Error Detection and Correction-Cont

Types of Errors ™   Single Bit & Burst o    In a burst error multiple bits are changed o    For  Example,  a  0.01  second  burst  of  impulse  noise  on  a  TX  with  a  data  rate  of 1200 bps might change all or some of 12 bits of information

o    In a single-bit error, a 0 is changed to a 1 or a 1 to a 0

o    The  term single  bit  error  means  that  only one  bit  of  a  given  data  unit  (such  as  a byte, character, or a packet) is changed from 1 to 0 or from 0 to 1 o    Figure shows the effect of a single bit error on a data unit o    ASCII  character  000000010  (ASCII  STX)  is  sent  but  00001010  (ASCII  LF)  is received o    Single bit errors are the least likely type of error in serial data TX o    To see Why? Imagine a sender sends data at 1Mbps o    This means that each bit lasts only 1/1,000,000 seconds or 1 microsecond o    For  single  bit  error  to  occur,  the  noise  must  have  a  duration  of  1  microsecond which is very rare, noise lasts much longer than that o    However, single bit error can occur if we are sending data using parallel TX o    For Example, if 8 wires are used to send all of the eight bits of a byte at the same time and one of the wires is noise, one bit can be corrupted in each byte

™   Burst Errors The term burst error means that two or more bit sin the data unit have changed from 1 to 0 or from 0 to 1 o    Figure shows the effect of a burst error on a data unit o    In this case 0100010001000011 was sent but 0101110101000011 was received o    Note that a burst error does not necessarily mean that error occur in consecutive bits o    The  length  of  the  burst  is  measured  from  the  first  corrupted  bit  to  the  last corrupted bit o    Some bits in b/w may not have been corrupted o    Burst error is most likely to happen in a serial TX o    The  duration  of  the  noise  is  normally  longer  than  the  duration  of  a  bit  which means that when noise affects data, it affects a set of bits o    The number of bits affected depends on the data rate and duration of noise o    For example, if we are sending data at 1 Kbps, a noise of 1/100 seconds can affect 10 bits o    If we are sending data at 1 Mbps, the same noise can affect 10,000 bits Error Detection o    Even if we know what type of errors can occur, will we recognize one when we see it? o    If we have a copy of the intended TX for comparison, of course we will o    But what if we don't have a copy of the original o    Then  we  will  have  no  way  of  knowing  we  have  received  an  error  until  we  have decoded the TX and failed to make sense of it o    For a device to check for errors this way will be Costly and Slow o    We  don't  need  a  machine  that  decodes  every  thing  and  then  sits  and  decides whether a specific word makes sense or not o    We need a mechanism that is Simple and Completely objective ¾   Redundancy o    One error detection mechanism that would satisfy these requirements would be to send every data unit twice o    The  receiving  device  would  then  be  able  to  do  a  bit-for-bit  comparison  b/w  two TXs o    Any   discrepancy   will   indicate   an   error   and   an   appropriate   error   correction mechanism could be set in place o    This  system  will  be  completely  Accurate  because  the  odds  of  error  affecting  the same bits in both version will be infinitesimally small o    But this system will be extra ordinarily SLOW o    Not only will the TX time double, but the time it takes to compare two data units is also added up o    The  concept  of  including  extra  information  in  the  TX  solely  for  the  purpose  of comparison is a good one o    But  instead  of  repeating  the  entire  data  stream,  a  shorter  group  of  bits  may  be appended to the end of each unit o    This  technique  is  called  REDUNDANCY  because  the  extra  bit  are  redundant  to the  information  and  are  discarded  as  soon  as  the  accuracy  of  TX  has  been determined

o    Fig shows the process of using redundant bits to check the accuracy of data unit o    Once  the  data  stream has  been  generated, it  passes  through  a  device  that  analyzes  it and adds on an appropriately coded redundancy check o    The data unit now enlarged by several bits (7) travels over the link to the receiver o    The receiver puts the entire stream through a checking function o    If the received bit stream passes the checking criteria, the data portion of the data unit is accepted and the redundant bits are discarded