signalling optimization

1

I. Tnh ton knh iu khin trn giao din v tuyn

1. Gii thiu v h thng knh iu khin trn giao din v tuyn.

Trong h thng GSM c bn loi knh iu khin:

+ Broadcast Control Channel (BCCH).

+ Common Control Channel (CCCH).

+ Standalone Dedicated Control Channel (SDCCH).

+ Cell Broadcast Channel (CBCH), s dng mt knh SDCCH.

Khi s dng thm tnh nng GPRS/EGPRS th h thng c thm mt vi knh iu khin v knh lu lng cho d liu gi trn giao din v tuyn nh sau:

+ Packet Common Control Channel (PCCCH): bao gm nhng knh logic sau:

+ Packet Access Grant Channel (PAGCH): dng cho downlink n nh mt hoc mt vi knh PDTCH. C th truyn trn knh AGCH hoc PDCH.

+ Packet Broadcast Control Channel (PBCCH): dng cho downlink, c th c truyn trn knh BCCH hoc PDCH.

+ Packet Notification Channel (PNCH): dng cho downlink (knh ny khng dng trong phin bn u ca GPRS).

+ Packet Paging Channel (PPCH): dng cho downlink, c th c truyn trn knh CCCH hoc PDCH.

+ Packet Random Access Channel (PRACH): dng cho uplink yu cu n nh knh cho mt hoc mt vi knh PDTCH.

+ Packet Data Traffic Channel (PDTCH): l ti nguyn v tuyn n nh cho mt MS trn mt knh vt l truyn d liu ca thu bao.

+ Packet Dedicated Control Channel (PDCCH): bao gm nhng knh logic sau:

+ Packet Associated Control Channel (PACCH): l knh iu khin bo hiu gia MS v PCU trong qu trnh truyn d liu.

+ Packet Timing Advance Control Channel (PTCCH/U): dng cho uplink c s dng truyn burst truy cp ngu nhin. Khi thu pht s dng nhng burst ny nh gi timing advance cho MS trong qu trnh truyn d liu.

+ Packet Timing Advance Control Channel (PTCCH/D): dng cho downlink truyn thng tin cp nht timing advance ti mt vi MS trong cng mt khong thi gian.

2. Mt vi vn cn nhc khi tnh ton.

Khi tnh ton cu hnh knh iu khin trong h thng GSM/GPRS/EGPRS chng ta cn quan tm ti cc loi knh iu khin sau:

+ Yu cu v bo hiu ca loi knh CCCH.

+ Yu cu v bo hiu ca loi knh PCCCH (nu kch hot tnh nng ny)

+ Yu cu v bo hiu ca loi knh SDCCH.

S lng knh SDCCH c th c tnh ton mt cch c lp cn vic tnh ton s lng knh CCCH li ph thuc vo vic tnh ton knh PCCCH. Trn mng Vinaphone khng s dng ring bit loi knh PCCCH (tham s pccch_enabled = 0) iu ny c ngha l ton b bo hiu iu khin cho GSM v GPRS u c thc hin qua knh CCCH.

Khi tnh ton n s lng knh CCCH cn thit cho mt cell chng ta cn quan tm thm n cu trc knh iu khin BCCH l combined (cha 3 block CCCH trong mt a khung knh iu khin) hay l non-combined (cha 9 block CCCH trong mt a khung knh iu khin). Trong trng hp cn thit p ng nhu cu dung lng ln chng ta c th s dng thm timeslot cho a khung iu khin ch cha knh CCCH, khi mi mt timeslot s cho thm 9 block CCCH. Vic s dng theo cu hnh combined, non-combined hay thm timeslot tng dung lng knh CCCH s c cn nhc da trn s lng block cn thit ca knh CCCH v SDCCH.

Quay tr li chc nng ca knh iu khin CCCH, CCCH bao gm ba knh iu khin:

+ Paging Channel (PCH): l knh logic trn downlink dng cho mc ch tm gi MS cho vic nhn cuc gi, tin nhn hoc cc dch v khc.

