IT-101 Section 001 Lecture #5 Introduction to Information Technology

 Overview Chapter 3: Error detection and correction Parity checking Repetition Redundancy check Chapter 4: Protocols

 Error Detection and Correction Many factors can lead to errors during transmission or storage When binary information is sent across a physical channel such as wire, coax cable, optical fiber or air, there is always the possibility that some bits will be received in error due to interference form other sources or signal attenuation due to long distances or events like rain or snow etc… Clever code construction or additional information added to the transmission can increase the odds of the sent information being received correctly. We will discuss 3 methods: Parity checking Repetition Redundancy code word checking

Adding redundancy to a code increases the number of bits that need to be transmitted, but leads to the detection and fundamental decrease in errors at the receiver Just about any system that uses digital information employs some form of error detection and/or correction. For example, information on a CD is encoded to allow the CD player to detect and correct errors that might arise due to smudged or scratched disks

Parity Checking Error detection can be accomplished by appending an extra bit called a parity bit to the code Even parity is when the parity bit is set so that the total number of 1’s in the word is even 11   10 1 Odd parity is when the parity bit is set so that the total number of 1’s in the word is odd 11   10 0 Parity bit

Parity Errors Even parity is set: is received Error present, but don’t know which one is the bit in error Parity can detect errors, but can’t correct them Odd parity is set: is received. Error? Even parity is set: is received. Error? Parity checking has a major disadvantage due to its inability to detect even number of inverted bits, e.g would go undetected even if the original transmission was , because there are 2 errors

Repetition Repetition is the provision of extra bits to ensure the information is received correctly Original data: Transmitted: Received: Errors in the first and third bits detected Errors in the first and third bits can be corrected

Redundancy Check Symbols are given a parity bit Total “word” is given a redundancy check The first bit of each symbol in the word is given parity, then the second bit of each symbol in the word is given parity The additional parity symbol is given its own parity and then appended to the transmitted information

Redundancy Check Information to be sent: With even parity, the above is converted to: First bits are: Odd parity bit: 1 Second bits are: Odd parity bit: 1 Odd parity bits: 1 1 Even parity bit: 0 Transmitted: Parity bits Symbol Word

Redundancy Check Error Transmitted: Received: Even parity tells us that the second symbol has an error Comparing Odd parity with the first bit in each symbol shows us that the first bit in the second symbol should be a 0 Comparing Odd parity with the second bit in each symbol shows us that everything is OK

In-class examples The following binary stream uses repetition to help detect errors. Find the erroneous 2-bit code word If an even-parity bit has been added to each 3-bit data unit for error detection, find the erroneous 4-bit code word in the following: Determine the check code word for the following stream of 2-bit words: Find and correct the error in the following received data sequence that is terminated in a redundancy check code word

 Protocols Believe it or not, you use protocols every day When you see a red, octagonal sign, you_____ When you pick up the telephone when it rings, you say _______ You know to wait in line at the DMV You understand how to mail a letter Protocols give structure and provide rules, but they aren’t based on anything other than human convention, agreement and understanding “Agreed upon sets of rules that provide order to different systems and situations.”

Protocols are a vital component of IT What type of connector or voltage level should be used by a device? How can information be formatted in a standard manner? Where in a bit stream do you begin? Which bits comprise the destination address? How can a document include bold, italics, different font sizes, etc. Interoperability requires sets of rules for communicating between various devices. Without agreed upon formats, we’d be drowning in a sea of 1s and 0s.

IT Protocols We’ve already discussed many IT standards. Internet addresses have 32 bits. The byte is commonly accepted as the smallest division of information for storage and manipulation. ASCII is the standard protocol for alphanumeric data IT is built on protocols Hypertext Markup Language (HTML) Hypertext Transfer Protocol (HTTP) Simple Mail Transfer Protocol (SMTP) Internet Protocol (IP) And just about everything else we will talk about To achieve interoperability, digital systems must organize, manage, and interpret information according to a set of widely accepted standards.

Who Sets Standards and Protocols? Technology Consortiums Internet Engineering Task Force (IETF) World Wide Web Consortium (W3C) International Organizations International Telecommunications Union (ITU) International Standards Organization (ISO) National Organizations American National Standards Institute (ANSI) Professional Organizations Institute of Electrical and Electronic Engineers (IEEE) Companies: Microsoft, Cisco, 3Com, others…..

Protocol example How do we know where the bit stream begins? Start bits; stop bits (These are protocols) Flags--used in Ethernet, High-Level Data Link Control (HDLC) Flag =

HDLC Protocol Challenges A protocol procedure like the HDLC flag byte would fail if that byte also occurred somewhere in the content bit stream. Recall that flag byte = ASCII ~ Furthermore, because the bit stream is read on a bit-by-bit basis, this pattern could appear under other circumstances. To fix this problem, a rule had to be incorporated into the protocol: Transmitter: Whenever you have five 1’s in a row, insert an unneeded 0 Receiver: Whenever you receive five 1’s in a row, followed by a 0, discard the 0 Note that this only happens in the data (or content) field AFTER the flag byte is transmitted. This procedure is known as Bit Stuffing, or Zero Bit Insertion

Bit Stuffing Example Original Data Insert 0 Transmitted Data

 Comments for next class Topics to be covered next class: Chapter 7: Compression Introduction Entropy Huffman coding Universal coding