IEEE 1394a

IEEE 1394 interface

The IEEE 1394 interface is a serial bus interface standard for high-speed communications and isochronous real-time data transfer, frequently used in a personal computer (and digital audio and digital video). The interface is also known by the brand names of FireWire (Apple Inc.), i.LINK (Sony), and Lynx (Texas Instruments). The 1394 standard also defines a backplane interface, though this is not as widely used.

FireWire has replaced Parallel SCSI in many applications, due to lower implementation costs and a simplified, more adaptable cabling system. IEEE 1394 has been adopted as the High Definition Audio-Video Network Alliance (HANA) standard connection interface for A/V (audio/visual) component communication and control. FireWire is also available in wireless, fiber optic, and coaxial versions using the isochronous protocols.

Almost all digital camcorders have included this connection since 1995, as have the vast majority of high end professional audio interfaces. Since 2003 many computers intended for home or professional audio/video use have built-in FireWire/i.LINK ports, including all Sony computers, all but one of Apple's computers (the MacBook Air), and many of its older iPods. It is also available on many retail motherboards.

History and development

FireWire is Apple Inc.'s name for the IEEE 1394 High Speed Serial Bus. It was initiated by Apple and developed by the IEEE P1394 Working Group, largely driven by contributions from Apple, although major contributions were also made by engineers from Texas Instruments, Sony, Digital Equipment Corporation, IBM, and INMOS/SGS Thomson (now STMicroelectronics).

Apple intended FireWire to be a serial replacement for the parallel SCSI (Small Computer System Interface) bus while also providing connectivity for digital audio and video equipment. Apple's development began in the late 1980s, later presented to the IEEE, and was completed in 1995. As of 2007, IEEE 1394 is a composite of four documents: the original IEEE Std. 1394-1995, the IEEE Std. 1394a-2000 amendment, the IEEE Std. 1394b-2002 amendment, and the IEEE Std. 1394c-2006 amendment. On June 12 2008, all these amendments as well as errata and some technical updates were incorporated into a superseding standard IEEE Std. 1394-2008. Publication of this standard is expected mid October 2008

Sony's implementation of the system, known as "i.LINK" used a smaller connector with only the four signal pins, omitting the two pins which provide power to the device in favor of a separate power connector. This style was later added into the 1394a amendment. This port is sometimes labeled "S100" or "S400" to indicate speed in Mbit/s.

The system is commonly used for connection of data storage devices and DV (digital video) cameras, but is also popular in industrial systems for machine vision and professional audio systems. It is preferred over the more common USB for its greater effective speed and power distribution capabilities, and because it does not need a computer host. Perhaps more importantly, FireWire makes full use of all SCSI capabilities and has high sustained data transfer rates, a feature especially important for audio and video editors. Benchmarks show that the sustained data transfer rates are higher for FireWire than for USB 2.0, especially on Apple Mac OS X with more varied results on Microsoft Windows.

However, the royalty which Apple Inc. and other patent holders initially demanded from users of FireWire (US$0.25 per end-user system) and the more expensive hardware needed to implement it (US$1–$2), both of which have since been dropped, have prevented FireWire from displacing USB in low-end mass-market computer peripherals, where product cost is a major constraint.

Technical specifications

FireWire can connect up to 63 peripherals in a tree topology (as opposed to Parallel SCSI's Electrical bus topology). 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. It is designed to support Plug-and-play and hot swapping. Its six-wire cable is more flexible than most Parallel SCSI cables and can supply up to 45 watts of power per port at up to 30 volts, allowing moderate-consumption devices to operate without a separate power supply. (As 4-wire Sony-branded i.LINK cables omit power wiring, i.LINK devices must acquire power through other means.)

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 the protocols it supports.

