In this tutorial you are going to learn about Error Detection in Computer Networks. You will learn about transmission errors, types of errors, error detection and error correction processes and about various error detection techniques.
What is an Error?
In a computer network data are transmitted in the network. When the data are transmitted over the network, we don't know what happens to the data once the data has left the node. The data can be corrupted during transmission. So, the errors that are caused because of this transmission are known as transmission error. For reliable communication, these errors must be detected and corrected.
In the OSI reference model, there are two layers that deal with error control. Error detection and correction are implemented either at the data link layer or the transport layer of the OSI model. In the data link layer as well as in the transport layer, we have error control as one of the services of those layers.
Types of Errors:
- Bit Error
- Burst Error
Bit Error
In Bit Error, only one bit is going to be changed. So, it is also known as single bit error and in a single bit error, only one bit in the data unit has been changed.
Example:
In this scenario given, there is a sender side and a receiver side. The sender is sending 00000010 but upon reception, the receiver is receiving 00001010. If you notice in the figure, the 0 bit has been changed to 1. We can notice here the sender is sending some information, but the receiver is receiving a different information and there is a change in exactly one bit. When there is only one bit change, we will call it as a bit error. Whereas, if there are a group of bits that are changed, then we will call it as a burst error.
Burst Error
In burst error, two or more bits in the data units have been changed.
Example:
Here also you can see the sender side and the receiver side. But we can notice that there are some bits that are corrupted during transmission. The sender has sent zero, but the receiver is receiving them as one. Now, when we calculate the length of the corrupted bits, i.e., the length of the burst error is eight bits because this is what the entire length of the corrupted bits.
How to Detect the Errors?
Generally, error detection will be done by the receiver. It means the receiver has to decide whether the received data is correct or not, without having a copy of the original message. The receiver normally will not have the original message because the original message itself has been corrupted by transmission errors.
When we send only the original message, it's very difficult for the receiver to understand or to know whether there is an error or not. So with the message, some additional information has to be sent by the sender. These additional information will help the receiver to understand or to find out whether there is an error or not in the data that is being transmitted. So, to detect or correct errors, we need to send some extra bits with the data and these extra bits are called as redundant bits.
Redundancy
In redundancy, there is a sender side and a receiver side. If the sender has some data to be sent, the sender gives the data to the generating function or an algorithm. This algorithm generates a code which we call as the redundant code or redundant bits or redundancy check. This information is appended with the message that has to be sent. The whole information is now sent to the receiver.
The receiver messages two parts in the message. One is the original message and the other one is the redundant bits. So it takes this original message and the checking function on the receiver side calculates redundant bits. It compares the redundant bits with the redundant bits that were sent by the sender. If both are matching, then the receiver understands that there are no errors in the transmission. So it accepts it, otherwise the receiver rejects this.
Error Correction
Error Detection in Computer Networks means, it's the job of the receiver to detect the error and if it is there, it will discard it, otherwise it will accept it. Error detection is very simple, just accept or reject. Whereas, Error Correction is a tedious job. The receiver has to correct the errors.
It can be handled in two ways:
- The receiver can have the sender retransmit the entire data unit. If the receiver detects the error, it can ask the sender to retransmit the entire information.
- The receiver can use an error correcting code, which automatically corrects certain errors.
Error Detection/Correction
Error detection and error correction both need redundancy because without additional information, it's very difficult to detect or correct the errors. So, when the sender sends the data or the message, there will be a generator or some algorithm to generate the redundancy and this redundant information and the message are generally sent over an unreliable transmission medium. We say it as an unreliable medium because it is prone to errors and the receiver receives this information.
Whatever the receiver receives, it gives that information to the checker and this checker says whether to accept the data or reject the data or whether to correct the data or discard the data. Correcting means, this checker part tells which part is affected with errors, and it can be corrected. Discarding means simply discarding the packet and we can ask the sender to retransmit. And the message is finally received by the receiver.
Error Detection Technique
There are basically four types of redundancy checks or techniques that are used in data communications.
- Vertical redundancy check (VRC)
- Longitudinal redundancy check(LRC)
- Checksum or Internet Checksum or Arithmetic Checksum
- Cyclic redundancy check (CRC)
This article on Error Detection in Computer Networks is contributed by Rajnish Kumar. If you like TheCode11 and would like to contribute, you can also write your article to us. Here is our mail id - thecode11info@gmail.com