computer networking-fuctions of datalink layer

1. Functions of data link layer
Subject: computer network
CE-B
 Maulik togadiya

2. Data link layer
 The data link layer transmit data packet(frames)
sequentially over the transmission medium.
 The data link layer provides reliable information
transfer capabilities by sending ack frame to the
sender after receiving data packet.

3.  The data link layer has two sub layers:
1) logical link control (LLC)
2) media access control (MAC).
LLC: It specifies which mechanisms are to be used for
addressing stations over the transmission medium and
for controlling the data exchanged between the
originator and recipient machines.
MAC: when several computer are trying to transmit data,
MAC determine which computer should get a
transmission medium to transfer the data.

4. Main functions
 Encapsulation of network layer data packets
into frames.
 Frame synchronization
 Error control and error detection
 Flow control

5. Frames
 A frame is a digital data transmission unit
in computer networking and telecommunication .
 A frame typically includes frame
synchronization features consisting of a sequence of
bits or symbols that indicate to the receiver the
beginning and end of the payload data within the
stream of symbols or bits it receives. If a receiver is
connected to the system in the middle of a frame
transmission, it ignores the data until it detects a
new frame synchronization sequence.

6. Framing Method Implemented By
Data Link Layer Are:-
 Character Count.
 Starting & Ending Characters With Character
Stuffing.
 Starting & ending Flags With Bit Stuffing.
 Physical Layer Coding Violation.

7. 3 0 41 1 57 7 02 41 26 12
Character count
Character Count Method
Frame 1 Frame 2 Frame 3 Frame 4

8. Starting & Ending Characters With Character
Stuffing
DLE ASTX B DC ETXDLE
Start Of Frame Data End Of Frame
A B DC Data From Network Layer

9. Character Stuffing
DLE ASTX B DC ETXDLE
Start Of
Frame
Data End Of
Frame
A B DCDLE
DLEDLE
A B DCDLE
data on
Sender
side
data on
Receiver
side

10. Starting & ending Flags With Bit Stuffing
0 1 10 0 10 1 11 1 01 1001111110 01111110
Starting flag Bit
Ending Flag BIT
Shifted Bit
BIT stuffing
Stuffing
Performed
By Data
Link layer
0 1 10 0 10 1 11 1 01 101
Data received by
Network layer On
receiver side After
Performing Destuffing
By data link Layer

11. Physical Layer Coding Violation
 In this method Bit 1 Is encoded into high-low(10)
Pair And Bit 0 Is Encoded Into low-high(01)pair
shown in figure.
0 1 10 01
Menchester Encoding

12. Physical Layer Coding Violation.
0 1 10 01
Menchester Encoding

13. Error detection and correction
Error detection
Error detection means to decide whether the received
data is correct or not without having a copy of the
original message.
Error detection uses the concept of redundancy, which
means adding extra bits for detecting errors at the
destination.

14. Four types of redundancy checks are used
in data communications

15. Vertical Redundancy Check
VRC

16. Longitudinal Redundancy Check
LRC

17. Cyclic Redundancy Check
CRC

18. Checksum

19. Error Correction
It can be handled in two ways:
1) receiver can have the sender retransmit the entire
data unit.
2) The receiver can use an error-correcting code,
which automatically corrects certain errors.

20. Hamming Code

21. Hamming Code

22. Hamming Code

23. Example of Hamming Code

24. Error
Detection