Framing
Definitely, the first service provided by the data-link layer is framing. The data-link
layer at each node needs to encapsulate the datagram (packet received from the network
layer) in a frame before sending it to the next node. The node also needs to decapsulate
the datagram from the frame received on the logical channel. Although we have shown
only a header for a frame, we will see in future chapters that a frame may have both a
header and a trailer. Different data-link layers have different formats for framing.