1. Ghp knh phn tn (FDM)

DMF l k thut tng t c th c ng dng khi bng thng ca lin kt ln hn bng thng kt hp ca cc tn hiu c truyn. Trong FDM, tn hiu c sinh ra mi khi thit b gi iu ch cc tn s mang khc nhau. Cc tn hiu iu ch sau c kt hp thnh mt tn hiu n c th truyn i qua mt link. Cc tn s mang c phn chia theo bng thng sao cho ph hp vi tn hiu iu ch. Bng thng c chia thnh cc knh theo phm vi (range) qua cc tn hiu khc nhau c th i qua. Cc knh phi c phn tch bi cc di bng thng khng bao gi c s dng (gi l di bo v - guard band) ngn cn s chng lp gia cc tn hiu. Ngoi ra, cc tn s mang khng c lm nhiu tn s d liu gc. Nu cc iu kin trn khng c tn trng c th dn n cc li truyn dn nghim trng.

Hnh 8.3 minh ha khi nim FDM. Trong minh ha ny, lin kt vt l c chia thnh 3 phn, mi phn biu din mt knh.

Hnh 8.3 Phng php ghp knh FDM

Qu trnh x l FDM:

Hnh 8.4 minh ha v mt thi gian k thut x l ghp knh. FDM l mt qu trnh x l tng t m chng ta minh ha thng qua h thng in thoi. Mi mt in thoi sinh ra mt tn hiu trong di tn ging nhau. Trong b ghp knh, cc tn hiu ging nhau c iu ch thnh cc tn s mang khc nhau (f1, f2 v f3). Sau cc tn hiu iu ch c kt hp thnh mt tn hiu hn hp truyn i qua mt lin kt c bng thng ph hp.

Hnh 8.4 X l ghp knh FDM theo thi gian

Hnh 8.5 minh ha v mt tn s khi nim FDM (lu rng trong trng hp ny trc honh biu din tn s ch khng phi biu din thi gian, v ti mi thi im c 3 tn s tn ti trong bng thng). Trong FDM, cc tn hiu c iu ch thnh cc tn s mang ring bit (f1, f2 v f3) s dng k thut iu ch AM hoc FM.

Hnh 8.5 X l ghp knh FDM (biu din theo tn s)

Cn lu rng, s dng lin kt vt l hiu qu, chng ta cn thm bng thng m rng (gi l di bo v) phn tch cc knh.

Phn knh

B phn knh s dng cc b lc phn tch cc tn hiu c ghp knh thnh cc tn hiu cu thnh n, sau cc tn hiu ny c truyn ti b gii iu ch thc hin vic phn tch thnh tn hiu gi v truyn ti cc thit b nhn.

Hnh 8.6 minh ha v mt thi gian k thut ghp knh FDM, xem 3 in thoi nh l cc thit b truyn thng. Hnh 8.7 minh ha v tn s k thut ghp knh FDM.

Hnh 8.5 X l phn knh FDM (biu din theo thi gian)

Hnh 8.7 X l phn knh FDM (biu din theo tn s)2. K thut ghp knh phn thi (TDM)

Ghp knh phn thi l k thut x l s, c th ng dng khi tc truyn d liu trung bnh ln hn tc truyn d liu yu cu bi thit b gi v nhn. Trong trng hp ny, nhiu truyn dn phc tp c th chim lnh mt lin kt vt l bng cch chia nh chng v chn vo cc khe khc nhau.

Hnh 8.10 minh ha khi nin TDM.

Hnh 8.10 Phng php ghp knh TDM

Ghp knh phn thi ng b

Trong ghp knh phn thi ng b, thut ng ng b c ngha khc vi thut ng ng b s dng trong lnh v truyn thng. Thut ng ng b y c ngha l b ghp knh phn chia cc khe thi gian ging nhau cho mi mt thit b ti tt c cc thi im khng phn bit v thit b c thc hin truyn dn hay khng. V d, khe thi gian A c gn cho thit b A, khi khe thi gian A s khng th c s dng bi cc thit b khc. Ti mi thi im, thit b c khe thi gian tng ng c phn s c hi gi mt phn d liu ca n. Nu nh mt thit b khng th thc hin truyn dn hoc khng c d liu gi th khe thi gian ca n s duy tr trng thi rng.

