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FireWire

From Academic Kids

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A 6-Pin FireWire 400 connector

FireWire (also known as i.Link or IEEE 1394) is a personal computer and digital video serial bus interface standard offering high-speed communications and isochronous real-time data services. FireWire can be considered a successor technology to the obsolescent SCSI Parallel Interface.

Almost all modern digital camcorders have included this connection since 1995. All Macintosh computers currently produced have built-in FireWire ports, as do all Sony PCs and many PCs intended for home or professional audio/video use. FireWire is also used on the Apple iPod music player, permitting new tracks to be uploaded in a few seconds and also for the battery to be recharged concurrently with one cable.

Contents

Standards and versions

FireWire color logo

FireWire was developed primarily by Apple Computer in the 1990s, after work defining a slower version of the interface by the IEEE 1394 working committee in the 1980s. IEEE proposed the standard as a serial replacement for the SCSI bus. Apple's development was completed in 1995. It is defined in IEEE standard 1394 which is currently a composite of three documents: the original IEEE Std. 1394-1995, the IEEE Std. 1394a-2000 amendment, and the IEEE Std. 1394b-2002 amendment. Sony's implementation of the system is known as i.Link, and uses only the four signal pins, discarding the two pins that provide power to the device in favor of a separate power connector on Sony's i.Link products.

The system is commonly used for connection of data storage devices and digital video cameras, but is also popular in industrial systems for machine vision and professional audio systems. It is used instead of the more common USB due to its faster speed, higher power distribution capabilities, and because it does not need a computer host. It also has native support for isochronous data transport (data that must be delivered with deterministic latency, such as audio or video). However, the small royalty that Apple Computer and other patent holders have initially demanded from users of FireWire ($0.25 per end-user system) and the more expensive hardware needed to implement it ($1–$2) has prevented FireWire from displacing USB in low-end mass-market computer peripherals where cost of product is a major constraint.

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A 4-Pin FireWire 400 connector. This connector is not powered.

FireWire can connect together up to 63 peripherals in an acyclic network structure (as opposed to SCSI's linear structure). It allows peer-to-peer device communication, such as communication between a scanner and a printer, to take place without using system memory or the CPU. FireWire also supports multiple hosts per bus, and IP networks can be formed through software between FireWire-linked computers. It is designed to support plug-and-play and hot swapping. Its six-wire cable is not only more convenient than SCSI cables but can supply up to 45 watts of power per port, allowing moderate-consumption devices to operate without a separate power cord. The Sony-inspired i.Link usually omits the power part of the cable/connector system and only uses a 4-pin connector.

FireWire 400 can transfer data between devices at 100, 200, or 400 Mbit/s data rates (actually 98.304, 196.608, or 393.216 Mbit/s, but commonly referred to as S100, S200, and S400). Cable length is limited to 4.5 metres but up to 16 cables can be daisy chained yielding a total length of 72 meters under the specification.

FireWire 800 (Apple's name for the 9-pin "S800 bilingual" version of the IEEE1394b standard) was introduced commercially by Apple in 2003, allows an increase to 786.432 Mbit/s with backwards compatibility to the slower rates and 6-pin connectors of FireWire 400.

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A 9-Pin FireWire 800 connector

The full IEEE 1394b specification supports optical connections up to 100 metres in length and data rates all the way to 3.2 Gbit/s. Standard category-5 unshielded twisted pair supports 100 metres at S100, and the new p1394c technology goes all the way to S800. The original 1394 and 1394a standards used data/strobe (D/S) encoding (called legacy mode) on the signal wires, while 1394b adds a data encoding scheme called 8B10B (also referred to as beta mode). With this new technology, FireWire, which was arguably already slightly faster, is now substantially faster than Hi-Speed USB.

FireWire devices implement the ISO/IEC 13213 "configuration ROM" model for device configuration and identification, to provide plug-and-play capability. All FireWire devices are identified by an IEEE EUI-64 unique identifier (an extension of the 48-bit Ethernet MAC address format) in addition to well-known codes indicating the type of device and protocols it supports.

