gsm network sdcch congestion & solutions-16

8/10/2019 GSM Network SDCCH Congestion & Solutions-16

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Contents

1 SD Seizure Signaling & Relevant Counters...............................................................................................1

1.1 SD seizure and release flow.................................................................................................................1

1.2 Definition of SDCCH congestion indicator........................................................................................2

2 Classification of SDCCH Congestion Causes............................................................................................5

3 Procedure of Checking SD Congestion SD.................................................................................................

! "#$ical Cases of SD Congestion.................................................................................................................11

4.1 SD congestion due to LAPD delay caused by transmission fault.....................................................11

4.2 SD congestion due to large uantity of L!C "caused by cross#LAC co$erage%..............................11

4.& SD congestion due to strong interference..........................................................................................12

4.4 SD congestion due to large uantity of L!C caused by HL' cuto$er.............................................14

4.( SD congestion due to s)ort message *ager.......................................................................................1+



1 SD Seizure Signaling & RelevantCounters

1.1 SD seizure and release flow

,ig 1#1 SD Seizure and 'elease ,low

As s)own in ,ig 1 #1- during L!C and t)e early stage of !C and /C *rocess- S

usually seizes SDCCH to e0c)ange signaling. /)e signaling flow is as follows

After recei$ing C)annel 'euired message- 3SC a**lies for SD from database-

and t)e counter for SD seizure attem*t times increases by 1

5)en database allocates SD- t)e number of successful SD seizure increases by

1

3SC sends SD acti$ation message to 3/S- t)en t)e number of SDCCH

allocation attem*ts increases by 1

3/S res*onds to 3SC wit) a res*onse message of SD acti$ation- 3SC increases

t)e number of successful SDCCH allocation by 1

3SC sends 6mmediate Assignment message to S t)roug) 3/S- 3SC increases



t)e number of SDCCH assignment attem*ts by 1

S re*ort SA3 frame- 3/S re*orts 7st 6nd message to 3SC- and 3SCincreases t)e number of successful SDCCH assignment by 1.

1.2 Definition of SDCCH congestion indicator

/able 1#1 SDCCH Congestion 6ndicator Definition

8P6 name SDCCH bloc9ing rate

Definition :umber of signaling c)annel bloc9ing times;1<<=> :umber of signaling c)annel

call attem*ts

Counter

formula

?2 "2.@% "C11+2(#C11+2+BC11+@%;1<<=>"C11+2(BC11+@+%

?& "+.2<%"C@<<<+<<<(BC@<<<+<<11BC@<<<+<<&@%;1<<=>

"C@<<<+<<<&BC@<<<+<<1<BC@<<<+<<&%

SD congestion means t)at u*on recei$ing C)annel 'euired message- 3SC a**lies for

SD from database- w)ile 3SC )as no SD for allocation "all SD resource are bloc9ed or

occu*ied%.

5)en 3SC fails in a**lying for SD- it sends a message 6mmediate Assignment 'eect

to S- commanding S to reuest for access after a *eriod of time "set on /&122% and

3SC increases t)e counter for SDCCH congestion by 1. essage flow of 6mmediate

Assignment 'eect can not sol$e SD o$erflow *roblem- but can )el* relie$e and

smoot)en it.

/able 1#2 /imer /&122

Protection *eriod of access attem*t（/&122-s）

?alue range < E 2((

Fnit s

Default $alue 1<

2



C)a*ter & Procedure of C)ec9ing SD Congestion SD

Descri*tion

6f networ9 )as no a**ro*riate c)annel to allocate to S after it )as recei$ed

c)annel reuest message from S- it sends 6mmediate Assignment 'eect to

S. 6n order to a$oid radio c)annel congestion w)ic) may be caused by

continual c)annel reuest from S- timer *arameter /&122 "t)e so called

5ait 6ndication message% is included in 6mmediate Assignment 'eect

message.

After it recei$es 6mmediate Assignment 'eect message- S can not start a

new call until /&122 e0*ires. /)is timer is one of t)e system control

*arameters- w)ic) is sent to S in t)e 6mmediate Assignment 'eect.

'ecommended normal $alue for /&122 is 1<E1(s- and 1(E2(s in areas wit)

dense traffic.

3



2 Classification of SDCCH CongestionCauses

,rom t)e as*ect of singling flow- t)ere are two main ty*es of causes of SD congestion.

/oo many C)annel 'euired e0ceeds networ9 ca*acity and all SDCCHs are occu*ied.

/oo#many C)annel 'euired means t)e cell is busy- w)ile few SDCCH are

configured- w)ic) results in freuent occu*ancy of SDCCH and o$erflow.

5)en c)ec9ing SD congestion- w)at s)all be considered is if t)ere is *ossibility

of traffic increase.

