TONG QUAN GSM

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    CHNG 4

    H THNG GSM

    4.1 Gii thiu chung Nm 1982, c quan qun l chnh v vin thng ca chu u (CEPT)

    thnh lp nhm c trch di ng (Groupe Special Mobile - GSM) vi nhim v tm ra h thng v tuyn t bo ton chu u hot ng trong di 900 MHz. H thng ny c hnh thnh khc phc nhng hn ch thy r v dung lng ca cc h thng tng t trin khai ti mt s nc chu u (v d NMT cc nc Bc u). Tiu chuNn t bo ton chu u phi h tr lu ng (roaming) quc t v to pht trin cho ngnh cng nghip vin thng chu u. Sau nhng tho lun ban u, ba nhm lm vic (WP) c thnh lp s l cc cng vic c th ca vic xc nh h thng, sau nhm th 4 c thnh lp tip. Nm 1986 b phn thng trc c lp ti Pari phi hp hot ng ca cc nhm lm vic v qun l vic to cc khuyn ngh h thng. Cc WP c yu cu xc nh cc giao din h thng sao cho my di ng, d dng cm tay hay lp trn xe, u c th lu ng qua cc nc c trin khai h thng mi v u c th truy cp tt c cc dch v. So vi h thng tng t hin c, h thng mi phi c dung lng cao hn, chi ph vn hnh bng hoc thp hn v cht lng ting ni phi bng hoc tt hn. Di tn chung ton chu u cho h thng mi l 890-915 MHz v 935-960 MHz.

    Cc nghin cu trong cc nc chu u khc nhau kt lun rng h thng s l ph hp hn h thng tng t, song vic la chn phng php a truy nhp vn cha c quyt nh. V th, ngi ta quyt nh th nghim mt s d n h thng khc nhau trn thc a (ti Paris, cui nm 1986). C 8 d n vi mt s c im c bn nh Bng 4.1.

    Trong nm 1987 cc kt qu th nghim c nh gi tho lun v cui cng t c tha thun v cc c trng ch yu ca h thng mi. n thng 6/1987 quyt nh chn TDMA di hp, vi 8 knh trn sng mang (c th m rng n 16 knh/sng mang). M ha ting ni c chn l loi RPE-LPC (d on tuyn tnh kch thch xung u) vi tc bt l 13 kb/s. M knh c chn l m xon, kiu iu ch c chn l GMSK nh hiu qu ph cao ca n.

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    Cc c t h thng GSM xut hin vo gia nm 1988. Tuy nhin, do khng th xc nh mi c im ca h thng cho kp khai trng vo nm 1991 nn c t c chia thnh 2 pha. Cc c t ca Pha 1 (cc dch v chung nht) hon thnh vo nm 1990. Pha 2 s xc nh cc dch v cn li (fax v.v..) ng thi sa li v ci thin cht lng ca h thng Pha 1. Theo yu cu ca Anh, mt phin bn ca GSM hot ng trong di 1800 MHz cng c a vo trong qu trnh xy dng c t. Phin bn ny c gi l h thng t bo s ti 1800 MHz (DCS1800). Pha 2 hon thnh nm 1993 v tip sau l Pha 2+, gm 1 s c im mi nh m ha ting ni bn tc, tng tc di chuyn ca my di ng (vn m bo lin lc tin cy).Ton b c t GSM dy hn 5000 trang.

    Trong chng ny ta tp trung ch yu vo giao din v tuyn ca GSM, v n c nh hng trc tip n dung lng h thng t bo.

    Bng 4.1. Mt s c im chnh ca cc h thng GSM mu Kiu a truy

    nhp Tc bt

    truyn (kb/s)

    Gin cch sng mang

    (kHz)

    Kiu iu ch

    S knh trn sng

    mang CD-900 CDMA/TDMA 7980 4500 4-PSK 63 MATS-D CDMA/TDMA

    FDMA 2496 19.5

    1250 25

    QAM GTFM

    32 1

    ELAB TDMA 512 600 ADPCM 12 DMS900 TDMA 340 300 GMSK 10 MOBIRA TDMA 252 250 GMSK 9 SFH-900 TDMA 200 150 GMSK 3 S900-D TDMA 128 250 4-FSK 10 MAX II TDMA 104.7 50 8-PSK 4

