Error checking

When data arrives at its destination, it may contain errors. These errors may be caused by interference with the signal or simply by errors in encoding and decoding the data. Errors must first be detected and then corrected. Three common methods for error detection are parity checking, checksum and cyclic redundancy check:

• Parity checking is a method of checking for errors in data transmission using an additional bit called a parity bit. This bit is used only for the purpose of identifying whether the bits being moved have arrived successfully. When data is represented using an 8-bit ASCII, then a ninth bit is used that holds a 0 or a 1. When an odd parity is chosen, the number of ones in the nine bits must be odd. This means that if the number of ones in the first eight bits is even, then the parity bit must be one to make it odd. If the number of ones in the first eight bits is odd, the parity bit must be zero to keep it odd. The sender and receiver can also decide to send an even parity, in which case the number of ones in the nine bits must be even. If an error has occurred in a single bit, then the parity will be different and an error in transmission has occurred.

• Checksum is a method of checking for errors in data transmission by counting the number of bits in a data packet. A data packet is created by dividing the total data into smaller groups. The count of the bits in a data packet is attached to the data packet. It is used by the receiver to check whether all the bits have arrived successfully. If the count matches, it is assumed that a complete transmission was received.

• Cyclic redundancy check (CRC) is a method of checking for errors in data transmission using a division process. The data is divided into predetermined lengths and divided by a fixed divisor. The remainder of the calculation is attached and sent with the data. When the data is received, the remainder is recalculated. If the remainders do not match, an error in transmission has occurred. There are a number of different standards for CRC. A 32-bit CRC achieves a 99.99% detection of all possible errors. The most common form of error correction is to retransmit the data. 

Checksum is used in the upper layers, while Parity checking and CRC is used in the physical layer. Checksum is also on the concept of redundancy.

Student Activity

1. Take notes on each of the three types of error detection in video
2. Prepare and present a 2 minute talk on each of these types of error detection