- bits <-> electrical / optical signals
- transmit individual bits over the cable: modulation, encoding
- bits <-> frames
- bit error detection
- packet boundaries
- in some cases: error correction by retransmission (802.11)
Physical Layer: The job of the physical layer is to move the individual bits within the frame from one node to the next. The protocols in this layer are again link dependent, and further depend on the actual transmission medium of the link (for example, twisted-pair copper wire, single-mode fiber optics). For example, Ethernet has many physical layer protocols: one for twisted-pair copper wire, another for coaxial cable, another for fiber, and so on. In each case, a bit is moved across the link in a different way.
Link Layer: The services provided at the link layer depend on the specific link-layer protocol that is employed over the link. For example, some protocols provide reliable delivery on a link basis, that is, from transmitting node, over one link, to receiving node. The process is analogous to the postal worker at a mailing center who puts a letter into a plane that will deliver the letter to the next postal center along the route. Examples of link layers include Ethernet and PPP; in some contexts, ATM and frame relay can be considered link layers. As datagrams typically need to traverse several links to travel from source to destination, a datagram may be handled by different link-layer protocols at different links along its route. For example, a datagram may be handled by Ethernet on one link and then PPP on the next link. The network will receive a
different service from each of the different link-layer protocols.
