CIT 307 Online Data Communications Error Detection Module 11 Kevin Siminski, Instructor.

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Presentation transcript:

CIT 307 Online Data Communications Error Detection Module 11 Kevin Siminski, Instructor

Data Link Layer

Error Correction Data can be corrupted during transmission. For reliable communication, errors must be detected and corrected.

Types of Errors Single-Bit Errors Burst Errors

Single Bit Errors In a single-bit error, only one bit in the data unit has changed.

Single Bit Error Example

Burst Error A burst error means that 2 or more bits in the data unit have changed.

Burst of errors (length 5)

Error Detection Redundancy Parity Check Cyclic Redundancy Check (CRC) Checksum

Error Detection Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.

Redundancy

Detection methods

Even Parity concept

Parity Check In parity check, a parity bit is added to every data unit so that the total number of 1s is even (or odd for odd-parity).

How does a parity check work? Suppose the sender wants to send the word world. In ASCII the five characters are coded as The following shows the actual bits sent

Parity Check continued… Now suppose the word world in Example 1 is received by the receiver without being corrupted in transmission The receiver counts the 1s in each character and comes up with even numbers (6, 6, 4, 4, 4). The data are accepted.

Parity Check continued… Now suppose the word world in Example 1 is corrupted during transmission The receiver counts the 1s in each character and comes up with even and odd numbers (7, 6, 5, 4, 4). The receiver knows that the data are corrupted, discards them, and asks for retransmission.

Simple Parity Check Simple parity check can detect all single-bit errors. It can detect burst errors only if the total number of errors in each data unit is odd.

Two-dimensional parity

Two-dimensional parity check In two-dimensional parity check, a block of bits is divided into rows and a redundant row of bits is added to the whole block.

Other types of Error correction Checksum CRC – Probably most common in network systems

Error Correction If an error is detected what must happen? – Receiver can try to correct error – Receiver can request a re-transmission – or the system can fail