    4.2 Cu trc tng qut ca mng GSM Phn ny xt vn tt cc thnh phn khc nhau to thnh mng GSM.

    Nhiu phn l chung vi cc mng t bo khc, song mt vi phn l ring ca GSM. S khi n gin ca mng GSM mt t cng cng (PLMN) nh hnh 4.1. 4.2.1 Trm di ng (MS)

    Thu bao s s dng MS gi v nhn cuc gi qua mng GSM. MS gm 2 phn c chc nng khc nhau l modul nhn dng thu bao (SIM) v thit

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    b di ng (ME). SIM l th thng minh tho lp c, cha cc thng tin lin quan n thu bao c th, cn ME chnh l bn thn in thoi di ng (khng c SIM).

    ME c th chia thnh 3 khi chc nng. Th nht l thit b u cui (TE), thc hin chc nng ring cho dch v c th v d my fax. TE khng s l mt chc nng no lin quan n h thng GSM. Khi chc nng th 2 l u cui di ng (MT), thc hin mi nhim v lin quan n truyn thng tin trn giao din v tuyn GSM. Cui cng khi chc nng th 3 l b phi hp u cui (TA) dng bo m s tng thch gia MT v TA. V d cn TA to giao din gia MT tng thch - ISDN v TA c giao din modem.

    Hnh 4.1 Cu trc mng GSM SIM: l th thng minh c kch thc th tn dng hoc nh hn, c thu bao s dng c nhn ha ME. Lu rng trong GSM, MS lun l t hp ca SIM v ME. SIM c vng b nh c nh nh thng tin lin quan n thu bao (gm c IMSI). S ny dng nhn dng tng thu bao trong mng GSM v di khng qu 15 ch s thp phn. Ba ch s u tin l m nc di ng (NMC), dng nhn dng nc m thu bao ng k. Hai ch s tip theo l m mng di ng (MNC) dng nhn dng mng PLMN thng tr ca thu bao trong nc. Cc ch s cn li ca IMSI l s nhn dng thu bao di ng (MSIN) dng xc nh duy nht tng thu bao trong mng PLMN. SIM cn

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    cha cha kha nhn dng mt thu bao, Ki, thut ton nhn dng A3 v thut ton to cha kha mt m A8. Tt c cc khon ny (bt buc phi c) u c lu trong SIM v c bo v rt cht ch. Ngoi ra SIM cn c th cha 1 s chc nng ty chn khc nh quay s tt, danh b v.v..Cng nh c SIM m thu bao c th d dng thay i cc ME khc nhau, v nh khi phi sa cha ME.

    Mt trong nhng ng lc chnh pha sau s pht trin ca h thng GSM l cho php thu bao lu ng t do khp chu u m vn gi nguyn kh nng gi v nhn cuc gi bng cng mt MS. iu ny ch c th khi tn ti cc mng tng thch nhau ti mi nc. SIM a ra khi nim lu ng SIM tc l thu bao c th lu ng gia cc mng khc nhau khng tng thch vi nhau bng cch thu ME thch hp v cm SIM vo. iu ny tr nn c bit hp dn khi nc M gii thiu h thng PCS1900 (thng gi l GSM1900).

    4.2.2 H thng trm gc (BSS) MS lin lc vi trm thu pht gc (BTS) qua giao din v tuyn Um. BTS

    thc hin tt c cc chc nng thu pht lin quan n giao din v tuyn GSM v s l tn hiu mc nht nh. V 1 s phng din c th coi BTS l modem v tuyn phc tp nhn tn hiu v tuyn ng ln t MS ri bin i n thnh d liu truyn i n cc my khc trong mng GSM, v nhn d liu t mng GSM ri bin i n thnh tn hiu v tuyn pht n MS. Cc BTS to nn vng ph sng ca t bo, v tr ca chng quyt nh dung lng v vng ph ca mng. Tuy nhin BTS ch ng vai tr ph trong vic phn phi ti nguyn v tuyn cho cc MS khc nhau. Vic qun l giao din v tuyn c giao cho BSC, vi cc nhim v l: phn phi cc knh v tuyn cho MS khi thit lp cuc gi, xc nh khi no th tin hnh chuyn giao HO, nhn dng BTS ch ph hp v iu khin cng sut pht ca MS sao cho va ti c BTS ang phc v. Cc BSC thay i theo nh sn xut, song thng mt BSC c th iu khin ti 40 BTS. Ngoi kh nng s l, BSC cn c kh nng chuyn mch (hn ch), cho php n nh tuyn cc cuc gi gia cc BTS khc nhau thuc quyn kim sot ca n. Giao din gia BSC v BTS lin quan c gi l giao din A-bis. Tp hp BTS v BSC c gi l h thng trm gc (BSS).

