Introduction to Data link layer
  1. It divides the stream of bits into small manageable packets called as frames.  
  2. The data link layer transforms the physical layer, a raw transmission facility, to a link responsible for node-to-node (hop-to-hop) communication. 
  3. It provides Hop-Hop connection.

Functionality :

  1. Access control
  2. Error control
  3. Flow control
  4. Framing
  5. Physical addressing.


Taking streams of bits from the network layer and divides into small manageable units is called frames.

A large stream of bits can be sent throughout the channel but doing so will have some issues like :

When an error occurred, you have to check the entire message and then retransmit the whole bitstream. When a message is carried in one very large frame, even a single-bit error would require the retransmission of the whole message. When a message is divided into smaller frames, a single-bit error affects only that small frame.


SFD (Start frame delimiter):- It tells the start of the frame.

EFD(End frame delimiter):-   these bit tells End of the frame.


Types of framing:-

1. Fixed-size Framing:

Size of each frame is fixed. Here we don't need any delimiter because the size of frames is confirmed and it itself acts as the delimiter.


2. Variable Size Framing:-

Here we have to define delimiter. We have to tell the beginning and end of each frame. For this, we have two approaches.

Character Stuffing and Bit Stuffing.