Post on 30-May-2018
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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