+ Access Grant Channel (AGCH): l knh logic trn downlink dng cho mc ch n nh ti nguyn v tuyn (knh SDCCH) theo yu cu t pha MS.

+ Random Access Channel (RACH): l knh logic trn uplink dng cho mc ch yu cu ti nguyn v tuyn t pha MS thc hin cuc gi, nhn tin hoc cc dch v khc.

Chng ta cho rng nu p ng nhu cu cho downlink th ng thi cng nhu cu cho uplink. Trong a khung knh iu khin BCCH vic n nh s block CCCH cho PCH v AGCH l tu thuc vo nhu cu v chc nng m hai knh ny thc hin.

Mi mt block CCCH (gm 4 khung lin tip trong a khung iu khin) mang mt bn tin. Chng ta bit thi gian ca mt a khung iu khin 51 khung l 235,38 ms nn trong mt giy mt block CCCH trung bnh truyn khong 4,25 bn tin.

Knh AGCH dng truyn hai loi bn tin:

+ Immediate assignment message: mi bn tin AGCH loi ny c th n nh knh ti a cho 2 MS.

+ Immediate assignment reject message: mi bn tin AGCH loi ny c th t chi n nh knh ti a cho 4 MS.

Knh PCH dng truyn cc bn tin paging message, mi bn tin ny c th nhn tm cng mt lc 4 MS nu nhn tm theo TMSI v 2 MS nu nhn tm theo IMSI.

2.1 Tnh ton s block CCCH

Nh cp trn CCCH bao gm bo hiu cho c GSM v GPRS do vy chng ta cn tnh n s block ca AGCH v PCH p ng cho c GSM v GPRS. Sau y l nhng tnh ton c th.

S lng block CCCH cho mi cell xc nh bi cng thc:

S lng block AGCH c xc nh bi cng thc sau:

Trong , s lng block AGCH c yu cu dng cho GSM l:

Nh cp trn th mt bn tin immediate assignment dng n nh knh cho 2 yu cu cng mt lc nn:

S lng block AGCH c yu cu dng cho GPRS l:

trong :

Tc chp nhn truy cp c xc nh bi:

trong , tc cuc gi (s cuc gi/giy) l:

tc Location update l:

tc tin nhn l:

S lng block PCH c xc nh bi cng thc:

S lng block PCH dng cho GSM l:

nu nhn tm theo TMSI th:

nu nhn tm theo IMSI th:

S lng PCH dng cho GPRS l:

Trong :

: h s s dng CCCH.

: tc chp nhn truy cp.

: s lng thu bao GPRS trong cell

: s lng trung bnh cc session xut pht t GPRS user trong cell.

: tc cuc gi (cuc gi/s)

: tc tin nhn (tin nhn/s)

: tc location update (Location update/s)

: lu lng ca cell tnh theo Erlang

: di trung bnh ca cuc gi (theo giy)

: s lng bn tin paging ca GSM truyn ti cell trong mt giy

: s lng bn tin paging ca GPRS truyn ti cell trong mt giy

2.2 Tnh ton s block SDCCH

Trong h thng knh iu khin trn giao din Um, SDCCH thc hin cc chc nng c bn sau:

+ Thit lp cuc gi.

+ Truyn, nhn tin nhn.

+ Thc hin location update.

Tuy nhin khi tnh ton s block SDCCH trn a khung knh iu khin chng ta vn cn phi quan tm n nhng vn sau:

+ S lng ti a block SDCCH ca mt cell l 44 (nu dng combined BCCH) hoc 48 (nu dng non-combined BCCH).

+ Nhu cu s dng knh SDCCH ph thuc vo nhiu yu t: cc tham s ca m hnh cuc gi, c s dng tnh nng truyn tin nhn qua GPRS hay khng ..

