FAILURES AFTER ADMISSION ON ERICSSON UMTS SYSTEM
Juan Berrizbeitia and others (see references)
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Contents
Background information Possible causes for failures after admission Troubleshooting aids
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Background information: pmNoFailedAfterAdm counter
Description :– Number of Radio Resource Control (RRC) or Radio Access
Bearer (RAB) establishment requests failed after being admitted by admission control.
Condition :– If either a RRC or RAB establishment procedure fails
AFTER the admission has been granted for the establishment (RRC or RAB), this counter will be incremented at the cell or cells where the UE is located.
– The counter is not pegged for D-RNC cells.– This counter will increment on both Circuit switched failures
and Packet Switched failures.
MO Class :– Utran Cell.
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Background information: pmNoFailedAfterAdm at Admission control counter flowchart
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Background information: When is this counter pegged?
When a new resource request is made, Admission control checks DL power, ASE, Codes ,etc.- to determine if the new resource request will be granted.
It then adds the new load to those monitors (as they need to be included in the next resource request processing).
If for some reason, the resource request fails then Admission Control must remove those loads (associated with the request) from the load monitors. It is at this time that pmFailedAfterAdm is incremented.
It is important to mention that transmission resources are not checked at admission control, therefore there is a high probability of having blocking due to lack of transmission resources.
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RRC/RAB Failures
This counter can include failures for reasons that are Transport Network (TN) related as well as not TN-related.
RAB and RRC establishment failures which occur after admission control can be due to
– timeout in the UE, RNC or RBS.– invalid parameter settings.– TN failure reasons. – If the connection is suddenly lost with a RAB you will get a
system RAB release – If it disconnects with RRC you may get an abnormal
Disconnect message which is failure to setup a call.
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This counter will be increment on all cells for a failure on all Radio links that have had admission granted. So, when we have soft/softer handover and 3 RL in the AS are granted admission, then
– RAB failure will be pegged on one link (serving cell) &– pmNoFailedAfterAdm will be pegged on all 3 cells.
pmNoFailedAfterAdm at RAB setup
RAB set-up attempt is on Cell-A, but no success, then : -pmNoFailedAfterAdm will be incremented on all three cells( A, B& C) in active set. But, -RAB failure will only increment in the best cell in the active set (Cell-A)
(pegged)
SoHo
SoHo
pmNoFailedAfterAdm
(pegged)pmNoFailedAfterAdm
(pegged)pmNoFailedAfterAdm
(pegged)pmNoRabEstablishAttempt
(Serving Cell)
CELL A
CELL B
CELL C
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General causes for failures after admission
The counter pmNoFailedAfterAdm is pegged for the reasons stated below :
1. NBAP RL setup failure (RAX or TXB congestion)2. AAL2 setup failure (due to congestion or miss
configuration)3. Channel Code Allocation Failure (code not available)
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Failure After Admission: Iub Congestion
Possible solutions for Iub congestion: Enable Directed retry (short term solution) Correct possible AAL2 miss configuration at Node B, RNC and RXI or MSN (AAL2
profile must match in all three entities) Order new T1s (long term solution) Change AAL2 QoS configuration depending on services request volume (CS voice,
R99 data, HSDPA, etc)
Iub congestion is a common reason for high number of failures after admission events
Depending on the volume of traffic per service or RAB, certain QoS could be congested at the AAL2. All RABs excluding HSDPA are configured tu use Class A or Class B being these two mostly impacted by congestion
Most of the time Class B is the first QoS to get congested leading to failures after admission events
Iub congestion could be caused by T1 issues Missconfigurations of AAL2 profile at the Node B, RNC or RXI/MSN can lead to Iub
congestion
Background information:
Possible indicators: Check for congestion at the Iub link Check for T1 issues and history of alarms of T1s (for intermittent T1 alarms). Ask
Utran help to check for T1 issues and alarms.
