When the physical layer does not have anything to send, it may send idle bits on the link. The idle bits are used to keep the link alive and to let the physical layer of all stations know that there are no packets being sent even though the physical link is up and functioning. When the data link layer has a packet for the physical layer to send, it marks the beginning of the packet with some specific sequence of bits. This sequence of bits informs the receiver that the link is not in the idle state anymore and that a packet is comming. The data link layer adds this sequence to frame the packet received from the upper layer. That is why a packet at the data link layer is called a frame.