Cng thc tnh ton lu lng SDCCH ca mt cell nh sau:

trong :

: Thi gian trung bnh thit lp cuc gi.

: Thi gian trung bnh thc hin location update.

: Thi gian trung bnh truyn, nhn tin nhn.

: Thi gian bo v ca SDCCH.

Cn c vo kt qu trn, s dng bng Erlang B vi mt gi tr GoS nht nh chng ta s c c s knh SDCCH cn thit.

Cng thc trn ch cho ta thy cch thc tnh ton lu lng knh SDCCH, tuy nhin ta cng c th ly c lu lng SDCCH t thng k ca h thng qua counter .......................

II. Tnh ton bo hiu iu khin bi BSC

Trong cu trc mng GSM, BSC thc hin giao tip vi cc thc th mng khc qua cc ng bo hiu sau:+ RSL (Radio Signalling Link): thc hin bo hiu gia BSC v BTS theo giao thc LAPD.+ MTL (Message Transfer layer Link): thc hin bo hiu gia BSC v MSC theo giao thc C7.+ XBL (RXCDR BSC Link): thc hin bo hiu gia BSC v TRAU theo giao thc LAPD.+ GSL (GPRS Signalling Link): thc hin bo hiu gia BSC v PCU theo giao thc LAPD.1. Tnh ton s lng knh RSL.Trong h thng BSS Motorola mt BTS phi c t nht l mt knh RSL trn mt E1 kt ni vi BSC. Tuy nhin, nu nhu cu v bo hiu gia BTS v BSC ln th phi tng thm s knh RSL. p dng cc cng thc tnh ton sau y nu tho mn cc iu kin sau:

+ Tc mt knh RSL l 64 kb/s.

+ Khng s dng knh PCCCH trn giao din v tuyn.

+ Truy cp GPRS s dng tnh nng one phase access.S lng RSL cn thit cho mt BTS l:

trong , nhu cu bo hiu cho GSM l:

v nhu cu bo hiu cho GPRS l:

Trong :

: s TCH ca BTS.

: thi gian trung bnh ca cuc gi.

: t s s SMS trn cuc gi.

: t s s Handover trn cuc gi.

: t s Location Update.

: phn trm s dng knh RSL.

: kch thc SMS trung bnh theo byte.

: s cell ca BTS.

: tc paging GSM trn giy.

: tc paging GPRS trn giy.

: S user GPRS trong BTS.

: s session trung bnh trn user.2. Tnh ton s lng knh MTL.Lu lng bo hiu thc hin trn MTL ph thuc vo cu hnh v m hnh lu lng ca BSS. i vi mt mng ang hot ng chng ta s tnh ton theo m hnh lu lng thc t (non-standard). S lng MTL cn ph thuc vo vic h thng c s dng tnh nng load share granularity hay khng. Tnh nng ny cho php nng cao kh nng chia s ti gia cc knh MTL bng vic tng s knh logic gia BSC v MSC t 16 ln 64. Nh , s knh MTL cn thit s st hn vi lu lng tng ng ca BSS. Khi cha s dng tnh nng load share granularity m bo cn bng ti gia cc knh MTL th s MTL phi thuc dy sau: 1, 2, 4, 8, 16. Nh vy, s c tnh hung khi lu lng ca BSS tng ln mt lng nh th chng ta phi tng s knh MTL t 8 ln 16. Nhng khi h thng s dng tnh nng load share granularity th s MTL cn thit c th nm trong dy rng hn t 1 n 16 m vn m bo cn bng ti chp nhn c gia cc knh MTL v nh trong tnh hung trn chng ta ch phi tng s knh MTL t 8 ln 10 thay v 16 nh trc.Vic cn nhc gi tr ln nht ca h s s dng knh MTL (UBSC-MSC) cng s quyt nh n s knh MTL cn thit. Khi s dng GPROC 1 th h s s dng MTL ti a l 0.2 nhng khi s dng GPROC 2 hoc GPROC 3 th h s s dng MTL ti a c y ln n 0.4. Tuy nhin chng ta phi xem xt n c nng lc s l ti MTL c pha thit b MSC.Sau y l cc bc tnh ton s knh MTL:+ Xc nh lu lng ti a (tnh theo Erlang) m mt knh MTL c th phc v:

