How FireWire Works

IEEE 1394 is another method for transferring data between computers and external peripheral devices. A 1394 connection includes these features:

• Data transfer speed up to 800 Mbits per second

• Support for up to 63 devices

• Plug and Play operation

• Support for hot swapping

• Support for peer-to-peer data exchange

Considering that they do very similar things, it's not surprising that the internal operation of an IEEE 1394 connection is very similar to a USB link. Both systems use enumeration to identify devices connected to the bus and to allocate bandwidth, but 1394 was never intended to be a one-size-fits-all solution like USB. It doesn't include the interrupt transfer method for mice, keyboards, and other low-impact devices.

Like USB, FireWire is a Plug and Play system: To connect a new device, just plug in the cable and Windows sets up the connection. To disconnect, pull the plug.

A single 1394 bus can support up to 63 devices through a single connection to a host computer. Many 1394 devices have two connectors, which makes it possible to connect several 1394 devices in a daisy-chain configuration; however, it's often better to use a multi-port 1394 hub because a hub allows you to disconnect one device without losing the rest of the chain. The total number of cables in an end-to-end chain should not exceed 16 cables or about 236 feet (72 meters).

Because an IEEE 1394 bus doesn't require a controller, it's also possible to connect two or more devices together without a host, in a peer-to-peer configuration. This can be useful for duplicating CDs or DVDs or copying from one camcorder or portable music player to another, or from one to many; just connect the source player and one or more destination recorders together. A 1394 link can also connect two or more computers as a network.

In theory, it should also be possible to use 1394 links to share a peripheral device among more than one computer. For example, you might want to read and write files on a hard drive from two different computers. However, Windows does not include controls for shared disk drives or other storage devices. If you have to share access to a disk drive, it's better to connect through a network.

IEEE 1394 uses two data transfer methods: isochronous and asynchronous. It can also support network connections among two or more computers.

Isochronous transfers

FireWire uses isochronous transfers for high-volume, time-sensitive streaming data from sources such as networks, DVDs, camcorders, and interfaces from audio production equipment. The IEEE 1394 standard assigns priority to isochronous data services in order to let the data move at a constant rate.

In an isochronous transfer, the data moves as a continuous stream, without error checking or other tests that could interrupt the transmission with re-feeds of corrupt data.

Asynchronous transfers

FireWire uses asynchronous transfers for load-and-store applications that don't require a continuous data feed, such as disk drives and printers. In an asynchronous transfer, the originating device holds data in a buffer until enough bandwidth is available to perform the transfer.

Network connections

In Windows XP (and Vista, when it becomes available), IEEE 1394 also supports local networking. 1394 is not as flexible as a traditional Ethernet network; but it's quick and easy to set up and use. This can also be a convenient way to connect a second computer to the Internet through a host computer that is already connected.

CROSS-REF 

For more about establishing a 1394 network, see Chapter 44.