OSI Model: Data Link Layer

The data link layer is layer 2 of OSI reference model. The data link layer is responsible for node to node delivery of frames. This layer receives packets from network layer, converts packets to frames and gives it to physical layer. It delivers frames between nodes on the same network. Data link layer analyse the bit pattern of data and ensures that received data is error free. If the bit pattern of received data is different, then it tries to correct data or ask to retransfer data.

There may be situation when different devices try to transfer data through same medium in LAN. This causes frame collision. Data link layer specifies how to recover frames or reduce frame collision.

The data link layer contains two sublayers:

1. Logical Link Control (LLC)

2. Media Access Control (MAC)

Functions of Data Link Layer

Framing: The data link layer converts the bit stream received from network layer to the manageable data units called frames.

Physical Addressing: To distribute frames to different devices on the network, physical address is necessary. For this purpose, data link layer adds a header to the frame to define physical address of sender and receiver. If the receiver is outside the sender network, then the receiver address is the address of the device that connects the network to the next one. 

Flow Control: If the data rate at receiver side is less than the sender side data rate, then there is a chance of data loss. For this, data link layer imposes a flow control mechanism to avoid traffic at receiver side.

Error Control: The data link layer adds trailer at the end of the frame to control error. It also uses mechanism to recognize duplicate frames. 

Access Control: If two or more devices try to transfer data through the same link on the network, it causes frame collision. To avoid this, data link layer protocols determines which device has control over the link at a given time.