How It Works: IEEE 1394

IEEE 1394: a high-speed, serial input/output bus for computer peripherals and consumer electronics, capable of transfer speeds of up to 400 megabits per second.

• PRO: fastest external input/output bus available; supports up to 63 devices at once; optimal for bandwidth-hungry devices such as digital video cameras and external storage devices.
  
  
• CON: not as widely implemented as Universal Serial Bus; needs fast CPU.
  
  

That rat's nest of parallel, serial, and audio cables spewing from the back of your PC could soon be a thing of the past. For more than two years now, most new PCs have come with Universal Serial Bus ports as standard equipment. But coming soon: systems and peripherals equipped with a fast connection euphoniously dubbed IEEE 1394.

Thanks to its speed (400 megabits per second, more than 33 times faster than USB), IEEE 1394 is especially well suited to demanding activities such as uploading video to your PC. So far, only high-bandwidth devices, such as digital video cameras and external storage devices, have adopted the standard; PC vendors have been slow to implement the technology.

Typically, a six-pin cable connects each device via a 1394 port. (Some devices, such as camcorders, come with four-pin connectors and require an adapter.) Inside this six-pin cable, two shielded, twisted-pair wires transmit data while another acts as a ground and supplies power. Data is sent along the twisted pairs in packets that contain both the data and the addressing information. The 1394 protocol achieves its fast data-transmission rates by reducing noise along the wires, which allows the signal to move along the line quickly and without error. But to support these high speeds, the cable can be only 4.5 meters long.

Because IEEE 1394 devices come with in- and out-ports, you can daisy-chain up to 63 of them together. The standard creates a peer-to-peer network among the devices on the chain, meaning no single node on the network is required to control the data traffic. So, unlike USB, 1394 doesn't require your PC to act as a server for connected peripherals--they can talk directly to each other. In addition, two computers can share a peripheral, which isn't possible with USB or other I/O protocols.

IEEE 1394 also has the advantage of being able to deliver data both asynchronously and isochronously. In asynchronous mode, transfers can be interrupted; in isonchronous mode, data is transferred without interruption.

When transferring data asynchronously, 1394 allocates bandwidth to devices as needed. When one device needs to communicate with another, it sends out a signal. It may be able to get on the bus or it may need to wait before getting the requested bandwidth. But this procedure doesn't work for applications such as streaming video: If another device requests space on the bus, the video data stream will be interrupted, degrading image quality. To deal with this, 1394 offers isochronous mode, which guarantees that video and similar devices consistently get the bandwidth they require.

Like USB devices, 1394 devices are hot-pluggable, meaning you can add them to your PC without rebooting. When a device is plugged in, it immediately broadcasts its unique identification number to other devices on the network and becomes part of that network. The devices are also plug-and-play, meaning they'll work without additional drivers--if your operating system supports the 1394 standard, that is. For true support, you'll need Windows 98 SE or the upcoming Windows 2000. Plain old Windows 95 and 98 don't support the hot-pluggable technology, although Windows 98 does offer base-level support for 1394.