A segment of data on a network or telecommunications link, generally consisting of a header with preamble (start of frame flag), destination and source addresses, data payload, and usually some form of error-checking information. Frames are assembled and generated by the data-link layer and physical layer of the Open Systems Interconnection (OSI) reference model.
This assembly process is called “framing.” In other words, packets from the network layer are encapsulated by the data-link layer into frames. Data segments generated by higher layers of the OSI model are generally referred to as packets, but the term “packet” is also sometimes used to include frames.
The format in which data frames are constructed depends on the particular data-link layer protocol being used. Thus, we can speak of Ethernet frames, Integrated Services Digital Network (ISDN) frames, X.25 frames, frame relay frames, and so on. Each particular local area network (LAN) or wide area network (WAN) data-link protocol has its own method of framing data for transmission over the network or telecommunications line. Frames can be fixed-length or variable-length, and have addressing information for multipoint connections or no addressing information for point-to-point connections. They can also have error correction, as in X.25, or no error correction, as in frame relay.