Finish Ch 2 and Review Dr. Clincy Professor of CS Exam 1 – Sept 13th 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 #) Dr. Clincy Lecture 8

Ch 2 Appendix: Codes for Data Recording and Transmission Line coding scheme categories Will cover these schemes Dr. Clincy Lecture 8

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. O K Dr. Clincy Lecture 8

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. 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. Dr. Clincy Lecture 8

D/D - Manchester Code 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). Dr. Clincy Lecture 8

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. Dr. Clincy Lecture 8

Frequency Modulation (Modified) 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. Dr. Clincy Lecture 8

Block coding concept The main challenge for data recording and transmission 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 Dr. Clincy Lecture 8

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 Dr. Clincy Lecture 8

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 Dr. Clincy Lecture 8

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. 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). A coding scheme that uses fewer magnetic transitions is more efficient than one with more magnetic transitions per character. Dr. Clincy Lecture 8

Review Begins Now NOTE: Reed-Solomon algorithm is not covered and will not be on the exam (Ch 2, page 113) Partial Response Maximum Likelihood Coding (PRML) is not covered and will not be on the exam (Appendix A, page 134) Rules for Review today: student controlled - must specify a specific issue (lecture # and slide #) Dr. Clincy Lecture 8