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 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.

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

Even and odd parity example 1For 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.

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

How it works

For example, we are sending data 1100 using even parity

1 1 0 0 0

During the transmission, one bit of data is corrupted.

1 1 0 1 0

The numbers of ‘one’ now become 3, therefore the last parity bit should be 1.

Flaws of parity checking

For example, using even parity to sent Data 00001001:

0 0 0 0 1 0 0 1

1 1 0 0 1 0 0 1

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.

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.

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.

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.

48 65 6C 6C 6F 20 77 6F 72 6C 64 2E

H e l l o W o r l d .

4865 + 6C6C + 6F20 + 776F + 726C + 642E = 71EA“Hello World.” is sent followed by 71EA

Handling errorChecksums 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 00113

0010 2 00000

0011 3 00011

0001 1 00113

Totals 77

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.

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.

Protocols

Some common protocols:

TCP/IP (Transmission Control Protocol/Internet Protocol

VOIP (Voice Over IP)

SOAP (Simple Object Access Protocol)

FTP (File Transfer Protocol)

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.