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RNC PS call drop rate increase after migration to Huawei SGSN

RNC PS call drop rate increase after

migration to Huawei SGSN

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1 Brief Description of the Issue

ssue occurred in H pro!ect w"en Huawei SGSN swap out NSN SGSN# $wo

RNC%Huawei& were migrated to Huawei SGSN and got same pro'lem on 'ot"# ($ is

implemented for 'ot" two RNC#

)fter RNC migration %from NSN SGSN to Huawei SGSN&* PS call drop rate increase

from 1+ to ,+#

igure 1 PS call drop rate increase after PS migration in .(G RNC

)fter RNC migration %from NSN SGSN to Huawei SGSN&* PS call drop rate increase

from 1+ to /+#

igure 2 PS call drop rate increase after PS migration in P RNC

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2 Alarm inforation

No additional alarm#

3 Root Cause Analysis

#1 S#R).#3oss#PS#G$P43oss description

S#R).#3oss#PS#G$P43oss detailed description is listed 'elow5

$"e measurement is triggered at point ) as s"own in t"e figure 'elow# n t"e case of

G$P4 failure%RNC receie RR8R9N( message&* RNC sends to t"e CN a R).

R3)S R:4S$ message# f RNC receies a 4 R3)S C8;;)N(message%wit" t"e cause ot"er t"an


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#2 RR8R9N( message description in GPP Specs

3.2.1 ERROR_IND message is described in 3GPP 23.060:

When the SGSN receives a GTP U PDU from the RNC for which no PDP context exists, the SGSN

shall discard the GTP U PDU and send a GTP error indication to the originating RNC. The RNC

shall locall release the R!".

When the GGSN receives a GTP U PDU for which no PDP context exists, it shall discard the GTP

U PDU and ret#rn an error indication to the originating SGSN. The SGSN shall mar$ the related

PDP context as invalid and send a Deactivate PDP Context Re%#est message to the &S. The &S

ma then reactivate the PDP context.

3.2.2 And the actin i! RN" recei#e that $ind ! message is described as %e&& in

3GPP 23.00':

GTP error indication message shall 'e handled as follows(

)When the RNC receives GTP error indication from the SGSN, it shall initiate the R!" Release

*roced#re with the error ca#se +GTP Reso#rces Unavaila'le+ and shall immediatel locall release

the R!" i.e. witho#t waiting for a res*onse from the SGSN-.

)f the RNC receives a GTP error indication from the GGSN i.e. if Direct T#nnel is esta'lished-, it

shall initiate the R!" Release *roced#re with the error ca#se +GTP Reso#rces Unavaila'le+ and

immediatel locall release the R!" i.e. witho#t waiting for a res*onse from the SGSN-.

Considering one tunnel is implemented for .(G RNC* we suspect GGSN send

RR8R9N( message to RNC some times#

4 Handling process

Compare t"e signaling flow from HA SGSN and NSN SGSN* t"e difference is

discoered#

@#1 Signaling Handing of G$P43oss w"en RNC connect to HA SGSN

Huawei traced su'scri'ers in t"e list and summari?ed t"e w"ole PS call flow w"ic"

"ae suc" issue# .elow is t"e w"ole flow c"art5

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igure @ .(G RNC PS call drop flow c"art

(uring t"e P(P actiation process* GGSN would sent CCR message to 8CS for

aut"entication wit"out c"ec>ing 'alance# )nd once GGSN receie an= data from

RNC* GGSN would send CCR-4 message to 8CS wit" corresponding RG for

'alance c"ec>ing# f su'scri'er donBt "ae enoug" 'alance* 8CS would send ,00

error code to GGSN# GGSN s"ould send delete P(P reuest message to SGSN and

delete su'scri'er P(P conteDt awa=# SGSN s"ould send deactiate P(P reuest

message to ;S since receie reuest from GGSN# .ut t"ere is a c"ance t"at ;S

>eep sending data to networ> after P(P actiation# f one data pac>et is sent to

GGSN after GGSN delete su's P(P conteDt* GGSN would response wit"

RR8R9N( message according to GPP spec# 8nce RNC receie RR8R9N(

message and receie u release C;P message wit" error code no remaining R).(*

S#R).#3oss#PS#G$P43oss item would plus one# )nd since u connection is

released* (eactiation P(P reuest message cannot send to ;S successfull=#

)ccording to a'oe description* we can conclude t"at t"e pre-condition for

G$P43oss error are su's wit"out enoug" 'alance E (irect $unnel networ> E 8nline

C"ange ena'le and would not affect an= normal serice#

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@#2 Signaling Handing w"en RNC connect to NSN SGSN

Huawei did a trace on P-RNC and anal=?ed t"e flow w"en RNC connect to NSN

SGSN# Please refer to 'elow flow5

igure , NSN SGSN flow c"art

$"e difference is w"en RNC send R).9release9re to NSN SGSN* NSN SGSN

donBt response t"is message immediatel= and >eep sending (eactiation P(P Re

message to ;S# )fter P(P deactiation flow finis"* NSN SGSN would response

R).9release9Re message wit" u release C;( w"ic" reason is normal-release# n

t"at case* RNC would not record t"is as G$P4#3oss error#

5 ummary

1# t will not affect t"e customer eDperience.

2# t doesnBt affect t"e t"roug"put since su's w"o "ae G$P43oss are no-'alance

su's#

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# t wouldnBt affect t"e 'illing since pro'lem onl= occur on no-'alance su's#

! "e#t action

Huawei would release a SGSN patc" for optimi?ation# Patc" num'er is SPH,10

w"ic" would 'e release '= 2@t" Fan 2010#

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