PHN I: M u1. t vn :Cc thit b trong mng khi truyn cho nhau i hi
tnh chnh xc. Tuy nhin d liu dng s khi c lu tr, x l hay truyn t my
ny qua my khc c th b li. Nh khi truyn d liu i xa qua mi trng in
thoi, dy cp, khng gian c th b nh hng bi nhit , nhiu ng dy, in t hay
do lu ngy cc b x l, b chuyn i c mt sai st nh s lm thay i d liu.Trn
ng truyn d liu c th b nhiu (in, t,..) dn n sai lc trong qu trnh
truyn.V vy, trc khi nhn c d liu th phi pht hin li v sa li ni nhn c
th nhn thng tin mt cch chnh xc nht.1. Cc phng thc truyn: C 2 phng
thc truyn d liu l : truyn song song v truyn ni tip.1. Truyn song
song:1. Mi bit dng mt ng truyn ring. Nu c 8 bits c truyn ng thi s
yu cu 8 ng truyn c lp.1. truyn d liu trn mt ng truyn song song, mt
knh truyn ring c dng thng bo cho bn nhn bit khi no d liu c sn
(clock signal)1. Cn thm mt knh truyn khc bn nhn bo cho bn gi bit sn
sng nhn d liu k tip.
PARALLEL DATA TRANSMISSIONBit 0Bit 1Bit 2Bit 0Bit 1Bit 2Bit 3Bit
4Bit 5Bit 6Bit 7Bit 3Bit 4Bit 5Bit 6Bit 7SenderMediaReceiver
1. V d: ISA, ATA, PCI,..1. Truyn ni tip:1. Tt c cc bit u c truyn
trn cng mt ng truyn, bit ny tip sau bit kia di tc ng ca xung nhp
clock.1. Khng cn cc ng truyn ring cho tn hin truyn i.1. Phi m bo c
vn thi gian gia bn pht v bn nhn -> trnh s xung t.1. Trong phng
thc truyn ni tip bao gm : truyn ng b v khng ng b1. Khng ng b: mi k
t c ng b bi bit start v bit stop.1. ng b: mi khi k t c bng b bi
c.
SERIAL DATA TRANSIMISSION1001 10011001 1001
SenderMediaReceiver
1. V d: RS232, SPI, I2C.
1. nh ngha v phn loi li.2. nh ngha: Trong thc t, mt phn ca thng
tin s b thay i trn ng truyn d liu (do t trng, nhit .). Chng c th lm
thay i hnh dng v thi im ca tn hiu. Nu d liu c m ha di dng nh phn th
mt hoc nhiu bit b li, c th chuyn t 1 sang 0 hoc ngc li hoc cng c th
chng b mt i.1. Li truyn thng2. Phn loi liC 3 loi li l: li n bit, li
a bit, li o bit.1. Li n bit.1. Khi truyn d liu th 1 bit ca khi d
liu (1byte, 1 k t,..) b thay i t 1 sang 0 v ngc li.1. Khi 1 bit b
li th cc bit xung quanh khng b nh hng.1. Thng xy ra li khi d liu b
nhiu trng.
0 changed 1
00000010
00001010
ReceiverSender
1. Thng xy ra khi truyn truyn d liu song song.1. Li a bit1. L li
m khi d liu truyn i c nhiu hn 2 bit b li. ( iu ny khng c ngha l phi
2 li xy ra lin tip nhau).1. Chiu di ca li tnh t bit b li u tin n
bit b li cui cng (mc d cc bit nm gia khng b li).1. SentLength of
burt error (5 bit)
0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
Bits corrupted by burts error
0 1 0 1 1 1 0 1 0 1 0 0 0 0 1 1
Receiver
1. Thng xy ra trong qu trnh truyn d liu ni tip.1. Thi gian ca
nhiu thng di hn ca 1 bit nhng n c th tc ng n nhiu bit. S bit b tc
ng ph thuc v khong thi gian ca nhiu v tc truyn.1. Li o bit1. L li
xut hin khi c 2 li lin tip nhau o gi tr cho nhau.1. V d 0 1 0 0 0 1
0 0 0 1 0 0 0 0 1 1
0 1 0 1 1 0 0 0 0 1 0 0 0 0 1 1
PHN II: Phng php pht hin v sa li.
1. Pht hin li.1. Khi truyn d liu nu gp li th khng th nhn ra c
trc khi gii m d liu nhn c. 1. Khi gi d liu i, n s c gn vo phn ui 1
phn kim tra li. Bn nhn s da vo xem d liu c b li hay khng.
Data
101000000001010101010
AcceptRejectChecking functionGenerating function 1011101
Detach Data
Redundancy check
1010000000010101010101011101Receiver
SenderData & redundancy check
1. C 4 phng php pht hin li: VRC, LRC, CRC, CHECKSUM
1. Phng php VRC ( bit chn l).1. Mi xu bit biu din k t truyn i c
thm v 1 bit, gi l parity bit hay l bit chn l. 1. Quy c:1. Chn: tng
s bit 1 c trong khi d liu (c bt parity) l s chn.1. L: tng s bit 1 c
trong khi d liu (c bit parity) l s l.1. u nhn d liu, mch s kim tra
tng khi d liu nhn c xem c tng s bit l chn(l). Nu khng th tc l c 1
bit no trong khi d liu b sai. Ngc li l mch truyn ng.1. V d: Truyn 1
khi d liu 4 bit 1101 c s dng mch to kim parity r sai c minh ho nh
hnh di y:
Hnh 2.3.28Mch to kim ParityNhn xt:1. Ch d c li sai s bit l.1.
Khng sa c li.1. t c dng trong truyn d liu i xa, c bit tc cao (hiu
sut truyn thng tin km).
1. Phng php LRC1. Vn s dng phng php ca VRC nhng theo dng 2 chiu
(vic kim tra c thc hin c chiu ngang ln chiu dc)1. V d:Parity
bit
1 0 1 1 0 1 1 11 1 0 1 0 1 1 10 0 1 1 1 0 1 01 1 1 1 0 0 0 01 0
0 0 1 0 1 10 1 0 1 1 1 1 10 1 1 1 1 1 1 0Data bits
LRC bits
1. Theo chiu ngang, mi k t c thm vo 1 bit kim tra chn l nh VRC,
v c gi l Kim tra chiu ngang VRC (Vertical Redundancy Check).1. Theo
chiu dc, cung cp thm 1 k t kim tra, c gi l LRC (Longitudinal
Redundancy Check).1. Khi d liu gi i c gn thm LRC. Ni nhn s em d liu
nhn c tp hp theo hng ngang v hng dc.Nu:1. Nu tt c cc ct(ngang v dc)
u =0 th d liu khng b li.1. Nu 1 gi tr bt k bng khng th d liu b
li.
V d khi d liu trn b li.VRC1 1 0 1 0 1 1 1 0 1 0 1 1 0 1 1 1
0
0 0 1 0 1 0 1 0 11 0 0 0 1 0 1 1 00 1 1 1 1 1 1 0 0 0 0 0 1 0 0
0 00 1 0 1 1 1 1 1 01 1 1 1 0 0 0 0 0
1. Vy c li xy ra sau khi d liu c truyn.1. Nhn xt1. Gim t ln li
khng pht hin c 2-4 bc so vi dng VRC.1. tin cy tng i cao.1. C kh nng
pht hin v sa li nu trng hp li n bit.
