Data-link Layer Introduction

• Data Link Layer is second layer of OSI Layered Model. This layer is one of the most complicated layers and has complex functionalities and liabilities. 
• Data link layer hides the details of underlying hardware and represents itself to upper layer as the medium to communicate.
• Data link layer works between two hosts which are directly connected in some sense. This direct connection could be point to point or broadcast. 
• Systems on broadcast network are said to be on same link. The work of data link layer tends to get more complex when it is dealing with multiple hosts on single collision domain.
• Data link layer is responsible for converting data stream to signals bit by bit and to send that over the underlying hardware. 
• At the receiving end, Data link layer picks up data from hardware which are in the form of electrical signals, assembles them in a recognizable frame format, and hands over to upper layer.

Data link layer has two sub-layers

• Logical Link Control: It deals with protocols, flow-control, and error control
• Media Access Control: It deals with actual control of media.

 Functionality of Data-link Layer

Data link layer does many tasks on behalf of upper layer. These are..

Framing

• Data-link layer takes packets from Network Layer and encapsulates them into Frames.Then, it sends each frame bit-by-bit on the hardware. 
• At receiver’ end, data link layer picks up signals from hardware and assembles them into frames.

Physical Addressing

• Data-link layer provides layer-2 hardware addressing mechanism. 
• Hardware address is assumed to be unique on the link. 
• It is encoded into hardware at the time of manufacturing.

Synchronization

• When data frames are sent on the link, both machines must be synchronized in order to transfer to take place.

Error Control (Error Detection and Correction)

• Sometimes signals may have encountered problem in transition and the bits are flipped.These errors are detected and attempted to recover actual data bits. It also provides error reporting mechanism to the sender.

Flow Control

• Stations on same link may have different speed or capacity. Data-link layer ensures flow control that enables both machine to exchange data on same speed.

Multi-Access Control

• When host on the shared link tries to transfer the data, it has a high probability of collision. 
• Data-link layer provides mechanism such as CSMA/CD to equip capability of accessing a shared media among multiple Systems.