D Ata and C Om Puter C

8/14/2019 D Ata and C Om Puter C

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Asynchronous and Synchronous

Transmission

Transm ission o f da ta requires co-operat ion a ndagreem ent be t. tran sm itter and rece iver

tim ing p rob lem s requ ire a m echa n ism to syn chron ize

the tran sm it ter and receiver

rece iver sam ples st ream at bi t in terva ls

i f the two clock s are n ot al ign ed and d r if ting, rece iver wi l lsam ple at w rong t im e a f ter suf fic ient bits are sen t

two solutions to synchroniz ing clocks asynchronous transm iss ion

synchronous transm iss ion


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Asynchronous T ransm iss ion


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Asynchronou s Transm iss ion

Send one ch aracter at a time each character begins with a start bit to alert the receiver receiver samples each bit in the character the data bits are usually followed by a parity bit

the final element is a stop element the stop element is the same as the idle state

Characterist ics simple

cheap overh ea d of 2 or 3 b its per cha r (~20% ) good for data w ith large gaps (key board)


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Synchronou s T ransm iss ion

block of data transm itted as aframe

clocks m ust be synchronized can u se sep arate clock l ine

or em bed clock signal in da ta need to indicate start and end o f block

use pream ble and postam ble

more efficient (lower overhead) than async


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Types of Error

an error occurs when a bit is altered be tween transm issionand recep tion

single b it errors

on ly one b it altered caused by w hite noise

bu rst errors cont iguou s sequence o fB bi ts in w hich firs t last and any num ber

of interm ed iate b its in error cause d by im pulse noise or by fading in w ire less

ef fect grea ter at highe r data rates


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E rror Detect ion

there w ill be e rrors introdu ced by the tx. system E rror d etect ion done using error-de tect ing cod e

funct ion of da ta b its, calculated by tran sm it ter and

appe nded to the f ram e

reca lcu la ted and checked by receiver

st ill cha nce of undetected e rror

E rror de tect ion using par ity pa rity b it set so cha rac ter has even (even pa rity) or odd

(odd parity) number of ones

even number of bit errors goes undetected


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E rror De tect ion P rocess


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Cyc lic Redundancy Check

one of mos t common and power fu l checks

for block ofk bi ts transm it ter gene rates an n b it

f ram e check sequence (FC S)

t ransmits k+n bits wh ich is exa ct ly div is ible by

some numbe r

rece iver div ide s frame by that num ber if no remainder, assume no e rror

for math, see Stal lings chapter 6


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E rror Correct ion

correct ion o f detected errors usual ly requ ires data block tobe retransm itted

not appropriate for wireless app l icat ions

bi t error rate is high cau sing lots of retransm ission s when prop agat ion de lay lon g (sa tellite) compa red w ith fram e

tran sm ission t im e, resu lt ing in retransm ission of fram e in error

p lus many subsequ ent frames

instead need to correct errors on basis of bits received

error correct ion provides this


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E rror Correc t ion P roce ss


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How E rro r Correc tion W orks

adds redundan cy to t ransm itted message

can deduce o r ig ina l de spite som e errors

eg . block e rror co rrection code map k bit inpu t onto an n bit codew ord

each d istinc tly differen t

if get error assum e codeword sent wa s closest to that

received

for math, see S talling s chap ter 6

mean s have redu ced e ffec t ive da ta rate


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L ine C on f igu rat ion - Topo logy

phys ica l arrangem ent of stat ion s on m ed ium

po int to point - two sta tions

such as b etwe en two rou ters / com puters

multi po int - multiple s tations

t radi tiona lly m ainfram e com puter and terminals

now typical ly a loca l area n etwork (LAN)


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L ine C on f igu rat ion - Topo logy


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L ine Con f igu rat ion - Dup lex

classi fy da ta exchange as half or full duplex

half dup lex (two-way a lterna te) only one stat ion m ay transm it at a t im e

requi res one data path full duplex (two-way sim ultaneous)

s im ul tane ous transm iss ion and recept ion be twee n two stations

requi res two data pa ths

sepa rate me dia or freque nc ies used for each direct ion or echo ca nce ling


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Summa r y

asynch ronou s verses synch ronou s transm iss ion

error detect ion and co rrect ion

line con f igu rat ion issues

D Ata and C Om Puter C

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