Security issues

Devices on a FireWire bus can communicate by direct memory access, where a device can use hardware to map internal memory to FireWire's "Physical Memory Space". The SBP (serial bus protocol) used by FireWire disk drives use this capability to minimize interrupts and buffer copies. In SBP, the initiator (controlling device) sends a request by remotely writing a command into a specified area of the target's FireWire address space. This command usually includes buffer addresses in the initiator's FireWire "Physical Address Space", which the target is supposed to use for moving I/O data to and from the initiator.

On many implementations, particularly those like PCs and Macintoshes using the popular OHCI interface, the mapping between the FireWire "Physical Memory Space" and device physical memory is done in hardware, without operating-system intervention. While this enables extremely high-speed and low-latency communication between data sources and sinks without unnecessary copying (such as between a video camera and a software video recording application, or between a disk drive and the application buffers), this can also be a security risk if untrustworthy devices are attached to the bus. For this reason, high-security installations will typically either purchase newer machines that map a virtual memory space to the FireWire "Physical Memory Space" (such as a G5 Macintosh, or any Sun workstation), disable the OHCI hardware mapping between FireWire and device memory, physically disable the entire FireWire interface, or do not have FireWire at all.

This feature can also be used to debug a machine whose operating system has crashed, and in some systems for remote-console operations.

Hot swap dangers

Although FireWire devices can be hot-swapped without powering down equipment, there are reports of cameras being damaged if the pins are accidentally shorted while swapping. Also, the potential difference between a computer and a camera sometimes results in arcing (sparks) when plugging the camera in. This can in turn "fry" the sensitive FireWire chip set of the camera, rendering the FireWire port unusable.

To ensure maximum protection of a consumer DV camera, both the camera and computer should be powered off before connecting a FireWire cable. Most commercial grade equipment is less sensitive to being hot-plugged, although care should still be taken.

History

According to Michael Johas Teener, original chair and editor of the IEEE 1394 standards document, and technical lead for Apple's FireWire team from 1990 until 1996:

  • When Apple was participating with the IEEE p1394 working group, they proposed licensing all of Apple's blocking patents for $3,000, a one time fee only for "the point of first use" or the integrated circuits that implement the protocols. Furthermore, there was a discount if a contribution was made to the IEEE undergraduate scholarship fund. Under that agreement, the IEEE agreed to include the appropriate patents in the standard.
  • The original FireWire project name was "Chefcat", the name of Michael Teener's favorite coffee cup.
  • The "FireWire" name was chosen by a group of engineers socializing before Comdex 1993, just before the project was about to go public. IBM, Apple, Texas Instruments, Western Digital, Maxtor and Seagate were all showing drives, systems and other various FireWire support technology. The marketing forces behind the FireWire project had originally considered a name like "Performa".
  • FireWire won the "most significant new technology" award from Byte Magazine at the Comdex 1993 show.
  • FireWire is a trademark of Apple Computer, Inc. The trademark was filed in 1993.
  • Apple never intended to charge for the use of the name "FireWire". It could be used by any party signing an agreement to use the name for a product that was compliant with IEEE 1394-1995, the original version of the standard.
  • Steve Jobs was convinced that Apple should ask for $1 per port for the patents that became part of the standard. The argument was that it was consistent with the MPEG patent fees.
  • The fallout from charging $1 per FireWire port was significant, particularly from Intel. Intel had sunk a great deal of effort into the standard with the improved 1394a-2000 standard being partially based on work contributed by Intel. A group within Intel used this as a reason to drop 1394 support and bring out the improved USB 2.0 instead.
  • Simultaneously, Sony and the other backers of the technology noted to Apple that they all had patents too and were entitled to per-port royalties. Under these circumstances, Apple would have to pay roughly $15 per port to the other FireWire technology developers. The end result was the creation of the "1394 Licensing Authority", a body which charges everyone $0.25 per end user system (like a car or computer) that uses any 1394 technology.

See also

External links

References


This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
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