Frames

Cc khe thi gian c nhm thnh cc frame. Mi mt frame gm mt hoc nhiu vng khe thi gian hon chnh, bao gm mt hoc nhiu khe dnh cho tng thit b gi (hnh 8.11). Trong h thng c n ng vo, mi mt frame c it nht n khe, mi mt khe c ch nh mang d liu t mt ng vo xc nh. Nu tt cc cc thit b vo chia s mt lin kt (link) ang truyn dn cng mt tc th mi thit b s c mt khe thi gian trn mt frame. Tuy nhin, chng ta c th iu chnh tc truyn. Thc hin truyn dn vi 2 khe thi gian/frame s nhanh hn 2 ln truyn dn vi 1 khe thi gian/frame. Cc khe thi gian c ch nh ng vi mt thit b s chim c (nm ) v tr ging nhau trong mi frame v to thnh knh ca thit b . Trong hnh 8.11, chng ta ch ra b ghp knh gm 5 ng vo s dng k thut TDM ng b. Trong v d ny, tt c cc ng vo u c cng tc truyn, v vy s khe thi gian trn mi mt frame s bng s ng vo.

Hnh 8.11 Ghp knh phn thi ng b TDM

Interleaving K thut chuyn dch lun phin

TDM ng b c th so snh vi mt ci cng tc chuyn mch quay cc nhanh. Khi m cng tc pha trc ca thit b, thit b c c hi gi mt s lng d liu xc nh (x bits) trn ng truyn (path). Cng tc chuyn dch t thit b ny n thit b khc vi tc khng i v theo mt trt t xc nh. K thut x l ny gi l interleaving.

Interleaving c th thc hin theo bit, byte hoc theo mt n v d liu bt k. Ni cch khc, b ghp knh c th nhn mt byte t mi thit b, sau ly mt byte khc t mi thit b, vv,,, Trong h thng nh sn, cc n v (units) interleaving s lun lun c cng kch thc.

Hnh 8.12 minh ha interleaving v cch xy dng cc frame. Trong v d ny, chng ta dch chuyn lun phin vic truyn dn cc k t khc nhau. B ghp knh s interleaving cc thng bo khc nhau v t chc chng thnh cc frame trc khi gi chng ln ng truyn (link).

Hnh 8.12 ng b TDM, qu trnh dn knh

b nhn, b phn knh s phn r tng frame bng cch rt trch ln lt cc k t. Khi k t c ly ra khi frame n s c chuyn ti thit b nhn tng ng (xem hnh 8.13).

Hnh 8.12 ng b TDM, qu trnh phn knhHnh 8.12 v 8.13 ch ra nhng nhc im chnh ca k thut TDM ng b. Bng cch gn mi mt khe thi gian ng vi mt ng u vo (input line), th khi cc ng truyn khng hot ng ng thi chng ta s c nhng khe thi gian rng, vn ny gy ra s lng ph ln v tn dng kh nng ca ng truyn. Trong hnh 8.12, ch c 3 frame u cha y d liu, 3 frame sau c 6 khe rng, c ngha l chng ta lng ph 6/24=1/4 kh nng ca ng truyn.

Framing bits bit ng b frame

Bi v th t khe thi gian trong h thng TDM ng b khng thay i gia cc frame nn chng ta cn thm mt cht thng tin vo mi frame. ???? Th t tip nhn bo cho b phn knh ch qun l (iu khin) tng khe thi gian do khng cn nh a ch. Tuy nhin c rt nhiu yu t c th gy nn s tri ngc nhau trong iu phi thi gian. V l do ny, ngi ta thng thm vo u mi frame mt hoc nhiu bit ng b. Nhng bit ny, c gi l framing bit, sau mt mu, t frame ny ti frame khc, m n cho php b phn knh ng b vi dng d liu n v vy n c th phn tch cc khe thi gian chnh xc. Trong phn ln cc trng hp, thng tin ng b ny gm 1bit/frame, hon chuyn gia 0 v 1 (01010101010) nh ch ra hnh 8.14

Hnh 8.14 Khun dng bt

V d v TDM ng b

Gi s chng ta c 4 ngun vo trong mt lin kt TDM ng b, qu trnh truyn dn s truyn cc k t c thc hin theo cch dch chuyn lun phin. Nu mi ngun to ra 250 k t/giy v mi mt frame mang mt k t t mi ngun th ng truyn dn (path) phi c kh nng mang 250 frame/giy (xem hnh 8.15).

