Introduction:

While USB has been popular for several years know, many thanks to the introduction of the original iMac, the IEEE1394 standards (IEEE1394a and IEEE1394b), also called Fire Wire, is also catching on in the PC world. A high speed serial standard that supports transfer rates up to a blistering 400 Mbps (50 MB/s) in its standard IEEE1394a form and 800 Mbps (100 MB/s) in its upgraded IEEE1394b format, IEEE1394 is already de rigueur in the digital video and audio world. This high speed technology enjoyed a rapid proliferation in other areas thanks largely to OS support finally being added as standard beginning with Windows XP, Windows 2000, Window98SE and Windows ME. Mass storage, networking, scanners and digital still photography are areas of USB dominance that IEEE1394 now shares.

Lets take a look at USB technology and compare it with the IEEE1394 standard.

USB

Universal Serial Bus, or USB for short, was originally designed in 1993 by a group of companies including Intel, Compaq, Digital, Microsoft and NEC. USB requires a PC, but is relatively cheap to implement since it leverages the power of the computer's CPU. This is much faster than the slow COM ports that the standard is intended to replace, but is far slower than IEEE1394. It is even a little slower than the modern LPT ports that USB is also targeted to supplant, although USB has a great edge in user friendliness. Its is considered a primary hardware interface for low-speed peripherals such as the keyboard, mouse, joystick, scanner, printer and telephony devices.

USB follows a tiered star topology where hubs are necessary, but can be nested. A USB port can support up to 127 devices, all sharing the same bandwidth. The standard allows for devices to be powered from the port as well as from external power sources. Some notebooks do not provide enough power to their USB ports to run external devices requiring port power. Hi-Speed USB technology offers transfer rates up to 480Mbps (megabits per second) compared to USB 1.1 devices, which transfer at speeds of 12Mbps. For example, it would take 1.1 minutes to download a 100 MB file with USB 1.1 devices in a sustained, optimal condition, while with USB 2.0 it only takes 1.6 seconds to transfer the same file.

Hot Swappable
USB's hot swap capability allows everything to be plugged in and unplugged without turning the system off. USB ports began to appear on PCs in 1997, and Windows 98, 2000 and ME fully support it. Devices are plugged directly into a four-pin socket on the PC or into a multi-port hub that plugs into the PC or into a device that also functions as a hub for other devices.

Power Needs
The USB bus distributes 0.5 amps (500 milliamps) of power through each port. Thus, low-power devices that might normally require a separate AC adapter can be powered through the cable. Hubs may derive all power from the USB bus (bus powered), or they may be powered from their own AC adapter. Powered hubs with at least 0.5 amps per port provide the most flexibility for future downstream devices. Port switching hubs isolate all ports from each other so that one shorted device will not bring down the others.

The USB Connectors
USB ports on the PC and hubs use a rectangular Type A socket. All cables that are permanently attached to the device have a Type A plug. Devices that use a separate cablehave a square Type B socket, and the cable that connects them has a Type A and Type B plug.

IEEE 1394 a.k.a Firewire

Briefly, IEEE 1394 is a high-speed serial bus developed by Apple and Texas Instruments that allows for the connection of up to 63 devices. Also known as the IEEE 1394 standard, the i.Link connector and the High Performance Software Bus (HPSB), the original spec calls for 100, 200, 400 and 800 Mbits/sec transfer rates.   In the future, it will be capable of providing even greater speeds, starting at 1600 and reaching as high as 3200 Mbits/sec speeds.

Ready since 1987!
The first completed specification was delivered by Apple in 1987. This standard was later absorbed into IEEE1394 in 1995. Requiring no hubs, each bus can support up to 63 devices and up to 1023 busses can be interconnected. A very compelling aspect of 1394 is that it is a peer-to-peer standard, resulting in that a PC is not needed to connect Fire Wire devices - in fact the PC is simply another "peer" in an IEEE1394 link. This is an unfamiliar notion for most PC users, but, in theory, a digital camera equipped with a Fire Wire port could be connected directly to an IEEE1394 hard drive and the camera should be able to directly save its files to the drive. At the recent IDF, components of a home entertainment center were entirely wired to one another using IEEE1394.

The Firewire IEEE 1394a Cable
A duo of two twisted pair conductors form an IEEE1394 cable. These cables are similar to those used for 10BaseT Ethernet. The cable is a 6-wire shielded twisted-pair type, where 2 pairs are used for communicating and the remaining pair is used to carry power up to 1.5 Amps at up to 40 Volt. Cable lengths are limited to 4.5 meters (15 feet), but can be boosted to 72 meters (235 feet) if a repeater is used. Using "fairness" arbitration to ensure that all devices obtain access to the pipe, IEEE1394 also multiplexes different types of digital signals like video, digitized audio, MIDI, as well as device control commands.

The Firewire IEEE 1394b Cable
Fireeire 800 cables promise a much higher total data speed, 800Mpbs, twice that of the original FireWire standard. They use a special 9-pin connector, compliant with the IEEE 1394b standard, and in addition to speed increases have some new features that improve reliability of data. You will use these cables with new, faster 800Mbps products.  There are versions of this cable that may have 4 or 6 pin standard IEEE1394a connectors, that are able to connect in a backward compatible fashion with older firewire devices.

Conclusion
While IEEE1394 is still faster than USB and uses very little overhead, it is more expensive to implement. For this reason, cheap, lower speed devices such as keyboards, mice, and digital camera are best suited for USB, while devices demanding premium, high-speed real-time service require Firewire service.