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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$er.............................................14
4.( SD congestion due to s)ort message *ager.......................................................................................1+
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1 SD Seizure Signaling & RelevantCounters
1.1 SD seizure and release flow
,ig 1#1 SD Seizure and 'elease ,low
As s)own in ,ig 1 #1- during L!C and t)e early stage of !C and /C *rocess- S
usually seizes SDCCH to e0c)ange signaling. /)e signaling flow is as follows
After recei$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
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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 $alue 1<
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C)a*ter & Procedure of C)ec9ing SD Congestion SD
Descri*tion
6f networ9 )as no a**ro*riate c)annel to allocate to S after it )as recei$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 $alue for /&122 is 1<E1(s- and 1(E2(s in areas wit)
dense traffic.
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2 Classification of SDCCH CongestionCauses
,rom t)e as*ect of singling flow- t)ere are two main ty*es of causes of SD congestion.
/oo many C)annel 'euired e0ceeds networ9 ca*acity and all SDCCHs are occu*ied.
/oo#many C)annel 'euired means t)e cell is busy- w)ile few SDCCH are
configured- w)ic) results in freuent occu*ancy of SDCCH and o$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$erflow.
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!rocedure of C"ecking SD CongestionSD
1. 'ead *erformance re*ort- c)ec9 if all cells or some cells under t)e 3/S are
congested
6t rarely )a**ens t)at all cells under t)e 3/S are congested. 6f it )a**ens- t)e
causes are usually related to terrestrial transmission and *arameter
configuration.
2. 'ead *erformance re*ort- c)ec9 if c)annel allocation failure "i.e. c)annel
acti$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.
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C)a*ter & Procedure of C)ec9ing SD Congestion SD
busy and its traffic $olume is )ig)- and e0*ansion is needed.
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,rom analysis of radio access measurement- itGs disco$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
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C)a*ter & Procedure of C)ec9ing SD Congestion SD
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$ed in freuency
Considering C)annel 'euest rules- engineers confirmed t)ere was an interference
signal w)ose freuency was t)e same as t)e siteGs- and t)e signal ust contained all t)e
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S :etwor9 SDCCH Congestion K Solutions
training seuence of A3 frame. /)e interference signal was *eriodical and it created
*eriodical de$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
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C)a*ter & Procedure of C)ec9ing SD Congestion SD
/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
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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.
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