Exam 1 – Sept 19th

1. Finish Ch 2 and Review Dr. Clincy Professor of CS Exam 1 – Sept 19th Format of exam and what it will cover (75 minutes, Units topic from Ch1, All of Ch 2, Short problems & problems, Closed book, no laptop or phone, can use calculator) Rules for Review today: student controlled - must specify a specific issue (lecture # and slide #) Lecture 8

2. Ch 2 Appendix: Codes for Data Recording and Transmission Line coding scheme categories Lecture 8

3. D/D - Non-Return-to-Zero Code (NRZ) • The simplest data recording and transmission code is the non-return-to-zero (NRZ) code. • NRZ encodes 1 as “high” and 0 as “low.” • The coding of OK (in ASCII) is shown below. • The problem with NRZ code is that long strings of zeros and ones cause synchronization loss. Lecture 8

4. D/D - Non-Return-to-Zero-Invert Code (NRZI) • The problem with NRZ code is that long strings of zeros and ones cause synchronization loss. • Non-return-to-zero-invert (NRZI) reduces this synchronization loss by providing a transition (either low-to-high or high-to-low) for each binary 1 – no change for 0. Lecture 8

5. D/D - Manchester Code • Although NRZI prevents loss of synchronization over long strings of binary ones, NRZI coding does nothing to prevent synchronization loss within long strings of zeros. • Manchester coding (also known as phase modulation) prevents this problem by encoding a binary one with an “up” transition and a binary zero with a “down” transition – or reversal of the phase (i.e starting point of the signal). • Toggle at start, at midpoint L-to-H for 1 and H-to-L for 0 Lecture 8

6. Frequency Modulation • For many years, Manchester code was the dominant transmission code for local area networks. • It is, however, wasteful of communications capacity because there is a transition on every bit cell. • A more efficient coding method is based upon the frequency modulation (FM) code. In FM, a transition is provided at each cell boundary. Only cells containing binary ones have a mid-cell transition. frequency turnovers per cycle for 1 – no turnovers for 0 Lecture 8

7. Modified Frequency Modulation - MFM • FM requires a transition at each cell boundary. • Modified FM (MFM) eliminates some of these transitions – thus, MFM is a more economical code. • It provides a cell boundary transition only when adjacent cells contain zeros. • An MFM cell containing a binary one has a transition in the middle as in regular FM. frequency turnovers per cycle for 1 – no turnovers for 0 – minus toggle at start Lecture 8

8. Block coding concept The main challenge for data recording and trans-mission is how to retain synchronization without chewing up more resources than necessary. Block coding provides redundancy for synchronization and error detection Block coding changes a block of m bits into a block of n bits (where n>m) Block coding is also called mB/nB encoding Lecture 8

9. Using block coding 4B/5B with NRZ-I line coding scheme Fixes that PROBLEM of long stream of 0s Use 4B/5B to change the long stream of 0s prior to using NRZ-I For example, for 4B/5B encoding, 4-bit groups or replaced with 5-bit groups and those 5-bit groups are re-combined – NOTE: the 5-bit code could be completely different from the original 4-bit code Lecture 8

10. 4B/5B mapping codes Because the 5-bit code has 25 = 32 codes, the extra codes can be used for control sequences and error detection For example, for 4B/5B encoding, 4-bit groups or replaced with 5-bit groups and those 5-bit groups are re-combined – NOTE: the 5-bit code could be completely different from the original 4-bit code Lecture 8

11. Run-Length Limited • Run-length-limited, RLL, is a code specifically designed to reduce the number of consecutive ones and zeros. • Some extra bits are inserted into the code. • But even with these extra bits RLL is remarkably efficient. • An RLL(d,k) code dictates a minimum of d and a maximum of k consecutive zeros between any pair of consecutive ones. • RLL(2,7) had been the dominant disk storage coding method for many years. • An RLL(2,7) code contains more bit cells than its corresponding ASCII or EBCDIC character. • However, the coding method allows bit cells to be smaller, thus closer together, than in MFM or any other code. Lecture 8

12. Run-Length Limited • The RLL(2,7) coding for OK is shown below, compared to MFM. The RLL code (bottom) contains 25% fewer transitions than the MFM code (top). RLL(2,7) Results A coding scheme that uses fewer magnetic transitions is more efficient than one with more magnetic transitions per character. Lecture 8

13. Review Begins Now • Rules for Review today: student controlled - must specify a specific issue (lecture # and slide #) Lecture 8