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Failure After Admission: Core Transport Network Congestion
Possible solutions: This issue will require support from the client in order to determine high utilisation
over Iu-cs and Iu-ps links
Accessibility issues are observed on all sites at the RNC without major issues with Iub congestion (especially at peak hour) Congestion will be observed at Iu-cs (RNC<-->MGW) or Iu-ps (RNC<-->SGSN) links
Possible indicators: Signalling bearer rab (SRB) issues for either CS or PS would increment on these
cases. SRB failures for CS: 1-(pmNoIuSigEstablishSuccessCs / pmNoIuSigEstablishAttemptCs). SRB failures for PS: 1-(pmNoIuSigEstablishSuccessPs / pmNoIuSigEstablishAttemptPs)
Congestion will be observed at Aal2 access point (Aal2Ap) entity starting with “g” (note: Aal2Ap entities starting with “b” correspond to node b, starting with “r” corresponds for Iur links,etc)
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Failure After Admission: Hardware Usage (Channel Elements)
Possible solutions: Check that ulHwAdm and dlHwAdm parameters are set to 100% Check numHsResources, numEulResources at the node B (long term solution) Reduce value of sf8Adm to 0 (short term solution) Order new RAX or TXB depending on the case Restart Node B and monitor performance if no obvious issues are found
Note: It is possible for lack of channel elements counters to increment even when there should be sufficient channel element capacity (for example due to a software bug in the software revision) so it is important to compare the channel element usage to the channel element capacity of the RBS to make sure that it makes sense for this to be the root of the problem
Lack of channel elements could be due to insufficient UL (RAX board) or DL (TXB board) hardware capacity Channel element capacity could be also software limited (software licenses configured at the RBS) Admission control is restricted by ulHwAdm and dlHwAdm parameters. They should be set at 100% so no hardware is limited for RRC/RAB setup.
Possible indicators: High number of events on LackDlChnlCode counter for lack of channel codes Congestion at RAX or TXB could be indicated by RBS counters (RBS universe in BO)
pmUlCredits and pmDlCredits respectively. Congestion per spreading factor (SF) can be also measured using pmSetupFailureSfXX RBS
counters from the BasebandPool (BBP) on the uplink (UL BBP) and the downlink (DL BBP).
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Failure After Admission: Other
Possible solutions: Check for hardware failures (for example, failures on RAX and TXB boards) Check for neighboring sites. Remember that neighboring sites having AAL2
congestion can cause other cells to have high number of failures after admission (due to soft handoff)
Check that software revisions are up to date at the Node B Restart Node B and monitor performance if no obvious issues are found
If a site shows none of the previously described issues, then it is likely to be a more complicated problem to resolve; often relating to a software/hardware fault, or perhaps an external source of interference in the area.
Possible indicators: High number of events on LackDlHw would indicate issues
with TXB board and LackUlHw would indicate issues with RAX board.
Congestion at RAX or TXB could be indicated by RBS counters (RBS universe in BO) pmUlCredits and pmDlCredits respectively
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Flowchart with proposed procedure
High number of pmNoFailedAfterAdm events
Iub congestion observed at
the site?
Double check presence of T1
issues with UTRAN
Address this issue with the customer
T1 issues at the site?
Lack of channel
elements at the site?
Other possible solutions: (when no T1 issues are present at the site)• Enable directed retry• Propose customer to consider changing AAl2 Qos configuration
Check with UTRAN for alarms or hardware
failures, specially at the TXB or RAX board
Alarm or hardware
failures at the TXB or RAX
board?
Re-start TXB or RAX board or
restart the site if issues not at the
boards
Channel Elements shortage
confirmed by Utran?
Recommend ordering new RAX
or TXB borads
Alarms present at the site?
Check if this alarms could be affecting accessibility at your
site and address alarm to the client
Check with the customer for
possible shortages at Iucs or Iups.
Note: shortages at Iucs and Iups would be affecting cs or ps accessibility on all sites at the RNC
Other possible solutions:• Reduce value of sf8Adm to 0
Shortages of Iucs or
Iups?
Note: channel element utilization must be greater than actual channel element capacity before recommending to order new hardware
Yes
Yes
Yes
Yes
Yes
Address this issue with the customer
YesNo
No
No
No
No
No
Yes
Check for neighboring sites, specially those having a high number of pmNoRabEstablishAttempt
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Summary Proposed actions
Try to see if you have any HW alarms or issues at the site. Check AAL2/transmission issues:
– T1 issues– AAL2 missconfiguration at the Node B, RNC or RXI / MSN
Check for Iub congestion at problem site and sites around. Check for lack of channel elements or congestion at RAX or TXB
boards. If UTRAN support is provided, confirm T1 issues and alarms with
them (In case that counters may not be pegging properly)
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Case study: Low PS accessibility on TXU04163
• Site TXU0416 had low PS accessibility on sector 3. Site had no alarms, no T1 issues and no Iub congestion. Only failures after admission are seen as explanation on low PS accessibility
• Neighbor site TXU0409 had T1 issues and packet drops on the same time frame possibly explaining low performance on TXU0416
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References
“pmNoFailedAfterAdmDescription and Detail”: Arun Verma, Soontorn Banchongsirichareon, Jian Shiou Yong
“3G Cell Optimization”: unknown author Counters Flowchart from Alex LNA 108 2040 R5B, WCDMA RAN Performance Management Workshop
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