Data Communication -Unit – III- Data Link Control- Part-A Questions with Answers

Unit – III- Data Link Control

PART A

1. Write the design issues of datalink layer?

1)   Services provided to network layer.

2)   Framing

3)   Error control

4)   Flow control

2. What is datalink?

When a datalink control protocol is used the transmission medium between systems is referred to as a datalink.

3. What is the main function of datalink layer?

The datalink layer transforms the physical layer, a raw transmission facility to a reliable link and is responsible for node to node delivery.

4. What is a datalink protocol?

Datalink protocol is a layer of control present in each communicating device that provides functions such as flow control, error detection and error control.

5. What is meant by flow control?

Flow control is a set of procedures used to restrict the amount of data that the sender can send before waiting for an acknowledgement.

6. How is error controlled in datalink controlled protocol?

In a datalink control protocol, error control is activated by retransmission of damaged frame that have not been acknowledged by other side which requests a retransmission.

7. Discuss the concept of redundancy in error detection.

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

8. Mention the types of errors and define the terms?

There are two types of errors

· Single-bit error

· Burst-bit error

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.

9.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.

10. What is the error detecting capability of parity check?

Parity check can detect an error but it cannot specify which bit is in error.

11. What are the three types of redundancy checks used in data communications?

-     Vertical Redundancy Check (VRC)

-     Longitudinal Redundancy Check (LRC)

-     Cyclic Redundancy Check (CRC)

12. How can the parity bit detect a damaged data unit?

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

13. How can we use the Hamming code to correct a burst error?

By rearranging the order of bit transmission of the data units, the Hamming code can correct burst errors.

14. Briefly discuss Stop and Wait method of flow control?

      In Stop and Wait of flow control, the sender sends one frame and waits for an acknowledgement before sending the next frame.

15. In the Hamming code for a data unit of m bits how do you compute the number of redundant bits ‘r’ needed?

In the Hamming code, for a data unit of m bits, use the formula 2r > = m + r + 1 to determine r, the number of redundant bits needed.

16. What are three popular ARQ mechanisms?

-     Stop and wait ARQ,

-     Go – Back – N ARQ and

-     Selective Report ARQ.

17. How does ARQ correct an error?

Anytime an error is detected in an exchange, a negative acknowledgment (NAK) is returned and the specified frames are retransmitted.

18. What is the purpose of the timer at the sender site in systems using ARQ?

The sender starts a timer when it sends a frame. If an acknowledgment is not received within an allotted time period, the sender assumes that the frame was lost or damaged and resends it.

19. What is damaged frame?

A damaged frame is recognizable frame that does arrive, but some of the bits are in error (have been altered during transmission)

20. What is HDLC?

HDLC is a bit oriented datalink protocol designed to support both half-duplex and full duplex communication over point to point and multiport link.

21. Give data transfer modes of HDLC?

1.   NRM – Normal Response Mode

2.   ARM – Asynchronous Response Mode

3.   ABM - Asynchronous Balanced Mode

22. How many types of frames HDLC uses?

1.   U-Frames

2.   I-Frames

3.   S-Frame

22. State phases involved in the operation of HDLC?

1.   Initialization

2.   Data transfer

3.   Disconnect

23. Define piggybacking?

The inclusion of an acknowledgment to a previously received packet in an outgoing data packet is known as piggybacking.

24. What is the meaning of ACK frame?

ACK frame is an indication that a station has received something from another.

25. What is CSMA?

Carrier Sense Multiple Access is a protocol used to sense whether a medium is busy before attempting to transmit.

26. Explain CSMA/CD

Carrier Sense Multiple Access with collision detection is a protocol used to sense whether a medium is busy before transmission but is has the ability to detect whether a transmission has collided with another

25. What is collision detection?

It is the ability of a station to determine when a collision has occurred.

26.  What is bit stuffing and why it is needed in HDLC?

Bit stuffing is the process of adding one extra 0 whenever there are five consecutive 1s in the data so that the receiver does not mistake the data for a flag. Bit stuffing is needed to handle data transparency.

27. What is the use of p/f bit in the HDLC frame?

P/F bit is subfield in HDLC frame. The P/F field is a single bit with a dual purpose, when it is set it can mean poll or final. It means poll when the frame is sent by a primary station to secondary station and it mean final when the frame is sent by a secondary to a primary station.