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AS Computing

AS Computing. Data Transmission and Networks. Transmission error. Detecting errors in data transmission is very important for data integrity. There are three methods that we are going to discuss: Parity checking Each Checksum. Parity checking.

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AS Computing

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  1. AS Computing Data Transmission and Networks

  2. Transmission error • Detecting errors in data transmission is very important for data integrity. • There are three methods that we are going to discuss: • Parity checking • Each • Checksum

  3. Parity checking • A technique used to detect memory or data communication errors. It is also called Vertical Redundancy Checking (VRC) • Parity checking involves bother devices deciding in advance whether they are going to use even parity or odd parity. • Parity checking is only useful when there are odd number bits get corrupted in data.

  4. Even and odd parity Even parity is when the parity bit is set so that the total number of 1’s in the word is even Odd parity is when the parity bit is set so that the total number of 1’s in the word is odd

  5. Even and odd parity example 1 For example, we are using data 1100 using even parity. Since there are 2 "1" bits, the parity bit will be 0. If we are sending data 1101 using even parity. Since there are now 3 "1" bits, and the parity bit will be 1 to make the total number of "1"s 4. For this data, Parity bit = 0 for even parity, Parity = 1 for even parity.

  6. Even and Odd Parity example 2 For example, we have: Odd parity code word = 11011010 Since the number of ones sums to 5 this is odd parity Even parity code word = 11011011 Since the number of ones sums to 6, this is even parity

  7. How it works For example, we are sending data 1100 using even parity During the transmission, one bit of data is corrupted. The numbers of ‘one’ now become 3, therefore the last parity bit should be 1.

  8. Flaws of parity checking For example, using even parity to sent Data 00001001: During the transmission, two of the bits are corrupted: Then there will still be an even number of ‘zero’ and the data will be accepted, despite it is wrong.

  9. Echo The sender sends a message, then the receiver returns the message to the sender and asks whether this is the message that has been sent. If it is, then the sender signals the receiver that the message was sent correctly. If it isn’t, then the message is re-sent. Echo requires ate to be sent twice and therefore takes long time.

  10. Checksum A checksum, is a character that is the sum of the binary values for each bit position for all transmitted bytes. Checksum requires the addition of a single byte (one character) after each string of transmitted bytes (characters). Data is sent in blocks because that is most efficient way of sending data.

  11. H e l l o W o r l d . 48 65 6C 6C 6F 20 77 6F 72 6C 64 2E Checksum example For example, to compute a checksum on the message “Hello World.”. Two characters are grouped together as a Hex number and added together to produce the checksum. 4865 + 6C6C + 6F20 + 776F + 726C + 642E = 71EA “Hello World.” is sent followed by 71EA

  12. Handling error • Checksums are easy to calculate since they use simple addition and this can be done quickly by implementing it in hardware. • The disadvantage with checksums is that they cannot detect all common errors Binary Checksum value Binary Checksum 0001 1 0011 3 0010 2 0000 0 0011 3 0001 1 0001 1 0011 3 Totals 7 7

  13. Communication protocols A communication protocol is the phrase used to describe a set of rules that communication equipment adhere to when they send data to each other.

  14. A set of rules in a communication protocol • What baud rate will be used • What parity checking will be used • Whether software or hardware ‘handshaking’ is to be used • What character set is to be used • How many bits will be used for data • How many control bits will be used to control data transfer.

  15. Protocols Some common protocols: • TCP/IP (Transmission Control Protocol/Internet Protocol • VOIP (Voice Over IP) • SOAP (Simple Object Access Protocol) • FTP (File Transfer Protocol)

  16. Handshaking Handshaking is a networking process when two devices establish a connection. An example of a handshake is when your modem dialup's to a computer network and agrees on baud rate, error correction and compression protocols. Handshaking usually occurs when packets of data are exchanged between two computers.

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