Question

1. What does the data link layer do? What are its primary responsibilities?

2. Describe two approaches to detecting errors, including how they work, the probability of detecting an error, and any other benefits or limitations.

3. Briefly describe how even parity and odd parity work. Give an example of even parity with a 7-bit ASCII code, for the following: 0110110

4. How does cyclical redundancy checking work (CRC)?

Answer 1

In OSI reference model,the data link layer(DLL) is the second layer.

DLL takes the packets from network layer and converts/encapsulates the packets into frames for the data transmission.

Responsibilities includes:

1. Providing services to network layer.
   • Unacknowledged connection less service
   • Acknowledged connection less service
   • Acknowledged connection-oriented service
2. Dealing with transmission errors.
3. Regulating the flow of data.

Two approaches to detecting errors:

1.Parity:

Simple parity check:

1 is appended to block of data if it contains odd number of 1’s, and

0 is appended to block of data if it contains even number of 1’s.

In the above example, 1 is added as there are odd number of 1's in the data.

2. Checksum:

The data will be divided into k segments each of m bits, in this checksum error detection method.

Sender side:

The segments of data will be added using 1's complement and finally the sum is complemented to get the checksum.

Along with the data segments, the checksum will be sent.

At receiver side, all received segments are added using 1’s complement and again the sum is complemented. If the final result is 0, then the data will be accepted.

If result is not 0, then the data will be discarded.

The major advantage is "easy to implement".

Probability of detecting an error by checksum is lesser than probability of detecting an error by CRC.

Even parity and odd parity:

The simple parity check described above is even parity check as it makes the total number of 1’s even.

(1 is appended to block of data if it contains odd number of 1’s, and

0 is appended to block of data if it contains even number of 1’s. )

Appended bit is the parity bot.

ODD parity:

If a number of 1's is odd then parity bit value is 0, else(i.e. number of 1's is even) parity bit value is 1.

Given example: 0110110, the parity bit will be 0 as there are even number of 1's in the data block.

Cyclic redundancy check (CRC):

CRC is mainly based on binary division.

In this CRC error detection method,

a sequence of bits called cyclic redundancy check bits will be appended to data at the end.

Let us understand with an example:

Here '000' is added to the original data.

Data+CRC bits will be sent to receiver.

As remainder is zero, it means data will be accepted.

This is how the CRC will work.