+ Xc nh lu lng ti a m mt GPROC qun l MTL c th phc v:

+ Nh vy, lu lng ti a m mt MTL thc s phc v phi l gi tr nh nht ca mt trong hai gi tr trn:

+ Xc nh lu lng m mt knh logic phi phc v:

+ Xc nh s lng knh logic trn mt knh MTL:

+ Cui cng, s lng knh MTL cn thit l:

Vi s knh MTL cn thit phi nh hn hoc bng 16.Trong :

: h s s dng knh MTL.

: Thi gian trung bnh ca cuc gi.

: t s SMS trn cuc gi.

: t s Handover trn cuc gi.

: t s intra-BSC handover trn tng s handover.

: kch thc trung bnh ca mt SMS tnh theo byte.

: t s location update.

: t s nhn tin trn cuc gi.

: s BTS c trong BSS.

: lu lng ca ton b BSS.

: s knh logic gia BSC v MSC.

: s knh MTL d phng.3. Tnh ton s lng knh XBL.Knh XBL thc hin bo hiu gia BSC v remote TRAU do vy lu lng bo hiu ph thuc vo nhng yu t sau:+ S knh trung k (CIC) gia BSC v TRAU.+ Nu h thng s dng tnh nng auto-connect hay enhanced auto connect th lu lng bo hiu trn XBL s tng ln v khi gn knh trung k gia BSC v TRAU theo kiu ng ch khng phi kiu tnh nh trc. Ngoi ra khi h thng a vo s dng thm cc kiu m ho thoi nh: enhanced full rate, GSM half rate, AMR full rate, AMR half rate th i vi mi mt cuc gi sau khi MSC n nh knh cho BSC th BSC phi bo hiu vi TRAU xem knh c kh nng h tr nhng loi m ho thoi no tr li cho MSC. iu ny cng lm tng lu lng bo hiu trn XBL.S knh XBL ti a gia mt BSC v mt TRAU l 20 v mt BSC c th ni ti a vi 10 TRAU v ngc li. H s s dng knh XBL ti a l 0.4Sau y l cng thc tnh ton s knh XBL cho m hnh non-standard:

trong :

: s trung k gia BSC v TRAU (CIC).

: thi gian trung bnh cuc gi.

: s bn tin XBL trung bnh cho mt cuc gi mi.

: s bn tin XBL trung bnh cho mt handover gia full rate v half rate.

: s handover gia half rate v full rate cho mt cuc gi.

: chiu di trung bnh ca bn tin XBL theo byte.

: h s s dng knh XBL.4. Tnh ton s lng knh GSL.Trong h thng GPRS, BSC ch ng vai tr trung gian gia PCU v cc cell c trin khai GPRS. Ton b bo hiu GPRS t cell c ng gi v truyn qua knh RSL ln n BSC sau c tp hp v truyn n PCU qua knh GSL. Mi mt PCU yu cu phi c ti thiu mt lung E1 mang bo hiu GSL v mt lung E1 khc d phng. Mi mt PCU c th h tr ti a l 30 knh GSL 64 kb/s v 30 knh khc d phng. Bo hiu GSL thc hin theo giao thc LAPD v hot ng theo phng thc cn bng ti trn c hai E1 cho mi PCU.S lng knh GSL c ch ra bi cng thc sau:

Trong qu trnh h thng khi to cc tin trnh ban u th cn t nht l 6 knh GSL nh vy: .Sau khi h thng hot ng n nh th s knh GSL cn thit s c tnh ton bi cng thc sau:

vi bo hiu GSL cho paging l:

v khi s dng ch truy cp one phase access th bo hiu GSL cho truy cp GPRS l:

Trong :

: h s s dng knh GSL.

