HARQ feedback on PUCCH
• DTX detection of CSI reports on PUCCH
• Radio Link Control (RLC) failure – maximum number of retransmissions is reached
•
SA Retainability (gNB and AMF Release Scenario)
Cause: DTX detection of HARQ feedback and CSI reports (1/3)
Process:
• gNB counts the number of consecutive DTX detections
for requested DL HARQ feedback or CSI reports
• If the number of consecutive DTX HARQ or CSI exceeds a
threshold rlpDetDlHarqThreshold or rlpDtcCsiBsiThreshold
the gNB starts the RLF guard timer
• While the RLF guard timer (tRLFindForDU) is running,
HARQ feedback and CSI reports are monitored for
possible recoveries of the radio link
• If the number of consecutively received non DTX for DL
HARQ is greater than rlpRecDlHarqThreshold or
rlpRecCsiBsiThreshold the radio link is assumed to be
recovered
SA Retainability (gNB and AMF Release Scenario)
Cause: DTX detection of HARQ feedback and CSI reports (2/3)
Process (cont.):
• If RLF guard timer expires and the radio link is not
recovered, gNB-DU informs gNB-CU about RLF by
sending the F1: UE Context Modification Required
and RLF_INITIATED_RNL (M5516C00004) is
incremented
• gNB-CU suspends data transfer and starts the
tWaitingRlRecover timer
• While tWaitingRlRecover is running:
•
the UE context is not released in order to allow
the RRC Reestablishment
•
HO or measurement reports are not handled
SA Retainability (gNB and AMF Release Scenario)
Cause: DTX detection of HARQ feedback and CSI reports (3/3)
Process (cont.):
• If the tWaitingRlRecover expires the gNB-CU sends to
the AMF the UE Context Release Request and the AMF
replies with the UE Context Release Command to
inform the gNB-CU to release the UE context also in
the gNB-DU.
• If the SRB is still established then an RRC Release
message is sent to the UE.
• Finally, the AMF is informed of the completion of the
UE Context Release.
SA Retainability (gNB and AMF Release Scenario)
Cause: RLC Failure
Process:
• gNB triggers RLC failure when the max. number of RLC
Retransmissions is reached
• Max. number of retransmissions is defined with
dlMaxRetxThreshold/ulMaxRetxThreshold
• In case of RLC failure, gNB-DU informs gNB-CU using
the UE Context Modification Required message and
N1CC_UECTXT_MOD_REQRECD (M55113C01006) is
incremented
• The CU suspends the data transfer, confirms the UE
Context modification to the DU and starts the
tWaitingRlRecover timer
• When the timer expires its context is released
SA Retainability (gNB and AMF Release Scenario)
gNB detected RLF
•
Counters related to gNB-detected radio link failure
Counter id
Counter name
Comment
M55125C00501
M55125C01001
M55129C00003
M55129C00004
M55129C00005
M55125C09003
M55125C09005
NNGCC_UE_CTXT_REL_CMD_RECD
NNGCC_UE_CTXT_REL_REQ_SENT
RLF_NG_UE_CTX_REL_REQ_RLF_OTH
RLF_NG_UE_CTX_REL_REQ_RLCRETX
RLF_F1_UE_CONTEXT_REL_CMD
NG_FLOW_REL_GNB_RLF
NG_FLOW_REL_GNB_UE_LOST
All UE context releases (gNB and AMF initiated)
All gNB-initiated UE context releases
Counts releases due to RLF (due to HARQ, CSI or other reasons)
Counts releases due to RLF (due to Max RLC retrans exceeded)
All releases due to gNB-detected RLF
QoS flow releases, see text below*
QoS flow releases, see text below**
* Before PR644597: Improper NGAP cause against gNB initiated RLF for a SA UE was corrected, a radio link failure detected by the
gNB would be signalled to the AMF by sending a release request with radio network layer cause “Unspecified”, and the
NG_FLOW_REL_GNB_RLF would be incremented.
** With the PR644597 correction (which happened in one of the early 22R2 packages), the radio network layer cause is set to
“Radio connection with UE lost” and NG_FLOW_REL_GNB_UE_LOST will be incremented.
