COMPUTER NETWORKS

DATA LINK LAYER

1.What are the responsibilities of data link layer?

Specific responsibilities of data link layer include the following.

a) Framing

b) Physical addressing

c) Flow control

d) Error control

e) Access control

2. Mention the types of errors.

There are 2 types of errors

a) Single-bit error.

b) Burst-bit error.

3. Define the following terms.

Single bit error: The term single bit error means that only one bit of a given data unit

(such as byte character/data unit or packet) is changed from 1 to 0 or from 0 to 1.

Burst error: Means that 2 or more bits in the data unit have changed from 1 to 0 from

0 to 1.

4. What is redundancy?

It is the error detecting mechanism, which means a shorter group of bits or extra

bits may be appended at the destination of each unit.

5. List out the available detection methods.

There are 4 types of redundancy checks are used in data communication.

a) Vertical redundancy checks (VRC).

b) Longitudinal redundancy checks (LRC).

c) Cyclic redundancy checks (CRC).

d) Checksum.

6. Write short notes on VRC.

The most common and least expensive mechanism for error detection is the vertical

redundancy check (VRC) often called a parity check. In this technique a redundant bit

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called a parity bit, is appended to every data unit so, that the total number of 0’s in the unit

(including the parity bit) becomes even.

7. Write short notes on LRC.

In longitudinal redundancy check (LRC), a block of bits is divided into rows and a

redundant row of bits is added to the whole block.

8. Write short notes on CRC.

The third and most powerful of the redundancy checking techniques is the cyclic

redundancy checks (CRC) CRC is based on binary division. Here a sequence of redundant

bits, called the CRC remainder is appended to the end of data unit.

9. Write short notes on CRC generator.

A CRC generator uses a modulo-2 division.

°

In the first step, the 4-bit divisor is subtracted from the

first 4 bit of the

dividend.

°

Each bit of the divisor is subtracted from the

corresponding bit of the

dividend without disturbing the next higher bit.

10. Write short notes on CRC checker.

A CRC checker functions exactly like a generator. After receiving the data appended

with the CRC it does the same modulo-2 division. If the remainder is all 0’s the CRC is

dropped and the data accepted. Otherwise, the received stream of bits is discarded and the

dates are resent.

11. Give the essential properties for polynomial.

A polynomial should be selected to have at least the following properties.

a) It should not be

b) It should be divisible by(x+1).

12. Define checksum.

The error detection method used by the higher layer protocol is called checksum.

Checksum is based on the concept of redundancy.

13. What are the steps followed in checksum generator?

The sender follows these steps

a) The units are divided into k sections each of n bits.

b) All sections are added together using 2’s complement to get the sum.

c) The sum is complemented and become the checksum.

d) The checksum is sent with the data.

14. List out the steps followed is checksum checker side.

The receiver must follow these steps

a) The unit is divided into k section each of n bits.

b) All sections are added together using 1’s complement to get the sum.

c) The sum is complemented.

d) If the result is zero.

15. Write short notes on error correction.

It is the mechanism to correct the errors and it can be handled in 2 ways.

a) When an error is discovered, the receiver can have the sender

retransmit the entire data unit.

b) A receiver can use an error correcting coder, which automatically

corrects certain errors.

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16. Mention the types of error correcting methods.

There are 2 error-correcting methods.

a) Single bit error correction

b) Burst error correction.

17. What is the purpose of hamming code?

A hamming code can be designed to correct burst errors of certain lengths. So the

simple strategy used by the hamming code to correct single bit errors must be redesigned

to be applicable for multiple bit correction.

18. Define flow control.

Flow control refers to a set of procedures used to restrict the amount of data. The

sender can send before waiting for acknowledgment.

19. What is a buffer?

Each receiving device has a block of memory called a buffer, reserved for storing

incoming data until they are processed.

20. Mention the categories of flow control.

There are 2 methods have been developed to control flow of data across

communication links.

a) Stop and wait- send one from at a time.

b) Sliding window- send several frames at a time.