Encoding Neil Tang 9/15/2008

1. Encoding Neil Tang9/15/2008 CS440 Computer Networks

2. Outline • Encoding/Decoding • Non-Return to Zero (NRZ) • Non-Return to Zero Inverted (NRZI) • Manchester • 4B/5B CS440 Computer Networks

3. Encoding • Signals: Different signals correspond to different power levels, frequencies and so on. • Encoding/Decoding: the procedure to transfer binary data (the signals) to the signals (binary data). CS440 Computer Networks

4. NRZ • 1 →High Signal • 0 → Low Signal CS440 Computer Networks

5. Problems • Cause: long strings of 1s and 0s • Baseline Wander: Too many consecutive 1s or 0s cause the average signal strength to change, making it more difficult to detect a significant change in the signal. • Clock Drift: A long period of time without a transition leads to clock drift. CS440 Computer Networks

6. NRZI and Manchester NRZI: • 1 → a transition from the current signal • 0 → stay at the current signal Manchester • 1 → high-to-low transition • 0 → low-to-high transition Differential Manchester • 1 → the first half signal equals to the last half of the previous bit’s signal • 0 → the first half signal is opposite to the last half of the previous bit’s signal CS440 Computer Networks

7. NRZI and Manchester CS440 Computer Networks

8. Problems • NRZI: consecutive 0s • Manchester: inefficient because link bit rate is half of the baud rate. • Baud Rate (symbol rate): the number of distinct symbolic changes (signaling events) per second. CS440 Computer Networks

9. 4B/5B Basic Idea • Every 4 bits of actual data are encoded in a 5-bit code that is transmitted using NRZI. • No more than one leading 0 and no more than two trailing 0s in a 5-bit code, i.e., no more than 3 consecutive 0s. CS440 Computer Networks