A Sharp Fall of AMR RAB Assignment Success Rate Caused by CN MGW Failure

A Sharp Fall of AMR RAB Assignment Success Rate Caused by CN MGW Failure INTERNAL

Author Zhong Xiaoxu Employee ID 51447

Local Office,

Representative Office, or

Regional Division

South

American

Network

Planning

Product Name WCDMA RNP

Date 2008-11-12

Product Version

(VxxxRxxxCxxxBxxx

)

BSC6810V200R009C01B072

Approved by

Case When the CN MGW fails, the AMR RAB assignment success rate falls greatly.

Sympto

m

In office C in country B, according to the daily report of performance, the AMR RAB assignment

success rate of RNC331 sharply fell from about 99% to 85% on October 28.

Alarm

Informat

ion

No RNC-related or NodeB-related alarm is reported.

Cause

Analysis

1. We categorized the causes for RAB assignment failure in traffic statistics, and found most cause

types are VS.RAB.FailEstabCS.TNL.

2008-12-29 Huawei Confidential Page 1 of 5

A Sharp Fall of AMR RAB Assignment Success Rate Caused by CN MGW Failure INTERNAL

For VS.RAB.FailEstabCS.TNL, the explanation in Performance Item is as follows: The

measurement is triggered at point B as shown in the figure, when the RNC sends the CN a RAB

ASSIGNMENT RESPONSE message indicating that the operation fails and the failure cause is

"Transport Layer Cause." Therefore, the failure is caused by the transport layer. Possible causes:

(1) Iu-CS transmission failure

(2) Improper Iu-CS configuration on the RNC

(3) Improper Iu-CS configuration on the CN

(4) Faulty Iu-CS equipment

2. We analyzed CHR logs as follows:

The explanation for Error Code 184945361 is , which

indicates that the cause is AAL2 error at the Iu-CS interface.

3. We selected several cells on RNC331 for IOS tracing. According to IOS analysis, the cause is as

follows:

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A Sharp Fall of AMR RAB Assignment Success Rate Caused by CN MGW Failure INTERNAL

RAB assignment succeeds on MGW5 and MGW7, that is, the failure occurs on MGW4.

4. According to the communication with a CN engineer, at night on October 27, the network

topology was changed as follows:

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A Sharp Fall of AMR RAB Assignment Success Rate Caused by CN MGW Failure INTERNAL

MGW5 and MGW7 are connected to the router, but MGW4 is not connected to the router. For future

convenience, the CN engineer activated MGW4, MGW5, and MGW7 on the MSC.

The control plane is set up between the RNC and MSC, and the user plane is set up between the

MGW and RNC. Therefore, the RRC connection setup success rate is about 99%. That is, all

signaling processes are set up successfully before service assignment, but the failure occurs in

service assignment. If the CN assigns services on MGW4, the failure occurs due to no connection

between MGW4 and the RNC. If the CN assigns services on MGW5 and MGW7, the service can be

set up successfully.

Handlin

g

Process

1. Considering AAL2 error is found in CHR logs; so the CN engineer deleted MGW4-related AAL2

paths on the MSC. The AMR RAB assignment success rate is still about 85%.

2. According to the check, MGW4, MGW5, and MGW7 are in the same MGW group; so the

customer deleted MGW4 from the MGW group. Now, the problem is solved.

3. After changing the network topology, the customer shall notify the information to the RF side.

Suggest

ion and

Summar

y

When a problem occur, multiple products may be associated with it. Therefore, it is necessary to

fully communicate with product-related departments.

Attachm

ent

2008-12-29 Huawei Confidential Page 4 of 5

A Sharp Fall of AMR RAB Assignment Success Rate Caused by CN MGW Failure INTERNAL

Referen

ce

2008-12-29 Huawei Confidential Page 5 of 5