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Week 7– CST– 311 Intro to Computer Networks

Module 7 – The Link Layer and LANs

This week we covered the link layer and LANs. In section 6.2 we studied the many simple error-detection and error-correction techniques that are used in practice in the link layer. Some of these techniques include parity checking, internet checksum, and cyclic redundancy check. With parity checking, the sender includes a parity bid with the data sent. In an even parity scheme, the total number of 1s in the d + 1 bits is even. With odd parity schemes, the parity bit makes an odd number of 1s. If the receiver finds an odd number of 1-valued bits with an even parity scheme, then they know that at least 1 bit error has occurred. There is also a two-dimensional parity error technique that can help identify and correct the corrupted bit.

The internet checksum can be implemented by summing 16-bit integers. Then the 1s complement of the sum is used as the internet checksum and send in the header segment. The receiver then checks the checksum by taking the 1s complement of the data and makes sure that the result is all 0 bits. The presence of any 1 bits indicates an error.

Lastly, cyclic redundancy check or CRC is an error detection technique that uses polynomial codes. The CRC codes are created by the sender and receiver agreeing on an r + 1 pattern known as the generator (G). The process of CRC is done by appending the data D to r additional bits and using modulo 2 arithmetic to make sure d + r is exactly divisible by G. The receiver can check for errors by dividing the d + r by G. If the remainder is zero, then the data is accepted as correct.

It is essential that multiple protocol layers of the internet protocol stack provide error detection and correction. This allows each layer to independently verify and correct errors ensuring data integrity at different levels throughout the entire transmission process.

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