In computer networking, a frame is a data packet of fixed or variable length which has been encoded by a data link layer communications protocol for digital transmission over a node-to-node link. Each frame consists of a header frame synchronization and perhaps bit synchronization, payload (useful information, or a packet at higher protocol layer) and trailer. Examples are Ethernet frames and Point-to-point protocol (PPP) frames. In telecommunications, the frame structure refers to the way a multiplexer divides the underlying communication channel into several channels, so that it can be used simultaneously for transferring more than one data stream. In statistical multiplexing, a frame may be a container which could be filled by other frames, in a hierarchical structure of multiplexed channels. A frame may also be a data packet of fixed or variable length, including a header with a channel identification number or a destination address. One example of statistical multiplexing is the MPEG transport stream used in digital TV transmission, where the channel number is denoted Packet ID (PID). Another example is the TCP and UDP multiplexing, where the channel number is denoted port number. Other examples are the Frame Relay protocol, where the channel number is called Data link connection identifier (DLCI), and Asynchronous Transfer Mode (ATM), where virtual circuits are identified hierarchically by a Virtual channel identifier and Virtual Path Identifier (VCI/VPI pair). In time-division multiplexing (TDM), a frame is a time interval of constant length, consisting of a fixed set of periodically repeated time slots of fixed length. Each time slot corresponds to one circuit switched (constant bitrate) channel. Not all time slots are necessarily in use. Normally a dedicated time slot is used for frame synchronization. The other timeslots only include payload data, typically one byte byte timeslot. No channel identification numbers are required, since the channel number is obvious from the position of the time slot relative to the frame synchronization. The term in-frame is used to indicate that a time-division multiplexer is properly synchronized with the demultiplexer on the other end of the link, so that (barring in-flight data corruption) packets will be properly received. One example of a TDM system is the pulse-code modulation (PCM) system, in which each time slot consist of one byte of data, corresponding to a digitized sample of an analog signal, for example a phone call.
