rlc error recovery

Telkomsel Network Performance Analysis

May 14th, 2012

Analyze of RLC Unrecoverable Error : Definition

Tektronix Communication - Confidential2

If the maximum number of tranmissions of a RESET PDU is equal to MaxRST-1,

then RLC unrecoverable error shall be indicated to upper layers. The concept of RLC unrecoverable error is valid only for RLC-AM entities.

The UE will initiate a RRC Cell update procedure, when a RLC unrecoverable error is detected. The cell update cause will be set to "RLC unrecoverable error".

Analyze of RLC Unrecoverable Error

OSS counters


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-Number of cell updates attempts are calculated when RNC receive cell update message from the UE

- Number of successful cell updates are calculated when the RNC sends Cell update confirms to the RNC


OSS counters


- There are different reasons for cell updates (paging response, reselection..).

-Here we are interested in Number of Cell Updates Due to SRB and TRB RLC Unrecoverable Error for Cell

-VS.AttCellUpdt.ErrSRB and VS.AttCellUpdt.ErrTRB


Reestablishment process


When RNC receives cell update message due to to RLC recoverable Error, the process of reestablishment should be triggered, in order to reestablish the call and not to drop.

In huawei, Call reestablishment is only triggered by UE cell update due to Radio Link failure or TRB reset (RLC Unrecoverable Error on Traffic Radio Bearer TRB)


Reestablishment process


When SRB reset happens, RNC will send “IU Release Request” to CN and the call is dropped.

RNC does not consider the cell updates due to RLC Unrecoverable Error in SRB, and proceeds with immediate Iu release and RRC connection release

Cell update success rate from OSS counters : RLC Unrecoverable Error


- Around 60% success rate for cell update due to RLC error recovery when reset occurs in Traffic Radio Bearer- Considering that these counters take into account retransmissions (up to N302=3), this value is quite reasonable

RLC Unrecoverable Error


-Workaround : R13 version contains algorithm for Reestablishment in case of SRB reset

--> Expecting Drop rate improvement in both CS and PS

--> This option can be proposed in order to improve cell update succ rate and drop rate

Reestablishment – improved version


we recommend also to activate the reestablishment for the UPLINK synchronization failure, and TRB/SRB reset in UL, as it is not the case currently. T313 starts only when the RIF or reset occurs in DL. In Uplink, the UE does not know about the issue occurring in UL, and do not start cell update process. So immediate Iu release is triggered. But in R13, there is possibility that when getting lost of synchro in UL, the NodeB will start switching off power to the concerned UE, and then this UE will detect a RIF in DL, and starts looking for another cell. Following reserved bits should be set to OFF in order to enable reestablishment due to SRB reset in DL, UL SRB/TRB resets and UL synchro loss, for both CS and PS : SET URRCTRLSWITCH:RsvdPara1:RSVDBIT1_BIT22-0RSVDBIT1_BIT23-0RSVDBIT1_BIT24-0RSVDBIT1_BIT25-0RSVDBIT1_BIT26-0RSVDBIT1_BIT28-0


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rlc error recovery

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