The datalink layer

Question 1

Role of the datalink layer

Answer 1

The Data Link Layer is responsible for:
- Framing
- Error handling
- Flow control

Question 2

What is a frame

Answer 2

Its is the transmitted element in Data Link layer . A frame is regarded either as a block of bytes, or as a sequence of bits

Question 3

Framing (to frame)

Answer 3

• The sender assembles/breaks an incoming sequence of packets (from network layer) into frames
• The receiver does the opposite

Question 4

Framing extraction

Answer 4

The receiver must determine the first and last bit/byte of the frame so that the frames can then be extracted

Question 5

Byte count

Answer 5

• Payload is transmitted as a variable length block of bytes preceded by a byte count
• this method uses a field in the header to specify the number of bytes in the frame

Question 6

Data transparency problem

Answer 6

It may happen that the flag byte occurs in the data .
This would cause the protocol software to incorrectly assume that the frame was finished at that point

Question 7

Solution to data transparency problem

Answer 7

insert a special escape byte (ESC) just before each ‘‘accidental’’ flag byte in the data. This is known as Byte stuffing

Question 8

Redundant data

Answer 8

In the data link layer, redundant data refers to extra bits added to the data being transmitted to detect and correct errors. This helps to improve the reliability of the communication.

Question 9

Error detection techniques

Answer 9

• Parity bit
• Checksums
• Cyclic Redundancy Check (CRC)

Question 10

Flow control protocol

Answer 10

It controls the sending of transmission frames at a faster pace than they can be accepted e.g. Simple Stop and Wait Protocol

Question 11

Stop and Wait protocol

Answer 11

• Sender sends one data frame
• Receiver receives the data frame and send an acknowledgment
• Sender receives the Acknowledgment and sends the next data frame

Question 12

Disadvantages of stop and wait protocol

Answer 12

• Slow
• Cannot be used in noisy channel

Question 13

Problems with stop and wait protocol

Answer 13

1. Damaged frame
2. Lost data frame
   3.Lost ACK

Question 14

Timeout problem with Stop and wait protocol

Answer 14

• If it is too long, (A) will wait unnecessarily long before
  retransmission, causing the link to be idle
• if it is too short, the protocol may fail due to the
  occurrence of duplicate ACKs.

Question 15

Automatic Repeat ReQuest Protocols

Answer 15

• A group of error control protocols incorporated with stop and wait flow control protocol for transmission of data over a noisy or unreliable network
• They provide automatic retransmission of frames that are corrupted or lost during transmission

Question 16

3 types of Automatic Repeat Request Protocols

Answer 16

• Stop – and – Wait ARQ
• Go – Back – N ARQ
• Selective Repeat ARQ

Question 17

Stop and wait ARQ

Answer 17

• “A” sends a frame with a unique sequence number, then waits for “B” to acknowledge it (flow control)
• If a timeout occurs ,the frame is retransmitted again (error control)
• frames with CRC errors are rejected by “B” and a NAK is sent
• Duplicate frames or Acks are also rejected

Question 18

piggy backing

Answer 18

• Data frames sent by a receiver carry acknowledgements of its received frames.
• Similar to Sop and Wait ARQ

Question 19

Advantages of piggy backing

Answer 19

• Better channel utilisation: fewer bits
  transmitted
• Less computation by the hosts: less work to
  pack, unpack and check the extra frames

Question 20

disadvantages of piggy backing

Answer 20

• If the receiver waits “too long” to send a
  piggybacked acknowledgement (waiting for
  a data frame to be available), the sender
  will time-out and retransmit the frame!

Question 21

Go back N protocol

Answer 21

• It allows for sending N frames before receiving the acknowledgement for the first frame
• The frames are sequentially numbered
• It uses the concept of sliding windows at the sender and the receiver, and so is also called Go
  -Back-N sliding window protocol

Question 22

Selective Repeat ARQ Protocol

Answer 22

• This protocol also provides for sending N frames before receiving the acknowledgement
  for the first frame
• The frames are sequentially numbered
• It uses the concept of sliding windows at the sender and the receiver, and so is also called
  Selective Repeat sliding window protocol

Question 23

What if the frame is a block of bytes

Answer 23

• Byte count
• Start and end flag bytes with byte stuffing

Question 24

What if the frame is a sequence of bits

Answer 24

• Start and end flag bytes with bit stuffing

Question 25

Byte stuffing

Answer 25

It is a method of inserting special escape characters into a data stream to signal the start and end of a frame. It is used to ensure that certain sequences of bytes, known as flag sequences, do not occur within the data.

Question 26

Bit Stuffing

Answer 26

Bit stuffing is a method of inserting extra bits into a data stream to ensure that certain bit patterns do not occur within the data. It is used to maintain the integrity of a frame structure in communication protocols.

Question 27

Error Handling

Answer 27

Detecting and correcting errors from the physical layer

Question 28

how to solve stop and wait timeout problem

Answer 28

• This can be solved by including the sequence number of
  the acknowledged frame into the ACK frame itself.
• However, all frames are still sent twice, causing 50% loss of
  transmission capacity