Some operations at OMCR (such as HLR configuration or re-planning of LAC) may

lead to traffic increase in networ!

/oo long occu*ancy *eriod of SDCCH due to non#in#time ending of signaling flow

6f signaling flow doesnGt end in time- w)ic) means c)annel acti$ation>release

*eriod is too long due to some reason "say- transmission fault%- it will lead to

long *eriod of SDCCH occu*ancy and reduce SDCCH resource- and e$entually

result in SDCCH o$erflow.

/oo#long c)annel acti$ation>release *eriod causes S to re*eat C)annel 'euest

again and again "t)e number of re*etition is decided by system *arameter

a0'etrans%- and to occu*y SDCCH re*eatedly- w)ic) worsens SDCCH

o$erflow.

5



!rocedure of C"ecking SD CongestionSD

1. 'ead *erformance re*ort- c)ec9 if all cells or some cells under t)e 3/S are

congested

6t rarely )a**ens t)at all cells under t)e 3/S are congested. 6f it )a**ens- t)e

causes are usually related to terrestrial transmission and *arameter

configuration.

2. 'ead *erformance re*ort- c)ec9 if c)annel allocation failure "i.e. c)annel

acti$ation e0*ires or fails% )a**ens during SD congestion

Note#

"oo large LA#$ flow will lead to LA#$ transmission delay% which causes timer

e&piration 'efore channel actiation is completed! "his ind of timer e&piration shall

'e differentiated from that caused 'y transmission fault!

&. C)ec9 radio access measurement- analyze t)e access reason of C)annel 'euest

w)ic) causes SD congestion- count t)e number of C)annel 'euest attem*ts and

success times due to different reasons- and com*are indicators wit) t)ose in

normal *eriod..

/)e access cause of C)annel 'euest falls into 4 ty*es

!C "obile#!riginating Call%

/C "obile#/erminating Call%；

L!C "Location F*date%；

!t)er reasons "call re#establis)ment%

enerally- t)e number of L!C attem*ts ta9es &(= of t)e total C)annel 'euest times

/)e *ercentage may increases to <=- if t)e site is located at LAC boundary.

:umber of !C C)annel 'euest and t)at of /C C)annel 'euest are almost eual-

w)ic) are related to local subscribersG call conditions

!t)er reasons rarely )a**en in normal conditions.

7




busy and its traffic $olume is )ig)- and e0*ansion is needed.

9



,rom analysis of radio access measurement- itGs disco$ered t)at t)e SD congestion was

caused by large uantity of L!C.

5)en c)ec9ing t)e site ma*- engineers found t)e site was located between two location

areas "LAC%- t)e cell belonged to LAC<@(2&. A road- 1.(9m from t)e site- was under

t)e co$erage of LAC<@(&&.

/)roug) signaling tracing- we could see t)e large uantity of L!C under t)e cell was

from LAC<@(&& to LAC<@(2&.

,ig 4#& Signaling S)owed Large Iuantity of L!C under t)e Cell

After D/ on site- itGs disco$ered t)at t)e road s)ould be co$ered by LAC<@(&&- but t)e

congested cell signal was $ery strong t)ere- w)ic) means it stretc)ed its co$erage to

t)e road- and resulted in freuent L!C on t)e road and SD congestion.

$. SD congestion due to strong interference

!n one nig)t- large amount of SD congestion occurred at a 3/S "2% in a town- w)ic)

lasted for a long time.

:ormal condition didnGt return e$en after reset of C and /'

Congestion disa**eared after adustment of freuency and 3CC

Congestion *)enomenon a**eared again after freuency and 3CC were c)anged

2




bac9

Congestion totally disa**eared &< minutes after adustment on /A accesst)res)old.

!bser$ed signaling- disco$ered t)e SD congestion was caused by large uantity of

abnormal C)annel 'euest- and all t)e 6mm Assign generated from t)ese C)annel

'euest ended in failure.

All abnormal C)annel 'euest a**eared once e$ery four frames all 'A was <1 /A

diminis)ed from +& to <- t)en restarted from +& after 1( frames le$el $alue remained

+&.

,ig 4#4 Case 666#Large Iuantity of C)annel 'euest

:ormal condition didnGt return e$en after t)e reset of C and /'- w)ic) means

t)e *roblem was irrele$ant to 3/S )ardware and software

/)e *roblem disa**eared after adustment of freuency and 3CC- but reoccurred w)en

t)e freuency was c)anged bac9- t)is means t)e *roblem in$ol$ed in freuency

Considering C)annel 'euest rules- engineers confirmed t)ere was an interference

signal w)ose freuency was t)e same as t)e siteGs- and t)e signal ust contained all t)e

3



S :etwor9 SDCCH Congestion K Solutions

training seuence of A3 frame. /)e interference signal was *eriodical and it created

*eriodical de$iation to timeslot < window. Just because of t)is de$iation- / c)anged

*eriodically. 3esides- t)e interference signal ust interfered timeslot<. /)erefore-

adustment on /A access t)res)old could only relie$e t)e *roblem- but couldnGt sol$e it

com*letely.

