The Data Link Layer

   Frames Across a Link

While the Physical Layer streams bits one at a time onto a link, the Data Link Layer addresses groups of bits to a particular node that is attached somewhere on that physical link. Each group of bits the Data Link Layer transmits is called a "frame." Frames are typically generated by the NIC of the sending computer.

Packets Within Frames

The Network Layer handles data in units called "packets." Thus, the Network Layer passes a packet to the Data Link Layer, which encapsulates the packet by placing a header and trailer around it, just as we encapsulate a letter within an envelope. The encapsulated data (header, packet, and trailer) forms the frame. As it encapsulates the packet passed down from the Network Layer, the Data Link Layer is not aware of the meaning of that data.

Frame Addressing

The frame header includes the unique NIC addresses of the sending and receiving nodes. Thus, each frame is addressed to an individual computing machine. The frame trailer contains error-checking information that allows the receiving Data Link Layer process to determine whether the frame has been damaged during transit.

The Data Link Layer passes each frame down to the Physical Layer for transmission across the link as a stream of bits. The receiving Data Link Layer process receives the bit stream from the Physical Layer and determines where each frame begins and ends. It then removes the frame header and trailer, then passes the packet up to the Network Layer.