What is Data Link Layer | Layer-2

Ø Data Link Layer is responsible for end-to-end delivery of frames between devices on a LAN Network segment

Ø  Data link layer comprises of 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

Ø  It deals with hardware addresses (MAC addresses)

Ø  Devices work at Data Link layer are “Switches”

Ø  Data Link Layer is second layer of OSI Layered Model.

Ø  Data link layer hides the details of underlying hardware and represents itself to upper layer as the medium to communicate.

Ø  Data link layer works between two hosts which are directly connected in some sense. This direct connection could be point to point or broadcast.

Ø  Data link layer is responsible for converting data stream to signals bit by bit and to send that over the underlying hardware.

Ø  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.

Functionality of Data-link Layer

a) Framing

Ø 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.

b) Addressing

Ø Data-link layer provides layer-2 hardware addressing mechanism. Hardware address(MAC address) is assumed to be unique on the link.

Ø It is encoded into hardware at the time of manufacturing.

c) Synchronization

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

d) Error Control

Ø Sometimes signals may have encountered problem in transition and the bits are flipped. These errors are detected and attempted to recover actual data bits.

Ø Adds reliability to physical layer by adding mechanisms to detect and retransmit damaged or lost frames.

Ø Uses mechanisms to recognize duplicate frames

Ø It also provides error reporting mechanism to the sender.

Ø Error control is normally achieved through a Trailer added to end of frames

e) Flow Control

Ø 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.

f)Multi-Access

Ø 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.