Operating system support

Full support for IEEE 1394a and 1394b is available for Microsoft Windows XP, FreeBSD, Linux, Apple Mac OS 8.6 through to Mac OS 9, and Mac OS X as well as NetBSD and Haiku. Historically, performance of 1394 devices may have decreased after installing Windows XP Service Pack 2, but were resolved in Hotfix 885222 and in SP3. Some FireWire hardware manufacturers also provide custom device drivers which replace the Microsoft OHCI host adapter driver stack, enabling S800-capable devices to run at full 800 Mbit/s transfer rates on older versions of Windows (XP SP2 w/o Hotfix 885222) and Windows Vista. At the time of its release, Microsoft Windows Vista supported only 1394a, with assurances that 1394b support would come in the next service pack. Service Pack 1 for Microsoft Windows Vista has since been released, however the addition of 1394b support is not mentioned anywhere in the release documentation.

Cable system support

Cable TV providers (in the US, with digital systems) must, upon request of a customer, provide a high-definition capable cable box with a functional FireWire interface. This applies only to customers leasing high-definition capable cable boxes from said cable provider after April 1, 2004. The relevant law is CFR 76.640 Section 4 Subsections i and ii. The interface can be used to display or record Cable TV, including HDTV programming.

Node hierarchy

FireWire devices are organized at the bus in a tree topology. Each device has a unique self-id. One of the nodes is elected root node and always has the highest id. The self-ids are assigned during the self-id process, which happens after each bus reset. The order in which the self-ids are assigned is equivalent to traversing the tree in a depth-first, post-order manner.

Standards and versions

All the individual standards are now incorporated into a superseding standard, IEEE 1394-2008, final paper due in October 2008. The old standard history below gives a good history on the development path.

FireWire 400 (IEEE 1394-1995)

FireWire 400 can transfer data between devices at 100, 200, or 400 Mbit/s half-duplex data rates (the actual transfer rates are 98.304, 196.608, and 393.216 Mbit/s, i.e. 12.288, 24.576 and 49.152 megabytes per second respectively). These different transfer modes are commonly referred to as S100, S200, and S400.

Cable length is limited to , although up to 16 cables can be daisy chained using active repeaters, external hubs, or internal hubs often present in FireWire equipment. The S400 standard limits any configuration's maximum cable length to 72 meters. The 6-pin connector is commonly found on desktop computers, and can supply the connected device with power.

The 6-pin powered connector adds power output to support external devices. Typically a device can pull about 7 to 8 watts from the port; however, the voltage varies significantly from different devices. Voltage is specified as unregulated and should nominally be about 25 volts (range 24 to 30). Apple's implementation on laptops is typically related to battery power and can be as low as 9 V and more likely about 12 V.

Enhancements (IEEE 1394a-2000)

An amendment IEEE 1394a was released in 2000, which both clarified and enhanced the original specification. It added in support for asynchronous streaming, quicker bus reconfiguration, packet concatenation, and a power saving suspend mode.

1394a also standardized the 4-pin connector already widely in use. The 4-pin version is used on many consumer devices such as camcorders, laptops, and other small FireWire devices. Though fully data compatible with 6-pin interfaces, it lacks power connectors.

FireWire 800 (IEEE 1394b-2002)

FireWire 800 (Apple's name for the 9-pin "S800 bilingual" version of the IEEE 1394b standard) was introduced commercially by Apple in 2003. This newer 1394 specification (1394b) and corresponding products allow a transfer rate of 786.432 Mbit/s full-duplex via a new encoding scheme termed beta mode. It is backwards compatible to the slower rates and 6-pin connectors of FireWire 400. However, while the IEEE 1394a and IEEE 1394b standards are compatible, FireWire 800's connector is different from FireWire 400's connector, making the legacy cables incompatible. A bilingual cable allows the connection of older devices to the newer port.

The full IEEE 1394b specification supports data rates up to 3200 Mbit/s over beta-mode or optical connections up to 100 metres in length. Standard Category 5e unshielded twisted pair supports 100 metres at S100. 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).

FireWire S1600 and S3200

In December 2007, the 1394 Trade Association announced that products will be available before the end of 2008 using the S1600 and S3200 modes that, for the most part, had already been defined in 1394b. The 1.6 Gbit/s and 3.2 Gbit/s devices will use the same 9-pin connectors as the existing FireWire 800 and will be fully compatible with existing S400 and S800 devices. It will compete with the forthcoming USB 3.0..

