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Link layer is the first lowest layer of the TCP/IP reference model, which describes how data should be physically generated and transmitted over the physical medium by the network devices.

Description

The physical layer and data link layer of OSI reference model is combined together to form a link layer.

Following are the functions and responsibilities of link layer :

At physical level

• At physical level data consist of a stream of bits (sequence of 0's and 1's).
• Defines how the bits are encoded into electrical signals by the network devices.
• Synchronizing the sender and receiver at the bit level.
• Line configuration - point-to-point or point-to-multipoint
• Transmission mode - simplex, half duplex or full duplex
• Topology - star, ring, tree, bus etc.
• Mechanical specification of cables and connectors.
• Specification of electronic factors like voltage, frequency, impedance etc.

At logical level

• At logical level data consists of frames.
• Encapsulates IP packets or message received from the higher layer into frames.
• Synchronization of frames.
• Flow control, error detection and correction of frames at LLC subl ayer.
• Physical addressing, access control at MAC sub layer.

Protocols used: Ethernet, Token ring, X.25, Frame relay, FDDI, GPRS.

Network Devices: Network adaptor, Modem, Hub, Switch.