    4.2.3 Trung tm chuyn mch di ng (MSC) T hnh 4.1 ta thy mi BSS c ni vi MSC. MSC m bo vic nh

    tuyn cc cuc gi n/i t MS. N c kh nng chuyn mch ln, thng

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    thng mt MSC iu khin vi chc BSC vi dung lng vi chc nghn thu bao. MSC tng t nh tng i trong mng c nh, song n cn c thm cc chc nng qun l di ng ca cc thu bao (ng k v tr v HO). c t GSM s dng thut ng vng MSC m t phn mng bao ph bi MSC v cc BSC, BTS lin quan. Giao din gia MSC v BSS c gi l giao din A, hon ton c xc nh trong c t. V th, cc nh khai thc mng c th t do la chn MSC v BSS t cc nh sn xut khc nhau. Giao din gia cc MSC khc nhau c gi l giao din E. Nh khai thc c th la chn 1 hoc mt vi MSC dng lm MSC ca ng (GMSC). GMSC cung cp giao din gia PLMN v cc mng ngoi. Khi c cuc gi n t mng ngoi, GMSC lin lc vi cc c s d liu mng lin quan bo m cuc gi c nh tuyn n MS thch hp.

    4.2.4 C s d liu mng GSM Trong phn trn ta xt cc b phn khc nhau trong GSM dng to ng lin lc gia mt MS v mt MS khc hoc ngi dng trong mng khc. Khng km phn quan trng trong cc mng thng mi l cc phng tin thanh ton tnh cc, duy tr s liu ng k chnh xc v ngn nga cc xm nhp mng tri php. Trong mng t bo, cc thu bao c t do lu ng trong vng ph sng, v th mng phi c cch no theo di cc MS nhm nh tuyn chnh xc cuc gi n chng. Tt c cc chc nng ny c thc hin nh s dng kt hp cc c s d liu hoc cc b ghi. B ghi v tr thng tr (HLR) lu gi cc thng tin ring ca cc thu bao, cc chi tit v ng k ca ngi dng (v d dch v) v thng tin v tr ca tng thu bao (v d: chi tit v vng MSC m thu bao hin thi ng k). C th truy cp thng tin trong HLR nu dng IMSI hoc MSISDN ca thu bao. Mi thu bao trong GSM u c u mc (entry) trong HLR ca mng ch. Giao din gia HLR v MSC c gi l giao din C.

    C s d liu khc lin quan cht ch vi HLR l trung tm nhn thc (AuC). AuC dng lu tr thng tin lin quan n kha cnh an ninh GSM, ngha l nhn thc ngi dng v mt m ha ng v tuyn. N cha cha kha nhn dng thu bao Ki v cc thut ton an ninh A3, A8. AuC ch lin lc vi HLR thng qua giao din H. C s d liu quan trng khc c dng trong GSM l b ghi v tr tm tr (VLR). VLR lin kt vi mt hoc vi MSC v cha thng tin lin quan n cc thu bao hin ang ng k trong vng MSC. Vng c phc v bi VLR c th c gi l vng VLR. Chc nng chnh ca VLR

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    l cung cp bn sao thng tin cc b (local) ca thu bao nhm mc ch s l cuc gi v trnh truy nhp lin tc HLR ( ly thng tin v thu bao c th). VLR cng cha cc thng tin cho php mng tm thy thu bao khi c cuc gi n.

    Qu trnh nh v thu bao tr nn d dng bng cch chia vng ph sng ca mng thnh mt s vng nh v (LA), mi LA gm