FireWire S800T (IEEE 1394c-2006)

IEEE 1394c-2006 was published on June 8 2007.

It provides the following improvements

Though the potential for a combined Ethernet and FireWire RJ45 port is intriguing, as of December 2007, there are no products or chipsets which include this capability.

Future enhancements

Besides the short term shoring up of S3200 over the beta connector already discussed, future iterations of FireWire should bring a bump in speed to 6.4 Gbit/s, use of single-mode fiber, and additional connectors such as the small multimedia interface.

Comparison to USB

Although high-speed USB 2.0 nominally runs at a higher signaling rate (480 Mbit/s) than FireWire 400, typical USB PC-hosts rarely exceed sustained transfers of 280 Mbit/s, with 240 Mbit/s being more typical. This is likely due to USB's reliance on the host-processor to manage low-level USB protocol, whereas FireWire delegates the same tasks to the interface hardware. For example, the FireWire host interface supports memory-mapped devices, which allows high-level protocols to run without loading the host CPU with interrupts and buffer-copy operations.

FireWire 800 is substantially faster than Hi-Speed USB.

Alternative uses for IEEE 1394

Aircraft

IEEE 1394b is used in military aircraft, where weight savings are desired. Developed for use as the data bus on the F-22 Raptor, it is also used on the F-35 Lightning II. NASA's Space Shuttle also uses IEEE 1394b to monitor debris (foam, ice) which may hit the vehicle during launch. This standard should not be confused with the unrelated MIL-STD-1394B.

Automobiles

IDB-1394 Customer Convenience Port (CCP) is the automotive version of the 1394 standard.

Networking over FireWire

FireWire can be used for ad-hoc (terminals only, no routers) computer networks. Specifically, 2734 specifies how to run IPv4 over the FireWire interface, and 3146 specifies how to run IPv6.

Mac OS X, Linux, FreeBSD, Windows ME, Windows 2000, Windows XP, and Windows Server 2003 all include support for networking over FireWire. A network can be set up between two computers using a single standard FireWire cable, or by multiple computers through use of a hub. This is similar to Ethernet networks with the major differences being transfer speed, wire length, and the fact that standard FireWire cables can be used for point-to-point communication.

On December 4, 2004, Microsoft announced that it would discontinue support for IP networking over the FireWire interface in all future versions of Microsoft Windows. Subsequently, support for this feature was removed from both Windows Vista and Windows Server 2008.

The PlayStation 2 console had an i.LINK-branded 1394 connector. This was used for networking until the release of an Ethernet adapter late in the console's lifespan, but was poorly supported by software.

IIDC

IIDC (Instrumentation & Industrial Digital Camera) is the FireWire data format standard for live video, and is used by Apple's iSight A/V camera. The system was designed for machine vision systems, but is also used for other computer vision applications and for some webcams. Although they are easily confused since they both run over FireWire, IIDC is different from, and incompatible with, the ordinary DV (Digital Video) camcorder protocol.

DV

Digital Video (DV) is a standard protocol used by some digital camcorders. Formerly, all DV cameras had a FireWire interface (usually a 4-pin), but recently many consumer brands have switched to USB. Labeling of the port varies by manufacturer, with Sony using either its i.LINK trademark or the letters 'DV'. Many digital video recorders have a "DV-input" FireWire connector (usually a 6-pin connector) which can be used to record video from a directly-connected DV camcorder ("computer-free").

The protocol also allows remote control (play, rewind, etc.) of connected devices.

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-2 (Serial Bus Protocol 2) used by FireWire disk drives uses this capability to minimize interrupts and buffer copies. In SBP-2, 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 Macs using the popular OHCI, the mapping between the FireWire "Physical Memory Space" and device physical memory is done in hardware, without operating system intervention. While this enables 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 which map a virtual memory space to the FireWire "Physical Memory Space" (such as a Power Mac G5, 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. On FreeBSD, the dcons driver provides both, using gdb as debugger. Under Linux, firescope and fireproxy exist.

See also

References

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