Why y Encode? 01 o 1 How many more ones? ecture 6 15441@2008
Why Encode? Lecture 6 15-441 © 2008 0 1 0 1 How many more ones? 6
Non-Return to zero(nrz 0 0 00 85 0 -85 1->high signal; 0-> low signal Long sequences of 1's or os can cause problems Sensitive to clock skew, i.e. hard to recover clock Difficult to interpret o' s and 1s ecture 6 15-441e2008 7
Non-Return to Zero (NRZ) •1 -> high signal; 0 -> low signal •Long sequences of 1’s or 0’s can cause problems: – Sensitive to clock skew, i.e. hard to recover clock – Difficult to interpret 0’s and 1’s V 0 .85 -.85 0 1 0 0 0 1 1 0 1 Lecture 6 15-441 © 2008 7
Why do we need encoding? Keep receiver synchronized with sender Create control symbols besides reqular data symbols E.g. start or end of frame, escape Error detection or error corrections Some codes are illegal so receiver can detect certain classes of errors Minor errors can be corrected by having multiple adjacent signals mapped to the same data symbol . Encoding can be done one bit at a time or in multi-bif blocks, e. g, 4 or 8 bits Encoding can be very complex, e.g. wireless ecture 6 15441@2008 8
Why Do We Need Encoding? •Keep receiver synchronized with sender. •Create control symbols, besides regular data symbols. – E.g. start or end of frame, escape, ... •Error detection or error corrections. – Some codes are illegal so receiver can detect certain classes of errors – Minor errors can be corrected by having multiple adjacent signals mapped to the same data symbol •Encoding can be done one bit at a time or in multi-bit blocks, e.g., 4 or 8 bits. •Encoding can be very complex, e.g. wireless. Lecture 6 15-441 © 2008 8
Non-Return to zero Inverted (NRZI) 01000 0 85 0 -85 1→ make transition:o→ signal stays the same Solves the problem for long sequences of 1's but not for o’s ecture 6 15441@2008 9
Non-Return to Zero Inverted (NRZI) •1 → make transition; 0 → signal stays the same •Solves the problem for long sequences of 1’s, but not for 0’s. V 0 .85 -.85 0 1 0 0 0 1 1 0 1 Lecture 6 15-441 © 2008 9
Ethernet Manchester Encoding 0 85 0 -85 1us Positive transition for o, negative for 1 Transition eve ycle communicates clock ecture 6 15441@2008 0
Ethernet Manchester Encoding •Positive transition for 0, negative for 1 •Transition every cycle communicates clock V 0 .85 -.85 0 1 1 0 .1s Lecture 6 15-441 © 2008 10