: tc paging GPRS trong mt giy.

: S lng LCF qun l RSL.

: s thu bao GPRS trong PCU vo gi cao im.

: trung bnh session cho mt user vo gi cao imIII. M hnh cuc gi.1. Thi gian trung bnh cuc gi -Call Duration (T).Thi gian trung bnh cuc gi c tnh ton t cc s liu thng k theo cng thc sau:

T c th tnh ton cho nhiu mc: cell (N=1), site (N=s cell ca site), BSS (N=s cell trong BSC), network (N=s cell ca mng).BUSY_TCH_MEAN: lu lng trung bnh tnh theo Erlang ca cell trong gi cao im.TOTAL_CALLS: s ln n nh thnh cng TCH trong giai on u thit lp cuc gi.

ASSIGNMENT_REDIRECTION: s ln n nh TCH c chuyn hng sang mt cell khc do qu trnh redirected retry handover, mutilband reassignment hoc handover trong qu trnh gn knh TCH.stat_interval_in_sec: khong thi gian m s liu thng k c thu thp tnh theo giy. Nu s liu thng k ly theo gi th tham s trn l 3600.2. T s bn tin SMS trn cuc gi Ratio of SMSs per call (S)T s bn tin trn cuc gi c tnh ton t cc s liu thng k theo cng thc sau:

Tng t nh T, S c th c tnh cho nhiu cp khc nhau: cell (N=1), site (N=s cell ca site), BSS (N=s cell trong BSC), network (N=s cell ca mng).

SMS_NO_BCAST_SMG: s ln cc bn tin c truyn trn knh CBCH.SMS_INIT_ON_SDCCH: s lng SMS c truyn trn knh SDCCH.SMS_INIT_ON_TCH: s lng SMS c truyn trn knh TCH.TOTALL_CALLS: s ln n nh thnh cng TCH trong giai on u thit lp cuc gi.ASSIGNMENT_REDIRECTION: s ln n nh TCH c chuyn hng sang mt cell khc do qu trnh redirected retry handover, mutilband reassignment hoc handover trong qu trnh gn knh TCH.

3. T s Handover trn cuc gi Ratio of handovers per call (H)Handover c phn thnh cc loi nh sau: inter BSS, intra BSS v intra cell. Do vy, t s handover trn cuc gi c tnh nh sau:

Trong :N: c hiu tu theo tng cp (tng t S v T).

out_inter_bss_req_to_msc: s lng request gi ln MSC thc hin interBSS handover.out_intra_bss_ho_atmp: s lng cc assignment command message c gi xung MS khi to mt qu trnh intra-BSS handover.intra_cell_ho_atmp: s lng cc assignment command message c gi xung MS khi to mt qu trnh intra-cell handover.TOTALL_CALLS: s ln n nh thnh cng TCH trong giai on u thit lp cuc gi.

ASSIGNMENT_REDIRECTION: s ln n nh TCH c chuyn hng sang mt cell khc do qu trnh redirected retry handover, mutilband reassignment hoc handover trong qu trnh gn knh TCH.

4. T s intra-BSS handover trn tng s handover Ratio of intra BSS handver to all handvers (i)T s intra-BSS handover trn tng s handover c tnh theo cng thc sau:

Trong :

N: c hiu tu theo tng cp (tng t T).

out_inter_bss_req_to_msc: s lng request gi ln MSC thc hin interBSS handover.out_intra_bss_ho_atmp: s lng cc assignment command message c gi xung MS khi to mt qu trnh intra-BSS handover.intra_cell_ho_atmp: s lng cc assignment command message c gi xung MS khi to mt qu trnh intra-cell handover.5. T s Location update trn cuc gi Ratio of Location updates per call (l)T s location update trn cuc gi c tnh theo cng thc sau:

Trong :N: c hiu tu theo tng cp (tng t T).

