lte idle mode

AT&T Best Practices: LTE Performance & Optimization

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Idle Mode Behavior

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Do not add objects or Ericsson Confidential | 2011-01-14 | Page 2

Abstract

This document discusses Idle Mode Support which is part of LTE Basic.

In idle mode the User Equipment (UE) has no active connection to the Radio Access Network (RAN). However it enables UE to access the network and be

reached from the network.

Keeping the UE in idle mode minimizes radio resource use in both the UE and the network and extends UE battery life.

Version Date Update

v1.0 June 20, 2011 Idle Mode Behavior

v1.1 July 30, 2011 Addition: UMTS to LTE Reselection

v1.2 Oct 04, 2011 Addition: Additional info on channels & parameters

Revision History

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Document has been divided into following sections:

Introduction: UE states & Channels

Initial System Acquisition

MIBs/ SIBs and UE categories

Cell Selection Criteria

Reselection Criteria

Appendix: MIB and SIB format

Idle Mode:

Appendix: UMTS to LTE Reselection

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Idle Mode - Introduction

UE states transitions:

EMM Registered

RRC Idle

TA List

Cell Reselection

Stand-by

Registration State

RRC State

UE Location

Mobility

Data Exchange

EMM* De-registered

RRC Idle

Unknown

PLMN/ cell selection

No data

EMM Registered

RRC Connected

E-Node B

Handover

Data ON

Power on Active UL/DL data Idle

Registration & Bearer Setup Data Inactivity

Data ActivityPower Off

* EMM (Evolved Mobility Management) is a NAS level protocol and is used for UE authentication

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Idle Mode - Introduction

Cell Selection Reselection Flow By UE In Idle Mode*

* This is a simplified version. Complete diagram can be found in 3GPP TS 36.304.

PLMN Selected

Cell Selection

Camped Normally

Any Cell Selection

Camped on any cell Cell Reselection

Evaluation Process

Cell Reselection

Evaluation ProcessConnected Mode

Return to idle mode

Suitable cell found

Leave idle

mode

Suitable cell

found

No Suitable cell found

No Suitable cell found

No acceptable cell found

Acceptable cell found

Acceptable

cell found

Trigger

Trigger

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PCCH BCCH CCH DCCH DTCH MCCH MTCH

PCFICH PBCH PHICHSCH PDSCH PDCCH PMCHDL-RS

PCH BCH DL-SCH MCH

CFI HI DCI

RLC Layer

MAC Layer

Physical Layer

Logical Channels

Transport Channels

Physical Channels

Highlighted physical channels (DL-RS, SCH, PBCH, PDSCH) are important to

understand system information acquisition by UE in Idle mode.

Idle Mode - Channels

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There are 3 types of DL Reference Signals:

Cell specific reference signals are also known as Pilots.

These are pseudorandom sequences and are used to estimate Channel Quality Indicator (CQI), Rank Indicator (RI) and Precoding Matrix Index (PMI).

One RS is transmitted per Downlink antenna port.

RSs consist of known reference symbol locations.

Cell Specific Reference Signals:

Cell specific Reference Signals

UE specific Reference Signals

MBSFN Reference Signals

Idle Mode Channels: DL-RS

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Time and frequency resource allocation of DL-RS for 2 Tx antenna is shown below:

Idle Mode Channels: DL-RS

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Synchronization signals are transmitted in every radio frame.

The signals are used for time (symbol) and frequency synchronization during initial power up.

As part of system acquisition process, the UE is synchronized to OFDM symbol, slot, subframe, half frame, and radio frame using synchronization signals.

There are 2 types of synchronization signals:

Primary Synchronization signal

Secondary Synchronization signal

PSS and SSS are transmitted in sub-frame 0 and 5 of every frame (see next slide for more information on frame and sub-frame)

Idle Mode Channels: SCH

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PSS helps in Sub-frame timing determination.

It also provides a unique cell ID index (0,1, or 2) within a cell ID group.

Primary Synchronization Signal:

SSS helps in Frame timing determination.

It provides a unique cell ID group number among 168 possible cell ID groups.