SA Retainability (gNB and AMF Release Scenario)
Redirection due to A2 event or VoLTE call
•
5GC000708: Coverage based blind redirect (SA option 2) enables blind redirection to either another 5G
frequency or to an LTE frequency
•
Also used to direct the UE to LTE when a VoLTE call is started (5GC000749: Voice fallback to LTE Redirection based)
There are two root causes for redirection:
•
•
1.
5G radio coverage: Either seen as potential problem or considered as normal, in cases where the LTE
layer has better coverage than 5G
2.
Voice call: UE and/or gNB does not support VoNR. This is not a problem
There are specific counters for the second case but not for the first case. To know the redirections due to A2
events, it is therefore needed to subtract the VoLTE redirections from the total number of redirections
SA Retainability (gNB and AMF Release Scenario)
gNB initiated Release
•
Counters related to redirection
Counter id
Counter name
Comment
M55125C00501
NNGCC_UE_CTXT_REL_CMD_RECD
All UE context releases (gNB and AMF initiated)
M55125C01001
NNGCC_UE_CTXT_REL_REQ_SENT
All gNB-initiated UE context releases
M55124C01501
NRRCC_RRC_RELEASE_REDIR_NR
Number of RRC Releases with NR frequency redirection information
M55124C01502
NRRCC_RRC_RELEASE_REDIR_LTE
Number of RRC Releases with LTE frequency redirection information
M55124C02501
NR_RRC_RELEASE_REDIR_VFB_LTE
Voice fallback to LTE
M55124C04002
NR_RRC_RELEASE_REDIR_EC_LTE
Emergency call fallback to LTE
M55124C03501
NR_RRC_RELEASE_REDIR_EF_LTE
Emergency call fallback to LTE
M55125C09002
NG_FLOW_REL_GNB_REDIRECT
QoS flows re-directed to LTE layer. Includes the voice fallback cases as well
as the A2 events
SA Retainability (gNB and AMF Release Scenario)
Releases due to Xn or NG Interface Reset
•
When a UE context is released due to a reset of the Xn or the NG interface, the counters in the below table
are incremented.
Counter id
Counter name
NetAct name
M55126C00018
Number of UE release due to incoming XN Reset
XN_UEREL_INCOMING_RESET
M55126C00019
Number of UE release due to outgoing XN Reset
XN_UEREL_OUTGOING_RESET
M55125C09001
Number of QoS Flows released due to NG interface Reset initiated by gNB
NG_FLOW_REL_GNB_NG_RESET
M55125C09017
Number of QoS Flows released due to NG interface Reset initiated by AMF
NG_FLOW_REL_NG_RESET_AMF
SA Retainability (gNB and AMF Release Scenario)
AMF-initiated Releases – Normal Cases (1/2)
•
•
•
•
The AMF-initiated releases which are regarded as normal include these release causes:
Radio Network Layer:
o
User inactivity
o
Redirection
o
IMS voice EPS fallback or RAT fallback triggered
o
Release due to CN-detected mobility"
o
Successful handover (only inter-RAT HO. Intra-NR HO cases not counted)
NAS
o
Normal release
o
Deregister
Miscellaneous
o
O&M intervention
SA Retainability (gNB and AMF Release Scenario)
AMF-initiated Releases – Normal Cases (2/2)
•
Counters related to AMF-initiated normal releases
Counter id
Counter name
Comments
M55125C00501
M55125C01001
NNGCC_UE_CTXT_REL_CMD_RECD
NNGCC_UE_CTXT_REL_REQ_SENT
M55125C08503
M55125C08504
M55125C09019
NG_FLOW_REL
NG_FLOW_REL_NORMAL
NG_FLOW_REL_AMF_NORMAL
Includes both AMF-initiated and gNB-initiated releases and all causes
gNB-initiated releases. Subtract this from the above counter to get AMFinitiated releases. Includes all causes
All QoS flow releases: AMF-initiated and gNB-initiated. Normal and abnormal
All normal QoS flow releases, AMF-initiated and gNB-initiated
All AMF-initiated QoS flow releases
SA Retainability (gNB and AMF Release Scenario)
AMF-initiated Releases – Abnormal Cases
•
The only specific abnormal release cause which is captured by a counter is the "Radio connection with UE
lost".
