rrc rejection due to radio link failure because of high wbbp cpu load

文档名称文档密级 :

Internal only

Author Ahsan Raza Staff Number 00725065

Department NTSProduct

Family:

Wireless Performance &

RNP & RNO

Update Time 2011-06-29Product

Vension: V200R012C00SPC300

Approver

Title： RRC Rejection due to Radio Link Failure because of high WBBP CPU Load

Phenomenon

Description:

Some Sites started having RRC Rejection due to Radio Link Failure, however,

when analyzed there was no congestion on any cell of these sites. And all

(power,code, Channel Element) utilization was also low. When the 2nd WBBP

board was added to these sites, RRC Rejection reduced to zero. So the main

question arised that why even with low CE utilization we are having RRC

Rejections and how adding another WBBP Board removed the RRC Rejection.

Alarm

Information：None

Cause

Analysis：None

Handling

Process：The first thing to understand is that Each WBBP card has 1 CPU and there is no

counter to see the CPU load. However, the CPU load can be approximately

calculated using following procedure.

With BTS3900 Equipment 1 WBBP CPU can support 45 CNBAP messages per

second.

3900

Version V200R012C00SPC110

WMPT+1*WBBP 45 CNBAP /s (WBBP capacity)

WMPT+2*WBBP 100 CNBAP /s



CNBAP messages are basically contributed by below messages:

RL SETUP Request

RL ADD Request,

RL Reconfigure Prepare and Commit,

NBAP measurement report

The actual live traffic CNBAP is as below:

Since we cannot calculate the real time traffic per each seconds, we are using

mean CNBAP to estimate the peak.

Normally peak traffic/average traffic ≈2, so the max CNBAP=2* mean

2023-4-17 华为机密，未经许可不得扩散第 1页, 共 5页


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CNBAP.

The formula for CNBAP messages is

Now, if the CPU load reaches to the value of 90-100%, NodeB starts to reject

the RRC due to RL failure. When further analyzed using CHR, the cause of

failure comes as NodeB resources unavailable. However, when the CE

utilization is checked is around below 80%. The cause of RRC rejections arte

basically the WBBP CPU is maximum utilized and has no more capacity to

process any more rrc requests. Below is one Such Case.

DXB2315 is having high RRC rejections specifically on weekends.

When analyzed in CHR the cause of RRC Failure was NodeB Resources

unavailable as shown in below snapshot.

.



Internal only

When CE utilization was calculated, CE utilization was observed to be around

maximum upto 60% on the weekend.

Now When WBBP CPU load was calculated using CNBAP message from

below formula. The CPU load was reaching to 90%.



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Also, when the SHO Factor was calculated for this site. It was found that some

of the cells are reaching upto 100% SHO Factor. This is very high causing too

many CNBAP messages and causing high WBBP CPU utilization.

Also, when compared to other cells with high SHO Attempts. It was found that

traffic of this site is not high as compared to the sites with same SHO attempts.

Hence, when the 2nd WBBP board was added the capacity of handling CNBAP

message increased from 45 messages per second to 100 messages per second

and RRC rejection reduced to zero.

Suggestions

and

summary：

This issue can be solved using following methods:

1. By addition of another WBBP Board.

2. By hard and soft parameter optimization to reduce the SHO Factor.

Attachments：Related

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rrc rejection due to radio link failure because of high wbbp cpu load

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