OK_ACC_PROC[Location_update]: s lng yu cu location update c MS gi ln mng.TOTALL_CALLS: s ln n nh thnh cng TCH trong giai on u thit lp cuc gi.


6. T s IMSI detach trn cuc gi Ratio of IMSI detachs per call (I)T s ny s bng khng nu tnh nng IMSI detach khng s dng, nu tnh nng ny s dng n s c tnh theo cng thc sau:

Trong :

N: c hiu tu theo tng cp (tng t T).

OK_ACC_PROC[imsi_detach]: s lng yu cu IMSI detach c MS gi ln mng.TOTALL_CALLS: s ln n nh thnh cng TCH trong giai on u thit lp cuc gi.


7. T s Location update Location update factor (L)T s location update (L) c tnh ton t hai thng s t s location update trn cuc gi (l) v t s IMSI detach trn cuc gi (I).Trong trng hp mng khng s dng tnh nng IMSI detach th:

L = l

Cn khi s dng tnh nng trn th cng thc ca L tu thuc vo h thng s dng bn tin IMSI detach loi 1 hay loi 2:

+ Nu l loi 1: L = l + 0.2*I

+ Nu l loi 2: L = l + 0.5*IS khc nhau gia hai loi IMSI detach ph thuc vo cch thc m MSC thc hin th tc xo kt ni vi BSS sau khi nhn c bn tin IMSI detach. Nu loi 1, MSC thc hin th tc xo kt ni vi BSS ch mt duy nht mt bn tin mi hng uplink v downlink th loi 2 phi mt 3 bn tin mi hng. Trong khi , th tc location update phi mt 5 bn tin uplink v 6 bn tin downlink. Do , khi s dng IMSI detach loi 1 th t l s dng bn tin gia th tc detach v location update l 2/11 (xp x 0.2) v khi s dng IMSI detach loi 2 th t l ny l 6/11 (xp x 0.5). 8. T l paging Paging rate (PGSM)Khi thc hin th tc tm gi mt MS no , MSC s gi bn tin PAGE_REQ_FROM_MSC ti tt c cc BSS c LAC hin thi ca MS v mi bn tin ny ch cha ng ni dung tm gi cho mt MS. Khi nhn c bn tin trn, BSS s nhn tm trn tt c cc cell ca n c cng LAC vi LAC hin thi ca MS cn nhn tm v mi mt bn tin nhn tm gi xung cc MS trn giao din Um s cha ng ni dng tm gi cho 2 MS (nu s dng IMSI paging) hoc 4 MS (nu s dng TMSI paging). Nh vy, nu trong mt BSC c hai hay nhiu LAC tr ln th s bn tin paging gi ti BSC s c cng gp t hai hay nhiu cell i din cho tng LAC ca BSC. V cng thc t l paging c tnh ton nh sau:

Trong :PAGE_REQ_FROM_MSC: l s lng cc bn tin nhn tm gi t MSC ti cc cell c cng LAC vi MS cn nhn tm.Stat_interval_in_seconds: khong thi gian thng k tnh theo giy (thng thng l 3600).

Khi tnh ton PGSM cho mc cell th N = 1, cn khi tnh ton cho mc site hay mc BSS th N bng s LAC c trong site hay BSS v nh trnh by trn s lng bn tin paging s c tnh cng gp t cc cell i din cho tt c cc LAC c trong site hoc BSS.9. S paging trn cuc gi Pages per call (PPC)S paging trn cuc gi c tnh cho mt BSC theo cng thc sau:

Trong :

N: c hiu theo ngha ging nh trng hp tnh PGSM cho mc BSS.PAGE_REQ_FROM_MSC: l s lng cc bn tin nhn tm gi t MSC ti cc cell c cng LAC vi MS cn nhn tm.

TOTALL_CALLS: s ln n nh thnh cng TCH trong giai on u thit lp cuc gi.


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signalling optimization

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