/)ere was army garrisoning in t)e area- t)e interference signal was *robably sent from

t)e army.

$.$ SD congestion due to large ,uantit of (-C caused * H(Rcutover

Large amount of SD congestion occurred in a S networ9 on :o$. 1@t)- 2<t)- 21st and

22nd- <&<#1<<am.

Performance re*ort s)owed t)ere was a large number of L!C 'euest during

congestion.

/)e measurement data of L!C at SC s)owed t)at at <&<am on :o$.2< t)- t)e number

of inter#SC L!C attem*ts um*ed from 2<<< to o$er 2<<<<- and it returned to

normal after )alf an )our

/able 4#& Case 6?#Dramatic 6ncrease in L!C :umber

At <<<am and <&<am on :o$. 21st- t)e number of inter#SC L!C attem*ts um*ed

from &<<< to o$er 1<<< twice.

4

Time

2008-11-19 23:15 2966 0 2081

2008-11-19 23:30 2773 0 2011

2008-11-19 23:45 2561 0 1812

2008-11-20 0:00 2775 0 1987

2008-11-20 0:15 2432 0 16342008-11-20 0:30 20197 0 18797

2008-11-20 0:45 7632 0 6695

2008-11-20 1:00 3215 0 2506

2008-11-20 1:15 2561 0 1787

2008-11-20 1:30 2475 0 1677

Number of inter-MSCLOC suess-!SM

Number of inter-MSCLOC "ttem#ts-!SM

Number of inter-MSCLOC suess-$MTS




/able 4#4 Case 6?# Dramatic 6ncrease in L!C :umber

At 1<<am on :o$. 22nd- t)e number of inter#SC L!C attem*ts um*ed from +<<< to

o$er 1+<<<- and it returned to normal on t)e early morning of 2&rd and 24t).

!n t)e early morning of 2( t)- t)e number of L!C attem*ts was 1(<<<.

/)ese L!C attem*ts matc)ed closely wit) t)e time *oint of SD congestion and t)e

cause of SD occu*ancy.

/)ere was no ob$ious c)ange on t)e number of inter#SC L!C attem*ts during SD

congestion.

During congestion *eriod- t)e number of /S6 re#allocation also increased

dramatically.

!n#site engineers re*orted t)at :o9ia was cutting o$er :ortel HL' w)en SD

congestion occurred. /)erefore- we concluded t)at t)e cuto$er of HL' caused S toma9e large uantity of inter#SC L!C at midnig)t- resulting in SD congestion. 6f itGs

3SS fault- t)en not only t)e number of inter#SC L!C- but t)at of intra#SC L!C

will be affected.

$. SD congestion due to s"ort +essage 'ager

!n :o$. 2<t)- SD congestion of different degrees occurred in + cells under one 3/S.

Performance re*ort s)owed allocation failures of different le$els in t)e cells.

5

2008-11-20 23:15 3725 0 2467

2008-11-20 23:30 3790 0 2532

2008-11-20 23:45 5460 0 4134

2008-11-21 0:00 17118 0 15562

2008-11-21 0:15 6322 0 4809

2008-11-21 0:30 11784 0 10138

2008-11-21 0:45 8859 0 7287

2008-11-21 1:00 3954 0 2657

2008-11-21 1:15 3462 0 2133

2008-11-21 1:30 3674 0 2311

TimeNumber of inter-MSCLOC suess-$MTS

Number of inter-MSCLOC "ttem#ts-!SM -!SM

Number of inter-MSCLOC suess-!SM



S :etwor9 SDCCH Congestion K Solutions

/ransmission was confirmed wit) no *roblem after in$estigation.

C)ec9ed *erformance data of radio access- disco$ered t)at t)e cause of large uantityof radio access was S sending s)ort message

,or e0am*le- at 1<&<- in cell& t)e number of !C Access 'euest reac)ed @@(@

wit)in 1( minutes- w)ile t)e number of successful access was 1<<- and 1<4( of t)e

1<< times were s)ort message reuests.

3y obser$ing ot)er daily data of t)e 3/S- engineers found t)e traffic $olume was not

)ig).

6tGs concluded t)at t)e *roblem was caused by )ea$y transmission load. ass#sending

de$ice of s)ort message caused )ea$y transmission load- w)ic) led to LAPD delay- and

t)en led to SD allocation o$ertime- wasted SD resource- and finally resulted in SD

congestion.

6

gsm network sdcch congestion & solutions-16

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