•
The rest of the AMF-initiated abnormal release causes are captured by the bucket counter “Other”.
•
Counters related to AMF-initiated releases due to abnormal cause
Counter id
Counter name
Comments
M55125C00501
M55125C01001
NNGCC_UE_CTXT_REL_CMD_RECD
NNGCC_UE_CTXT_REL_REQ_SENT
M55125C08503
M55125C09018
M55125C09020
NG_FLOW_REL
NG_FLOW_REL_AMF_UE_LOST
NG_FLOW_REL_AMF_OTHER
Includes both AMF-initiated and gNB-initiated releases and all causes
gNB-initiated releases. Subtract this from the above counter to get AMFinitiated releases. Includes all causes
All QoS flow releases: AMF-initiated and gNB-initiated. Normal and abnormal
AMF-initiated QoS flow releases due to "Radio connection with UE lost"
AMF-initiated QoS flow releases with other release causes than those
considered in the “Normal” counter” and in the “UE lost” counter
SA Retainability (gNB and AMF Release Scenario)
Performance Indicators
KPI id
KPI name
KPI formular
NR_5148d
5G SA Normal gNB-initiated release
ratio
NR_5356a
5G SA QoS Flow Drop Ratio - User
view
5G SA QoS Flow Drop Ratio - RAN
view
(NRRCC_RRC_RELEASE_REDIR_NR +NRRCC_RRC_RELEASE_REDIR_LTE +
NNGCC_UE_CTXT_REL_REQ_INACTIVE + SA_UE_CTX_REL_PREEM +
SA_UE_CTX_REL_UAI / NNGCC_UE_CTXT_REL_REQ_SENT
(NG_FLOW_REL - NG_FLOW_REL_NORMAL) / NG_FLOW_REL
NR_5358b
NR_5154a
5G SA QoS Flow Drop Ratio per cell
(NG_FLOW_REL_GNB_NG_RESET + NG_FLOW_REL_GNB_RLF +
NG_FLOW_REL_GNB_UE_LOST + NG_FLOW_REL_GNB_UNSPECIFIED +
NG_FLOW_REL_GNB_TRS + NG_FLOW_REL_PART_HO) / NG_FLOW_REL
NG_FLOW_REL - NG_FLOW_REL_NORMAL) / (NG_FLOW_REL +
NG_FLOW_REL_IANR_SUC_HO)
Index
NSA Retainability Introduction
NSA Retainability gNB Release Scenarios (gNB detected)
NSA Retainability gNB Release Scenarios (UE Detected)
NSA Retainability eNB Release Scenarios
NSA Retainability KPIs
SA Retainability (gNB and AMF Release Scenario)
SA Retainability (UE Detected Scenario)
Quiz/Exercise
SA Retainability (UE Detected Scenario)
gNB initiated Release - Cause: UE detected RLF
•
UEs can detect RLF based on parameters acquired from SIB1 (feature 5GC000742 System Information
Broadcast- intra-NR)
•
UE can detect a RLF upon detection of:
•
1.
DL out-of-sync: UE monitors the “out-of-sync” and “in-sync” indications from layer 1 and uses T310,
N310 and N311 parameters to detect radio link failures
2.
3.
Random access procedure failure using the rach-ConfigCommon IE
RLC failure: If the maximum number of RLC retransmissions is reached, a RLF is detected
When UE-detected RLF occurs, the UE will initiate RRC Re-establishment procedure to continue the RRC
connection.