Hnh 8.15 Tnh ton tc truyn d liu trong khung

Nu chng ta gi s rng m mt k t gm 8bits th mi frame s c 33 bits (32 bits dng cho 4 k t + 1 framing bit). Nh vy mi thit b to ra 2000bps (250 k t x 8bps), do ng truyn (line) phi c kh nng mang 8250bps (250 frame x 33bit/frame).

Bit Stuffing Nhi bit

Nh cp trong mc trc, chng ta c th kt ni cc thit b c tc truyn khc nhau truyn dn theo k thut TDM ng b. V d, thit b A s dng mt khe thi gian, trong khi tc truyn ca thit b B s dng 2 khe thi gian. S khe trong frame v cc ng vo (input line) s c gn c nh trong h thng, nhng cc thit b c tc truyn khc nhau c th kim sot s lng khe khc nhau. Cn nh rng chiu di ca khe thi gian l c nh. Do , v l thuyt, k thut ny lm vic, tc truyn d liu khc nhau phi l bi s ca nhau. V d, chng ta c th iu chnh mt thit b truyn nhanh hn cc thit b khc 5 ln bng cch gn cho n 5 khe v gn cho cc thit b khc 1 khe. Tuy nhin chng ta khng th iu chnh mt thit b truyn nhanh hn cc thit b khc 5,5 ln bng phng php ny bi v chng ta khng th a khe vo trong frame.

Khi tc truyn gia cc thit b khng phi l bi s ca nhau, chng ta vn c th thc hin vic truyn dn nh cch chng ta thc hin bng k thut bit stuffing. Trong k thut bit stuffing, b ghp knh s b sung cc bit m rng (extra bit) vo dng d liu ngun ca thit b m bo mi quan h v mt tc gia cc thit b l bi s (nguyn) ca nhau. V d, gi s thit b A c tc truyn nhanh hn thit b B 2.75 ln, chng ta c th thm mt s bit m bo t l v tc truyn gia 2 thit b l 3. Cc bt m rng (extra) s c loi b bi b phn knh.

Ghp knh phn thi khng ng b

Nh chng ta nghin c phn trc, k thut TDM ng b khng bo m tn dng ht kh nng ca lin kt (link) c s dng. Bi v cc khe thi gian c gn trc mt cch c nh nn mi khi c thit b kt ni khng truyn dn th khe tng ng ca n s rng v do c nhiu ng dn (path) khng s dng. V d, gi s chng c 20 my tnh u ra c ghp knh trn mt ng truyn n (single line). S dng TDM ng b, tc ca mi ng nh vy t nht cng phi bng 20 ln tc ca mi mt ng vo (input line). Nhng iu g s xy ra nu nh ch c 10 my tnh c s dng cng lc? Mt na dung lng ng truyn khng c s dng!

TDM khng ng b (TDM thng k) c thit k nhm trnh s l ph cp trn. Thut ng khng ng b c ngha mt ci g khc trong k thut ghp knh ngha l n cp n mt lnh vc truyn thng khc. y c ngha l linh hot v khng c nh.

Ging nh TDM ng b, TDM khng ng b cho php mt s cc ng vo (input line) c tc truyn thp ghp knh thnh mt ng c tc truyn cao hn. Khng ging nh k thut TDM ng b, trong k thut TDM khng ng b tc truyn tng th ca cc ng vo c th ln hn kh nng ca ng truyn (path). Trong h thng ng b, nu c n ng vo, s khe thi gian trong frame l c nh v t nht bng n. Trong h thng khng ng b, nu chng ta c n ng vo, mi frame s cha khng qu m khe, m