Secondary Synchronization Signal:

0 1 2 3 4 5 6 7 8 9

Radio Frame = 10ms

Subframe = 1ms

(2 Slots)0 1 2 3 4 5 6

Slot = 0.5ms LTE Frame Structure


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SS

SP

SS

6

100 R

eso

urc

e B

locks

10 ms

6 RBs = 72 Subcarriers

(1.08 Mhz)

5 ms subframe

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There are 504 unique physical layer cell identities.

0 1 2 3 . . . . . . . . . . . . . . . . 167

0 1 2 0 1 2 0 1 2

0 1 2 3 4 5 . 501 502 503

SSS*3

PSS

= Physical layer Cell ID

These identities are organized into 168 groups. Each group is uniquely identified by one of 168 (0 .167) possible sequences transmitted in SSS.

Each cell identity group contains 3 unique cell ID indexes. Cell identity index within the group is uniquely identified by one out of 3 (0,1,2) sequences

transmitted in PSS.

Physical layer cell identity is combination of cell ID group and cell ID index resulting in 1 out of 504 (3*168) possibilities.


+

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PBCH carries Master Information Block (MIB) which contains the most

important system parameters such as:

Channel bandwidth

PHICH configuration

System Frame Number.

PDSCH carries DL data transmissions for different users as well as upper layer

signaling.

Paging

SIBs

User data

PBCH:

PDSCH:

Idle Mode Channels: PBCH & PDSCH

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Idle Mode Channels: PBCH & PDSCH

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Appendix1: MIB and SIB format

Idle Mode:

This part of the document discusses Initial System Acquisition as well as MIBs/

SIBs and UE categories.

Appendix2: UMTS to LTE Reselection

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Once a UE is turned on it goes through Initial Acquisition phase.

Cell search involves UE to detect suitable Cell ID Index and Group. This is done using information broadcasted on PSS and SSS.

System information is acquired through MIBs and SIBs (Master Information Block & System Information Block) broadcasted on PBCH and PDSCH.

Both these steps are discussed in next slides.

The process can be divided into 2 stages:

1. Cell Search

2. Acquiring System Information

Idle Mode Initial System Acquisition

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Reads the USIM and stored info. Selects

PLMN & Band and searches for the

strongest cell in the DL band.

UE is switched on

UE has now determined:

Frame timing

Cell-ID group

Performs a rough

frequency

synchronization &

looks for PSS

UE attempts to

detect SSS

UE has successfully determined:

Exact carrier frequency

Cell ID index

Sub-frame timing

UE can now read PBCH and acquire system information

Step 1: Cell Search

Idle Mode: Initial Acquisition

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MIB/ SIB1 are read from PBCH and DLSCH

in a well defined scheduling instantUE reads MIB and

SIB1

MIB, SIB1 and SIB2 are required to

camp on a cell

After acquiring SIB1

UE checks if the cell

meets suitability

criteria

UE receives and

decodes SIB2 and

other SIBs

Confirms MCC/ MNC in PLMN elements

Also checks if PLMN is reserved, cell is

barred or cell belongs to forbidden TA.

PLMN selection is successful after this check

UE is camped on the cellS criteria is validated

Step 2: System Information

Idle Mode: Initial Acquisition

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Info. Blocks Description

MIBRepeats every 4 frames (10ms) and includes DL Tx bandwidth, PHICH

configuration and SFN.

SIB1Repeats every 8 frames (80ms) and includes PLMN id, Tracking Area Code,

Cell identity, Cell barring status and scheduling of other SIBs.

SIB2 Contains radio resource configuration information that is common for all UEs.

SIB3Contains cell reselection information common for intra-frequency, inter-

frequency cell reselection.

SIB4Contains neighboring cell related information relevant only for intra-frequency

cell reselection.

SIB5, 6, 7,8

Information relevant for interf-requency, UTRA, GERAN, CDMA2000

reselection respectively. This includes frequencies, and neighboring cells

relevant for reselection.

SIB9 Home eNB identifier

SIB10, 11 ETWS primary and secondary notification etc.

Following table gives a brief description of information contained in MIBs & SIBs*.

*Please see appendix for MIBs and SIBs format.

Idle Mode MIBs and SIBs

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UE Categories Description

Suitable Can Camp to obtain Normal service

AcceptableMay Camp to obtain limited service (e.g. Emergency calls). This cell is neither barred, reserved or in forbidden TA list.

