1.oeo000030 lte access fault diagnsis issue1

LTE Access Fault Diagnosis

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� After UE select a suitable cell, it initials attach flow, including the following procedures:


� Random access

� RRC connection setup (RRC cause: MO signaling)

� NAS authentication procedure

� E-RAB setup (Activate default EPS bearer)

� Setup dedicated bearer (Optional, only for some Smart UE that support VoIP)

� Above figure shows the detailed procedure


� If UE request service again while in idle status, it initials “Service Request” procedure


� The whole procedure are very similar as initial attach procedure except:

� The cause of RRC connection is “MO Data”

� No authentication and UE capability query procedure as UE context already exists

in MME


� TAU procedure is a UE idle behavior, ensure that UE location is known by MME. The


scenarios of TAU can be:

� UE initial attach

� UE move to a new TA area

� Periodic TA update


� There are five RA preamble formats, which are used for cells of different radii. LTE FDD


supports preamble formats 0-3, and LTE TDD supports preamble formats 0-4. The

preamble format can be set through the PreambleFmt parameter, and the cell radius can

be set through the CellRadius parameter.

� Format 4 is a special RACH preamble for LTE-TDD only.


Preamble Format

Length TCP TSEQ FDD Cell Radius (R) TDD Cell Radius (R)

0 1ms 103.1 µs 800 µs R ≤ 14.5km 1.4 km < R ≤ 14.5 km

1 2ms 684.4 µs 800 µs 29.5 km < R ≤ 77.3 km 29 .5km < R ≤ 77.3 km

2 2ms 203.1 µs1,600 µs

14 .5km < R ≤ 29.5 km 14.5 km < R ≤ 29.5 km

3 3ms 684.4 µs1,600 µs

77 .3km < R ≤ 100 km 77 .3km < R ≤ 100 km

4167.9 µs

14.58 µs133.33 µs

Not involved. R ≤ 1.4 km

� Upon receiving the UE preamble, the eNodeB transmits an RA response over DL-SCH.


� The UE monitors the Physical Dedicated Control Channel (PDCCH) in the Transmission Time

Interval (TTI) until it obtains the required RA response.

� The response contains RA-Preamble Identifier, Timing Alignment Information, Initial UL

Grant, and Temporary C-RNTI.

� If multiple-UE send the preamble with the same RA-RNTI, then eNodeB detects that the

conflict occurs. This conflict will be handled in contention resolution procedure.

� A message on the DL-SCH can carry multiple RA responses to be transmitted to multiple

use.

� For the UE ;

� If the received RA-Preamble Identifier is consistent with the identifier that the UE

previously sent, the UE infers that the response is successful. Then, the UE transmits

uplink scheduled data.

� Otherwise, the response fails. In this case, if the number of random access

attempts is smaller than the maximum, the UE attempts random access again.

Otherwise, random access fails.


� With the grant ,the UE transmits uplink scheduled data over the UL-SCH. This message is


scrambled by temporary C-RNTI. In this message, UE will carry its unique ID for contention

resolution if random access conflict.

� Based on different random access scenarios, the message could be

� RRC connection request

� RRC reestablishment request

� RRC reconfiguration complete (Handover confirmation)

� Other user data

� In case of “RRC connection request” message, UE sends its UE ID to identify the different

UE. This UE ID could be:

� If UE already get S-TMIS from MME, then use S-TMIS as UE ID

� If UE initial attach, UE generate a random number according to its IMSI as UE ID

� In the other case, UE at least send its C-RNTI to identify the different UE


� After the UE sends message 3,the contention resolution timer starts. The UE monitors the


PDCCH before the timer expires. In the following situations, the UE infers that the

contention resolution is successful and it notifies the upper layer and stops the :

� The UE obtains the C-RNTI when monitoring the PDCCH.

� Temporary C-RNTI is obtained when the UE monitors the PDCCH. In addition, the

MAC Packet Data Unit (MAC PDU) is successfully decoded.

� If the contention resolution timer expires, the UE infers that the contention resolution fails.

Then, the UE performs RA again if the number of RA attempts is smaller than the

maximum number of attempts.


� Purpose of RRC connection


� Setup SRB1

� Send NAS message to EPC

� Relevant counters:

� A: RRC connection attempts (L.RRC.ConnReq.Att)

� B: RRC connection setup successful (L.RRC.ConnReq.Succ)


� After UE succeed attach to MME, MME initial the ERAB setup procedure, to setup


dedicated S1 control plane for UE. At the same time, the default PDN connectivity and EPS

bearer will also be setup. After ERAB setup, there is already IP connectivity between UE

and gateway which is called “Always on line” function.

� Relevant counters:

� A: ERAB setup attempts (L.E-RAB.InitAttEst)

� B: ERAB setup successful (L.E-RAB.InitSuccEst)


� Trace of UE capability



� After SRB2 setup, all the NAS message on UU interface in carried by SRB2 with the


message RRC_DL_INFO_TRANSF or RRC_UL_INFO_TRANSF


� NAS procedure is coming after RRC complete, including


� Attach request (default)

� Default EPS bearer activity (default)

� Dedicate EPS bearer setup (optional)

� Authentication (optional)

� Identification (optional)

� NAS procedure is associated with ERAB setup procedure, for NAS delivery, on UU interface

NAS can be combined in common RRC message as NAS PDU or directly transferred as

“RRC_DL/UL_INFO_TRANSF” message; on S1 interface, NAS can be combined in common

S1AP message as NAS PDU or directly transferred as “S1AP_DL/UL_NAS_TRANS” message


� The barred cell only impacts on initial UE access, handover in is not affected.



� This failure is caused by eNodeB admission failure. If PUCCH or SRS resource is limited,


eNodeB should stop admit all services including RRC.


� Relevant RRC failure counter:



Counter name Description

L.RRC.SetupFail.ResFail Number of RRC connection setup failures due to

resource allocation failures

L.RRC.SetupFail.NoReply Number of RRC connection setup failures due to no

responses from the UE

L.RRC.SetupFail.Rej Number of RRC Connection Reject messages sent to

the UE in a cell

� This NAS procedure refers to the procedure between UE initial message and Initial UE


context setup response message, including attach, authentication, identification, default

bearer activity.

� If NAS procedure is failure, then MME will send a UE context release command, and the

release cause is NAS, so eNodeB will consider this release is normal release and doesn’t

count it in performance static.

� Possible scenarios:

� Authentication failure: Incorrect subscriptions user data in HSS

� Solution: Check subscription data in HSS

� Attach failure: Parameters between UE and EPC is not identical

� Solution: Check the if the following parameters are identical with EPC

1. PLMN

2. TAC

� PDN activity failure: Wrong configuration of APN in UE or EPC


� This failure refers the fault between “Initial context setup request” message and “Initial


context setup response” message. If any step occur fault, then ERAB setup procedure fails.


� Scneario: the L.E-RAB.FailEst.NoReply counter is incremented each time the eNodeB


receives no responses from the UE before the corresponding timer expires after sending an

RRC signaling message to the UE during the initial UE context setup procedure or the E-

RAB setup procedure

� Possible cause:

� Poor DL/UL coverage which cause message missing.

� UE internal process error


� Tracing message



� Message tracing



Cautioned WordsCautioned WordsCautioned WordsCautioned Words

1Tracing ( Signaling/ Message/Interface etc)

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1.oeo000030 lte access fault diagnsis issue1

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