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
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
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