phân biệt ghép kênh và Đa truy

Phn bit ghp knh v a truynhp15/11/2013HUNG CAO LI BNH LUN

y xt n trng hp c th TDM v TDMATDM=time division multiplexingTDMA=time division multiple accessTDM v TDMA l hai khi nim khc hn nhau. Tuy nhin, s na n nhau gia chng rt d gy nhm ln gia cc khi nim, k c i vi mt s thy ta. Ti s c gng trnh by tht vn tt, i ch s c th c cc v d two in one, mong cc bn chu kh suy ngh i cht.TDM l khi nimghp knh, thuc phm vitruyn dn, trong khi TDMA l khi nima truy nhp, thuc phm viaccess (truy nhp mng). Vic kim sot iu khin TDM thuc chc nng lp 1 (transmission layer) trong m hnh OSI cn kim sot iu khin TDMA thuc chc nng lp 2 (hoc 3),ch t cng khng thuc lp 1. Ghp knh l khi nim v t chc truyn dn gia hai nt ca mt mng, trong khi a truy nhp lin quan ti vic t chc kt ni t thu bao ti mng.TDM thng dng khi ni n ghp knh trn mt ng truyn point-to-point, hu tuyn nh T1/E1; cn TDMA th dng khi ni n cc h thng thng tin di ng, ng truyn v tuyn. TDM ch lin quan n 1 transmitter v 1 receiver cn TDMA th c nhiu transmitters.Mc d cng c im l chia khung thi gian thnh cc time slot (TS) cho nhiu ngi chia s ti nguyn song c nhng im khc bit c bn gia TDM vTDMA sau y:TDM l s phn nh dung lng tng cng ca knh tng theo thi gian cho cc ngi s dng khc nhau trn c s t u cui ti u cui v khng thay i TS dnh cho ngi s dng c nu khng config li ton b h thng. Ngha l mi mt ngi s dng s c n nh mt TS c nh truyn tin v d cho ngi s dng khng c traffic truyn trn knh vt l (TS) dnh cho mnh th knh s rng m ngi s dng khc khng th t ng chim knh truyn tin. Ngay c khi ta c c mun dng mt cch nht thi TS cho ngi s dng khc dng cng khng th c. Ly th d: Trong mt tuyn truyn dn quang t H Ni i Hi Phng gi d vi knh dung lng l lung STM-4 c 3 lung STM-1 c nh (3TS dnh cho 3 lung STM-1) c dnh cho kt ni t H Ni i Hi Phng, cn mt lung STM-1 dnh cho kt ni t H Ni n Hi Dng v t Hi Dng ti Hi Phng. Khi trn STM-1 lin lc H Ni Hi Dng khng c traffic th h thng cng s khng t ng ly knh cho truyn t H Ni i Hi Phng c v on Hi Dng Hi Phng li c th ang c traffic (on HNI HDG c th ang rng mt phn dung lng song an HDG HPG li ang y).a truy nhp l phng thc m mng s dng phn bit cc thu bao khc nhau ang truy nhp yu cu dch v vin thng. Vi mng in thoi c nh, mng (tng i) phn bit cc thu bao (vo lc truy nhp th l ngi s dng/user) nh cc i dy ti cc my l, gi l phng thc truy nhp bng cp.TDMA l phng thc m mng phn bit ngi s dng theo thi gian. Mng phn bit gia cc users khc nhau ang truy nhp mng yu cu dch v thng qua TS c n nh tm thi cho cc users. Mt khi ngi s dng chm dt phin lin lc ca mnh th ti nguyn (TS) s c gii phng tr li cho mng c th n nh cho ngi s dng khc (cho thu bao khc ang c gng truy nhp mng yu cu dch v). Cng xin lu lun vi cc bn v s khc bit gia cc khi nim thu bao (subscriber) v khi nim ngi s dng (user), ci m theo kinh nghim ca ti, cc SV, v thm ch nhiu thy dy vin thng ta, cn rt m m, d nhm ln. Trong ghp knh khng c khi nim thu bao m ch c user (cn phn bit r khi nim thu bao v khi nim ngi/t chc thu leased line).Vi cc h thng thng tin di ng th a truy nhp l phn access t cc thu bao ti mng m i din y l BTS (phn mng truy nhp access network), cn t nt mng BTS ti cc nt khc trong mng (BSC hay MSC) th knh c t chc bng ghp knh (phn mng/h thng truyn dn transmission network/system). Th d, vi h thngGSM th TDMA thuc phn t MS ti BS cn TDM thuc phn t BS v mng (v MSC). Trn tuyn t cc BS v MSC trong h thng GSM, gi nh t chc truyn dn theo cu hnh chain, d lung E1 t BS ny v BSC c rng (do trong cell m BS qun l ang khng c mt MS no lin lc) th t mt BS xa hn v BSC ngang qua BS cng khng th chim lung E1 truyn tin cho mnh nu khng config li h thng truyn dn.Cu tr li tng t cho phn bit FDM vs. FDMA hay CDM vs. CDMA (tuy nhin t nghe ni n CDM)Theo mnh hiu th ghp knh c chc nng lp 1, tc l ch x l (mang tnh cht vt l) truyn d liu, vic nhn d liu li cn phi phn knh, nh v d trn cng thy l n khng c thng minh cho lm khi khng chim c ng truyn ri (v mi c tnh nng ca 1 lp thi), ch truyn cc ch thu phn knh ra m ly d liu. V thy cng khng chuyn theo nhiu hng khc nhau song song c m nh trn th cn cc im tch ghp v ng truyn gia cc im ch c 1 chiu.Cn a truy nhp th ging nh ta nh dng modem share nhiu my hay access point (b pht wifi), cng truy cp, cng chia s, khi t ngi th chim c nhiu tc cn khi nhiu ngi cng dng th chia s tc thp thp thi. Cc bn c th xem yv cc thit b lp 2 v lp 3 cho n c lin h thc t.