How Does FireWire Work?

The IEEE-1394 High Performance Serial Bus is a remarkable engineering feat that has occupied many highly-creative digital circuit designers and software programmers for the past 10 years. FireWire is a very complex serial bus protocol, as evidenced by the hundreds of pages that comprise its standard specification. The following list, with links to pages providing more detailed information, is a very simplified description of the external and internal workings of the FireWire bus:

Standard cables and connectors replace the myriad of I/O connectors employed by consumer electronics equipment and PCs. FireWire multiplexes a variety of different types of digital signals, such as compressed video, digitized audio, MIDI, and device control commands, on two twisted-pair conductors. Multiplexing is used in virtually all analog and digital networking systems, but usually only a single type of signal is involved. As an example, Ethernet multiplexes digital data streams from workstations and servers over one (10Base2, "Thin" Ethernet) or two (10BaseT, 100BaseT) pairs of conductors. (FireWire cabling is quite similar to that of 10BaseT Ethernet.) Sending real-time, high-quality audio and video data over Ethernet, however, requires special protocols presently implemented only by proprietary multimedia networking systems. FireWire is much more flexible in its accommodation of different data types and topologies than alternative networking systems. FireWire uses a "fairness" arbitration approach to assure that all nodes having information to transmit get a chance to use the bus; standard Ethernet does not provide this type of arbitration. To implement home FireWire networks, bridges isolate local traffic on individual groups of nodes.
Special integrated circuit chips implement the FireWire protocol. Like Ethernet and other high-speed digital data transmission systems, FireWire is a layered transport system. The IEEE-1394 standard defines three layers: Physical, Link, and Transaction. The Physical layer provides the signals required by the FireWire bus. The Link layer takes the raw data from the Physical layer and formats it into recognizable 1394 packets. The Transaction layer takes the packets from the Link layer and presents them to the application. Link chips provide all link functions as well as a limited number of transaction functions. The remainder of the transaction functions are performed in software.
Consumer audio/video applications use logical "plugs and sockets," which are analagous to the physical RCA phono jacks and mini-DIN S-video connectors used by TV sets, VCRs, camcorders, receivers, amplifiers, and other audio/visual components. A "plug" corresponds to an audio or video output and a "socket" represents an input connector. The implementation of logical plugs and sockets is defined by the pending Digital Interface for Consumer Electronic Audio/Video Equipment specification, an extension to the IEEE-1394 standard proposed by members of the Japanese Digital Video Consortium (DVC), which is responsible for establishing the consumer DV standard. The Digital Interface specification has been prepared by the DVC for submission to ISO/IEC (International Standards Organization/International Electrotechnical Committee), rather than the IEEE.

If you're interested in a more technical explanation of how the IEEE-1394 works, read a longer article entitled Fire on the Wire: The IEEE-1394 High Performance Serial Bus.

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