Data Link Layer - Layer 2 of OSI model

The Data Link Layer is Layer 2 of the seven-layer OSI model of computer networking.
At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization.

The data link layer performs various functions depending upon the hardware protocol used, but has four primary functions:

- Communication with the Network layer above.
- Communication with the Physical layer below.
- Segmentation of upper layer datagrams (also called packets) into frames in sizes that can be handled by the communications hardware.
- The data link layer organizes the pattern of data bits into frames before transmission. The frame formatting issues such as stop and start bits, bit order, parity and other functions are handled here.
- It provides error checking by adding a CRC to the frame, and flow control.
- The data link layer is also responsible for logical link control, media access control, hardware addressing, error detection and handling and defining physical layer standards.
- The data link layer is divided into two sublayers: the media access control (MAC) layer and the logical link control (LLC) layer. The former controls how computers on the network gain access to the data and obtain permission to transmit it; the latter controls packet synchronization, flow control and error checking.
- The data link layer is where most LAN (local area network) and wireless LAN technologies are defined. Technologies and protocols used with this layer are Ethernet, Token Ring, FDDI, ATM, SLIP, PPP, HDLC, and ADCCP.
- The data link layer is often implemented in software as a driver for a network interface card (NIC). Because the data link and physical layers are so closely related, many types of hardware are also associated with the data link layer.
- Data link layer processing is faster than network layer processing because less analysis of the packet is required.
- The Data Link layer also manages physical addressing schemes such as MAC addresses for Ethernet networks, controlling access of any various network devices to the physical medium.