Reserved UEs with AC 11 and 15 are allowed to Camp in PLMN

Barred Not available for Camping

CSG Only UE of Closed Subscriber Group can Camp

Service Description

Normal Can receive Page and can transit to Connected state

Operator For UEs on reserved cell only

Limited Emergency call

In order to obtain Normal Service, UE needs to be camped on Suitable Cell.

Idle Mode UE Categories & Services

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Idle Mode:






Appendix: MIB and SIB format

This part of the document discusses Cell Selection and Reselection Criteria for

Intra, Inter and IRAT frequencies.

Appendix: UMTS to LTE Reselection

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Once a cell meets all requirements, Criterion S is considered for cell selection.

Pcompensation = [max (PEMAX PUMAX , 0)]

Srxlev = [Qrxlevmeas (Qrxlevmin + Qrxlevminoffset) Pcompensation] > 0

Parameter Description

QrxLevmeas Measured RSRP value by UE (dBm)

Qrxlevmin Required minimum RSRP level in the cell (dBm)

Qrxlevminoffset Offset to Qrxlevmin

PUMAX Maximum UE power according to its class

PEMAX Maximum UE power to be used in a cell. Ericsson equivalent is pMaxServingCell

where

Example: (Ignoring Offset and Pcompensation)

Srxlev = (Qrxlevmeas Qrxlevmin) => -105 (-120) = 15 db > 0

Idle Mode Cell Selection

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After cell selection UE performs measurement and carry out cell reselection of

new candidate cell. This is based on certain criterion.

Reselection process involves 2 steps:

Specifies when to start measuring neighboring cells. Once condition is met

UE measures the RSRP level of the E-UTRA cells on the current frequency and

any inter-frequency and inter-RAT frequency carriers.

1. Measurements Condition:

2. Reselection Condition:

Specifies reselection criteria for candidate cell.

There are two different mechanisms for cell reselection:

Legacy Based

Priority Based

Idle Mode Cell Reselection

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This is classical Ranking based system and is applied to intra-frequency reselection.

It is also applied to inter frequencies which are equally prioritized.

UE applies cell ranking R criterion on serving cell and neighboring cell.

Priority values in the range [0..7] are used, where 0 indicates lowest priority and 7 indicates highest priority.

Cell reselection can be either Legacy or Priority based.

Priority Based:

Legacy Based:

Inter-Rat Frequencies: Higher or Lower Priority

Inter Frequencies: Lower, Equal or Higher Priority

Equal priorities between different RATs are not supported.

Idle Mode: Cell Reselection

Priorities can be set for:

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Srxlev < SIntrasearch

R(serving) = Qmeas(s) + qHyst

R(neighbor) = Qmeas(n) qOffset(n)

As discussed earlier Legacy Reselection method is used for Intra Freq reselection.

When signal level goes below Sintrasearch UE starts measuring Intra Frequency

neighbors:

When camped on E-UTRA cell, UE performs a ranking of neighbor E-UTRA cells on the

same frequency and other frequencies with equal cell reselection priority, taking into

account the cells satisfies the cell selection criteria

R(neighbor) > R(serving) condition is fulfilled for tReselectionEutra time

Neighboring cell is reselected when:

Reselection Condition:

Measurement Condition:

Idle Mode Cell Reselection: Intra Frequency

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Rs = Qrxlevmeas + Qhyst

= -113 + 4

= -109 dBm


Sintrasearch Threshold for intra-frequency measurements

qHyst Cell reselection parameter that defines the hysterisis value in cell ranking criteria

qOffsetCellEutran Cell individual offset in intra-frequency and equal priority inter-frequency cell ranking criteria

qOffsetFreq Frequency specific offset in equal priority inter-frequency cell ranking.

TreselectionEUTRA Cell reselection timer value of an E-UTRA frequency.

