7 Layer OSI : Data Link Layer

As for serial links, the link layer provides data exchange between neighboring computers as well as data exchange between computers within a local network.
For the link layer, the basic unit of data transfer is the data link packet frame. A data frame is composed of a header, payload, and trailer.

A frame carries the destination link address, source link address, and other control information in the header. The trailer usually contains the checksum of the transported data. By using the checksum, we can find out whether the payload has been damaged during transfer. The network-layer packet is usually included in the payload.
The link layer does not engage in a conversation between DTE and DCE (the link layer does not see the DCE). It is engaged, however, in the frame exchange between DTEs. (It relies on the physical layer to handle the DCE issue.)
The following figure illustrates that different protocols can be used for each end of the connection on the physical layer. In our case, one of the ends uses the X.21 protocol while the other end uses the V.35 protocol. This rule is valid not only for serial links, but also for local networks. In local networks, you are more likely to encounter more complicated setups in which a switch that converts the link frames of one link protocol into link frames of a second one (for example, Ethernet into FDDI) is inserted between the two ends of the connection. This obviously results in different protocols being used on the physical layer.

A serial port or an Ethernet card can serve as a link interface. A link interface has a link address that is unique within a particular Local Area Network (LAN).