SA Retainability (UE Detected Scenario)
Cause: Downlink out-of-sync
•
The UE monitors the ‘out-of-sync- and ‘in-syncindications from Layer 1 (L1)
•
Upon receiving N310 consecutive
indications, the UE starts timer T310
•
T310 is stopped upon receiving N311 consecutive ‘insync’ indications and the connection continues
•
If T310 expires, RLF is detected
“out-of-sync”
SA Retainability (UE Detected Scenario)
Cause: RLC Failure
•
•
Triggered by the UE when the maximum number of Radio Link Control (RLC) Retransmissions is reached
Max. number of retransmissions is configured with the dlMaxRetxThreshold and ulMaxRetxThreshold
parameters and passed to the UE via RRCConnectionReconfiguration message
Note the new parameters in case of SA,
dlMaxRetxThreshold and
ulMaxRetxThreshold , instead of
maxRetxThreshold for NSA
SA Retainability (UE Detected Scenario)
Cause: Random Access Procedure Failure (1/2)
Process:
•
After receiving Msg. 1, gNB creates a temporary UE
context and schedules a grant for Msg2: Random
Access Response
•
If the RAR is not received within a time window
(raResponseWindow + 3 slots), UE retransmits the
preamble (Msg1) ramping up the power by a value
specified in the RRC Connection Reconfiguration
message
•
Msg. 1 retransmissions are repeated while the
number of preamble retransmissions
(preambleTransMax) is not reached
•
If this maximum is reached, then the Random
Access Procedure is started again
RAR: Random Access Response
SA Retainability (UE Detected Scenario)
Cause: Random Access Procedure Failure (2/2)
Process (cont.):
•
After receiving the UL grant in the Msg2 RAR, UE sends the RACH
Msg3 with the C-RNTI and the MAC Contention Resolution Timer
starts to monitor the reception of the Contention Resolution
•
After successful reception of RACH Msg3, gNB schedules the
Contention Resolution Grant for the UE using the C-RNTI
•
If the reception is not OK, the RACH Msg3 is retransmitted, and the
MAC Contention Resolution Timer restarted
•
If max. number of RA Msg3 transmission (maxHarqMsg3Tx) is reached
and still the CRC is not OK the UE sends again the preamble (if
preambleTransMax is not reached) otherwise, it declares RLF
•
If MAC Contention Resolution expired, the UE sends again Msg1
SA Retainability (UE Detected Scenario)
UE-detected 5G RLF
•
In all three cases when UE-detected RLF occurs, the UE will initiate the RRC Re-establishment procedure to
continue the RRC Connection
•
With 5GC002267: Early introduction of RRC Connection reestablishment via RRC Setup, the gNB initiates the
RRC Setup independently if a valid UE context is available or not
When UE starts the RRC Request procedure, it starts a Random Access Procedure, the gNB treats the UE as
a new UE and proceeds to establish the RRC connection
Once the new context is established, the old context in the gNB is released, either initiated by the gNB or by
the AMF
•
•
SA Retainability (UE Detected Scenario)
Counters: UE-detected 5G RLF
•
Counters related to UE-detected 5G radio link failures leading to RRC re-establishments:
Counter id
Counter name
Comments
M55125C00501
M5512400505
NNGCC_UE_CTXT_REL_CMD_RECD
RRC_REESTAB_OTHER_FAILURE
M55124C00502
RRC_REESTAB_FALLBACK
M55124C00512
M55124C00515
M55125C09005
M55125C09019
REESTAB_ACC_OTHER_FAIL
REESTAB_ACC_FALLBACK
NG_FLOW_REL_GNB_UE_LOST
NG_FLOW_REL_AMF_NORMAL
All UE context releases (gNB and AMF initiated). Incremented in “old” cell
If reestablishmentCause IE is “otherFailure”- this root cause most likely will be used
by the UE when sending RRC Reestablishment Request due to detected RLF.
Incremented in “new” cell
RRC Setup messages in response to re-establishment requests. Include all causes
for re-establishment, not only RLF. Incremented in “new” cell
Successful re-establishments
M55125C08504
NG_FLOW_REL_NORMAL
M55125C09018
NG_FLOW_REL_AMF_UE_LOST
Incremented in “old” cell if re-establishment fails
Expected to be incremented in “old” cell in case of successful inter-gNB reestablishment
Expected to be incremented in “old” cell in case of successful inter-gNB reestablishment
Hypothetically, this can be incremented in “old” cell in case of successful intergNB re-establishment if AMF does not follow 3GPP specification