How FireWire Works

FireWire is a method of transferring information between digital devices, especially audio and video equipment. Also known as IEEE 1394, FireWire is fast -- the latest version achieves speeds up to 800 Mbps. At some time in the future, that number is expected to jump to an unbelievable 3.2 Gbps when manufacturers overhaul the current FireWire cables.

You can connect up to63 devicesto a FireWire bus. Windows operating systems (98 and later) and Mac OS (8.6 and later) both support it.

Let's say you have yourdigital camcorderconnected to your home computer. When yourcomputerpowers up, it queries all of the devices connected to the bus and assigns each one an address, a process calledenumeration. FireWire isplug-and-play, so if you connect a new FireWire device to your computer, theoperating systemauto-detects it and asks for the driver disc. If you've already installed the device, the computer activates it and starts talking to it. FireWire devices arehot pluggable, which means they can be connected and disconnected at any time, even with thepoweron.

Now let's take a look at FireWire's specifications.

The original FireWire specification, FireWire 400 (1394a), was faster thanUSBwhen it came out. FireWire 400 is still in use today and features:

The release of USB 2.0 -- featuring transfer speeds up to 480 Mbps and up to 5 meters between devices -- closed the gap between these competing standards. But in 2002, FireWire 800 (1394b) started showing up in consumer devices, and USB 2.0 was left in the dust. FireWire 800 is capable of:

The faster 1394b standard isbackward-compatiblewith 1394a.

In the next section, we'll get deeper into the FireWire vs. USB debate.

The key difference between FireWire andUSBis that FireWire is intended for devices working with a lot more data -- things likecamcorders,DVD playersand digital audio equipment. FireWire and USB share a number of characteristics but differ in some important ways.

Implementing FireWire costs a little more than USB, which led to the adoption of USB as the standard for connecting most peripherals that do not require a high-speed bus.

Speed aside, the big difference between FireWire and USB 2.0 is that USB 2.0 ishost-based, meaning that devices must connect to a computer in order to communicate. FireWire ispeer-to-peer, meaning that two FireWire cameras can talk to each other without going through a computer.

Now let's get back to the implementation of FireWire. How do you connect?

FireWire devices can bepowered or unpowered. FireWire allows devices to draw their power from their connection. Two power conductors in the cable can supply power (8 to 30 volts, 1.5 amps maximum) from the computer to an unpowered device. Two twisted pair sets carry the data in a FireWire 400 cable using a 6-pin configuration.

Some smaller FireWire-enabled devices use 4-pin connectors to save space, omitting the two pins used to supply power.

FireWire 800 cables use a 9-pin configuration. Six of those pins are the same as the six pins in the 1394a connector (shown above). Two of the added pins provide a "grounded shield" to protect the other wires from interference, and the third added pin does nothing at this time [ref].

Because FireWire 800 is backward-compatible with FireWire 400, there are a variety of adapters available to facilitate the combination of both standards on the same bus. There are also two types of FireWire 800 ports available: a "bilingual" port accomodates both FireWire standards, while a b-only port accepts only a FireWire 800 connector.

FireWire uses 64-bit fixed addressing, based on theIEEE 1212standard. There are three parts to each packet of information sent by a device over FireWire:

The bus ID and physical ID together comprise the 16-bitnode ID, which allows for 64,000 nodes on a system. Data can be sent through up to 16hops(设备)。跳时devices aredaisy-chainedtogether. Look at the example below. Thecamcorderis connected to the externalhard driveconnected to Computer A. Computer A is connected to Computer B, which in turn is connected to Computer C. It takes four hops for Computer C to access the camera.

Assuming all of the devices in this setup are equipped with FireWire 800, the camcorder can be up to 400 meters from Computer C.

Now that we've seen how FireWire works, let's take a closer look at one of its most popular applications: streaming digital video.

FireWire really shines when it comes to digital video applications. Most digital video cameras orcamcordersnow have a FireWire plug. When you attach a camcorder to a computer using FireWire, the connection is amazing.

An important element of FireWire is the support ofisochronousdevices. In isochronous mode, data streams between the device and the host in real-time with guaranteed bandwidth and no error correction. Essentially, this means that a device like a digital camcorder can request that the host computer allocate enough bandwidth for the camcorder to send uncompressed video in real-time to the computer. When the computer-to-camera FireWire connection enters isochronous mode, the camera can send the video in a steady flow to the computer without anything disrupting the process.

You can easily edit and create custom video projects using fasthard drives, a digital camcorder and a computer. With the right software, the computer and the camera communicate, and the computer can download all of the video automatically and with perfect digital clarity. Since the content is digital from start to finish, there is no loss of quality as you work on successive generations.

For more information on FireWire and related topics, check out the links on the next page.