Rn = Qrxlevmeas - Qoffset

= -117 0

= -107 dBm

Example:

For Inter-Freq neighboring cell qOffsetFreq (frequency specific offset) is added to

qOffsetCellEutran in Offset calculation


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Signal

Strength

Qrxlevmin

Time

Sintrasearch

Starts measuring

Intra frequency

cells

Neighboring cell becomes

higher than serving cell

(including Hysteresis & Offset)

Reselection to Intra LTE cell

TreselectionEUTRA

Camped

Not CampedCell A

Cell B


Intra Frequency Cell Reselection

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Inter or IRAT reselection with equal or lower reselection priority:

Srxlev < SNonIntrasearch


Measurements on Inter frequencies or inter-RAT frequencies with equal or lower cell

reselection priority is performed when following criteria is met:


If Srx value of the serving cell falls below the threshServingLow value, UE attempts

to reselect a cell on an inter-frequency carrier or an IRAT carrier with cell reselection

priority lower than the current frequency.

Cell reselection occurs if UE finds a cell with an Srxlev value greater than

threshXlow value for that frequency.

Srxlev (source) < ThreshServingLow

Srxlev (target) > ThreshXLow

Idle Mode Cell Reselection: Priority Based

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CellReselectionPriority Absolute reselection priority for E-UTRA or IRAT frequency

sNonIntraSearch Threshold for inter and IRAT frequency

threshServingLow Threshold for Srx value of serving cell below which UE performs cell reselection towards a lower priority cell.

threshXLow Threshold for Srx value of target cell for cell reselection towards a lower priority Inter or IRAT frequency.

tReselectionEutra Cell reselection timer value for EUTRAN

tReselectionUtra Cell reselection timer value for UTRAN

Example:

SNonIntraSearch = 10 dB, ThreshServingLow = 8 dB, threshXlow = 6 dB

This means that UE will start low priority measurements when Srx (serving) falls below

10 dB. Once it crosses the threshold of 8 dB, it will be ready to reselect as soon as

target cell crosses 6 dB threshold (based on Reselection timer)


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TimeStarts measuring

IF or IRAT freq TreselectionUTRA

Threshxlow

Snonintrasearch

LTE High Priority)

Low Priority SignalSignal strength

Reselection

to UMTS

Threshservinglow

Qrxlevmin

Condition1 :


Condition2:

Srxlev (target) > ThreshXLow


High Priority LTE to Low Priority Inter/ IRAT Reselection

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If Inter Frequency or IRAT frequencies are of higher priority than the current

frequency, measurements are always performed.


threshXHigh value for that frequency.

Srxlev (target) > ThreshXHigh

The UE searches every layer of higher priority at every (60*Nlayers) seconds.

Nlayers is the total number of configured higher priority E-UTRA, UTRA, CDMA and

HRPD carrier frequencies.

Inter or IRAT with higher cell reselection priority:




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CellReselectionPriority Absolute reselection priority for E-UTRA or IRAT frequency

threshXHigh Threshold for Srx value of target cell for cell reselection towards a higher priority Inter or IRAT frequency.

tReselectionEutra Cell reselection timer value for EUTRAN

tReselectionUtra Cell reselection timer value for UTRAN

Example:

ThreshXHigh = 6 dB

UE will keep on measuring Higher Priority Inter or IRAT frequencies every (60*Nlayers)

seconds. Once it finds a candidate with Srx > ThreshXHigh it will handover after timer

value is reached.


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Time

TreselectionRAT

Signal

strength

ThreshxHigh

Reselection

to UMTS

Condition:

Srxlev (target) > ThreshXHigh

LTE Low Priority

High Priority Signal


Low Priority LTE to High Priority Inter/ IRAT Reselection

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Idle Mode:






Appendix1: MIB and SIB format

This part of the document discusses Master Information Block and System

Information Block format.

Appendix2: UMTS to LTE Reselection

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Info. Blocks Description

MIBRepeats every 4 frames (10ms) and includes DL Tx bandwidth, PHICH

configuration and SFN.

SIB1Repeats every 8 frames (80ms) and includes PLMN id, Tracking Area Code,

Cell identity, Cell barring status and scheduling of other SIBs.

SIB2 Contains radio resource configuration information that is common for all UEs.

SIB3Contains cell reselection information common for intra-frequency, inter-

frequency cell reselection.

SIB4Contains neighboring cell related information relevant only for intra-frequency

cell reselection.

