Error Detection at Data Link Layer
When data is transmitted from one device to another device then the data may change during transmission. If the sending data is not identical to receiving data then it is the error state.
For reliable communication error must be detected and then corrected.
Note: Error Deduction and correction are implemented either at data link layer (layer 2) or transport layer (layer 3) of OSI Model.
Types of Errors
Errors can be classified into two categories
1. Single-Bit Error
If only one bit of transmitted data unit is changed i.e. from 1 to 0 or from 0 to 1 then it is a single bit error. The following figure shows the one bit change,
2. Burst Error
If two or more bits are changed (i.e. from 0 to 1 or from 1 to 0) then it is a Burst Error. The Burst Error is calculated from the first corrupted bit to the last corrupted bit.
How to Detect the Errors?
Error deduction means to decide whether the received data is correct or not without having a copy of original data.
To detect or correct errors we need to send some extra bits along with original message (data). These extra bits are called redundant bits. To detect errors we use Generator and Checker.
Generator: Generator is used at sender end, it is used to generate redundancy bits. These redundancy bits are appended with sending data.
Checker: Checker is used at receiver end, it verify the data and redundancy bits. If sending information are unchanged then data is accepted otherwise rejected.
Block diagram of error deduction is given below
Error Detecting Techniques
The most popular Error Detecting Techniques are
- Single parity check
- Two-dimensional parity check
- Cyclic redundancy check
In the next lectures we will explain all types of error detecting techniques