SIB5, 6, 7,8

Information relevant for interfrequency, UTRA, GERAN, CDMA2000

reselection respectively. This includes frequencies, and neighboring cells

relevant for reselection.

SIB9 Home eNB identifier

SIB10, 11 ETWS primary and secondary notification etc.

Following table gives a brief description of information contained in MIBs & SIBs.

Appendix1 MIBs & SIBs

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Log M

essages

Type Direction Message

BCCH_ DL MasterInformationBlock

BCCH_ DL SystemInformationBlockType1

BCCH_ DL SystemInformation

RACH UL Random Access Preamble

PCCH DL Random Access Response

CCCH- UL RRC Connection Request

MIB:

Bandwidth Info: Resource Blocks

PHICH Configuration

System Frame Number

Appendix MIBs & SIBs

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SIB1:

Cell Status, Tracking Area

ID, Cell ID

Selection Info

PLMN ID

Scheduling of

other SIBs


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SIB2:

PCCH, PRACH,

PDSCH

Configuration

Preamble

Supervision


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SIB3:

Reselection

Information


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SIB4: SIB5:

Intra Frequency

Neighbors Information

Inter Frequency

Neighbors Information


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SIB6: SIB7:

UTRA Frequency

Neighbors Information GERAN Frequency

Neighbor Information


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Just like LTE 2 reselection mechanisms are used in UMTS

Appendix2: UMTS To LTE Reselection

This is ranking based system and it applies to intra-frequency, inter-frequency as

well as for GSM reselection.

This mechanism applies for cell reselection from WCDMA to LTE

Both legacy based and priority based cell reselection can be active at the same time.

Priority Based:

Legacy Based:

High Priority UMTS to Low Priority LTE

Low Priority UMTS to High Priority LTE

Priority based method results into 2 different scenarios:

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If LTE frequency is of higher priority than the current frequency, measurements

are always performed.


ThreshHigh.

Srxlev (target) > ThreshHigh

The UE searches every layer of higher priority at every Thigher_priority_search =

(60*Nlayers) seconds.

Nlayers is the total number of configured higher priority E-UTRA frequencies

Reselection from Low Priority UMTS to High Priority LTE



Appendix2: UMTS TO LTE Cell Reselection

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Low Priority UMTS to High Priority LTE Reselection

Time

Treselection

Signal

strength

ThreshHigh

Reselection

to LTE

Condition:

Srxlev (target) > ThreshHigh

UMTS Low Priority

High Priority Signal


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The UE only starts measurement for candidate LTE when following threshold is met.

Reselection from High Priority UMTS to Low Priority LTE



Srxlev (serving) < SPrioritySearch1

Squal (serving) < SPrioritySearch2

If Srx value of the serving cell falls below the threshServingLow value, UE attempts

to reselect a cell on LTE.

Cell reselection occurs if UE finds a cell with an Srxlev value greater than threshLow

value for that frequency.


Srxlev (target) > ThreshLow

As of now only Srxlev is used for ATT.

OR


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TimeStarts measuring

LTE freq Treselection

Threshlow

SPrioritySearch1

UMTS High Priority

LTE Low PrioritySignal strength

Reselection

to LTE

Threshservinglow

Qrxlevmin

Condition1 :


Condition2:

Srxlev (target) > ThreshLow

High Priority UMTS to Low Priority LTE Reselection


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Scheduling Block 19 (SIB19):

{

utra-PriorityInfoList

{

utra-ServingCell

{

priority 3

s-PrioritySearch1 8

threshServingLow 8

}

}

eutra-FrequencyandPriorityInfoList

{

{

earfcn 5780

priority 7

qRxLevMinEUTRA -62

threshXHigh 3

threshXlow 3

eutraDeteection TRUE

}

}

SIB 19 Format

In order to activate UMTS to LTE Reselection, SIB19 is introduced in the

UMTS system

This also impacts SIB scheduling which is needed to be changed to accommodate

SIB 19.

A maximum of 8 EARFCNs can be listed.

A maximum of 9 black listed cells per UTRAN cell can be included.

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Parameters discussed in this presentation can be found via AMOS

Appendix3: Parameters in AMOS

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For questions:

Fraz Tajammul

[email protected]

lte idle mode

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