FireWire

Also known as IEEE-1394, I.link, and DV, FireWire is a serial interface that's aimed at high-throughput devices such as a hard disk and tape drives, as well as consumer-level multimedia devices such as digital camcorders, digital VCRs, and digital televisions. Originally it was conceived as a general-purpose interface suitable for replacing legacy serial ports, but with blazing speed. However, it has been most used in digital video—at least so far. Promised new performance and a choice of media may rekindle interest in FireWire as a general-purpose, high-performance interconnection system.

For the most part, FireWire is a hardware interface. It specifies speeds, timing, and a connection system. The software side is based on SCSI. In fact, FireWire is one of the several hardware interfaces included in the SCSI-3 standards.

As with other current port standards, FireWire continues to evolve. Development of the standard began when the Institute of Electrical and Electronic Engineers (IEEE) assigned a study group the task of clearing the murk of thickening morass of serial standards in September 1986. Hardly four months later (in January 1987), the group had already outlined basic concepts underlying FireWire, some of which still survive in today's standard—including low cost, a simplified wiring scheme, and arbitrated signals supporting multiple devices. The IEEE approved the first FireWire standard (as IEEE 1394-1995) in 1995, based on a design with one connector style and two speeds (100 and 200Mbps).

In the year 2000, the institute approved the standard IEEE 1394a-2000, which boasts a new, miniaturized connector, a higher speed (400Mbps), and streamlined signaling that makes connections quicker (because of reduced overhead) and more reliable.

As this is written, the engineers at the institute are developing a successor standard, IEEE 1394b, that will quadruple the speed of connections, add both fiber-optic and twisted-pair wiring schemes, and add a new, more reliable transport protocol.

For now, FireWire is best known as a 400Mbps connection system for plugging digital camcorders into computers, letting you capture video images (live or tape), edit them, and publish them on CD or DVD.

Overview

FireWire differs from today's other leading port standard, USB, in that it is a point-to-point connection system. That is, you plug a FireWire device directly into the port on your computer. To accommodate more than one FireWire device (the standard allows for a maximum of 63 interconnected devices), the computer host may have multiple jacks or the FireWire device may have its own input jack so that you can daisy-chain multiple devices to a single computer port.

Although FireWire does not use hubs in the network or USB sense, in its own nomenclature it does. In the FireWire scheme, a device with a single FireWire port is a leaf. A device with two ports is called a pass-through, and a device with three ports is called a branch or hub. Pass-through and branch nodes operate as repeaters, reconstituting the digital signal for the next hop. Each FireWire system also has a single root, which is the foundation around which the rest of the system organizes itself.

You can daisy-chain devices with up to 16 links to the chain. After that, the delays in relaying the signals from device to device go beyond those set in the standard. Accommodating larger numbers of devices requires using branches to create parallel data paths.

Under the current standard (1394a), FireWire allows a maximum cable length of 4.5 meters (about 15 feet). With 16 links to a daisy-chain, two FireWire devices could be separated by as much as 72 meters (almost 200 feet).

Each FireWire cable contains two active connections for a full-duplex design (signals travel both ways simultaneously in the cable on different wire pairs). Connectors at each end of the cable are the same, so wiring is easy—you just plug things together. Software takes care of all the details of the connection. The exception is that the 1394a standard also allows for a miniaturized connector to fit in tight places (such as a camcorder).

FireWire also allows for engineers to use the same signaling system for backplane designs. That is, FireWire could be used as an expansion bus inside a computer as well as the port linking to external peripherals. Currently, however, FireWire is not used as a backplane inside personal computers.

The protocol used by FireWire uses 64-bit addressing. The 63 device limitation per chain results from only six bits being used for node identification. The rest of the addressing bits provide for large networks and the use of direct memory addressing—10 bits for network identifications and 48 bits for memory addresses (the same as the latest Intel microprocessor, enough for uniquely identifying 281TB of memory per device). A single device may use multiple identifications.

Signaling

To minimize noise, data connections in FireWire use differential signals, which means it uses two wires that carry the same signal but of opposite polarity. Receiving equipment subtracts the signal on one wire from that on the other to find the data as the difference between the two signals. The benefit of this scheme is that any noise gets picked up by the wires equally. When the receiving equipment subtracts the signals on the two wires, the noise gets eliminated—the equal noise signals subtracted from each other equals zero.

The original FireWire standard used a patented form of signal coding called data strobe coding, using two differential wire pairs to carry a single data stream. One pair carried the actual data; the second pair, called the strobe lines, complimented the state of the data pair so that one and only one of the pairs changed polarity every clock cycle. For example, if the data line carried two sequential bits of the same value, the strobe line reversed polarity to mark the transition between them. If a sequence of two bits changed the polarity of the data lines (a one followed by a zero, or zero followed by a one), the strobe line did not change polarity. Summing the data and strobe lines together exactly reconstructed the clock signal of the sending system, allowing the sending and receiving devices to precisely lock up.

FireWire operates as a two-way channel, with different pairs of wire used for sending and receiving. When one pair is sending data, the other operates as its strobe signal. In receiving data, the pair used for strobe in sending contains the data, and the other (data in sending) carries the receive strobe. In other words, as a device shifts from sending and receiving, it shifts which wire pairs it uses for data and strobe.

The 1394b specification alters the data coding to use a system termed 8B/10B coding, developed by IBM. The scheme encodes eight-bit bytes in 10-bit symbols that guarantee a sequence of more than five identical bits never occurs and that the number of ones and zeros in the code balance—a characteristic important to engineers because it results in no shift of the direct current voltage in the system.

Configuration

FireWire allows you to connect multiple devices together and uses an addressing system so that the signals sent through a common channel are recognized only by the proper target device. The linked devices can independently communicate among themselves without the intervention of your computer. Each device can communicate at its own speed—a single FireWire connection shifts between speeds to accommodate each device. Of course, a low-speed device may not be able to pass through higher-speed signals, so some forethought is required to put together a system in which all devices operate at their optimum speeds.

FireWire eliminates such concern about setting device identifications with its own automated configuration process. Whenever a new device gets plugged into a FireWire system (or when the whole system gets turned on), the automatic configuration process begins. By signaling through the various connections, each device determines how it fits into the system, either as a root node, a branch, a pass-through, or a leaf. The node also sends out a special clock signal. Once the connection hierarchy is set up, the FireWire devices determine their own ID numbers from their location in the hierarchy and send identifying information (ID and device type) to their host.

You can hot-plug devices into a FireWire tree. That is, you can plug in a new device to a group of FireWire devices without switching off the power. When you plug in a new device, the change triggers a bus reset that erases the system's stored memory of the previous set of devices. Then the entire chain of devices goes through the configuration process again and is assigned an address. The devices then identify themselves to one another and wait for data transfers to begin.

Arbitration

FireWire transfers data in packets, a block of data preceded by a header that specifies where the data goes and its priority. In the basic cable-based FireWire system, each device sharing a connection gets a chance to send one packet in an arbitration period that's called a fairness interval. The various devices take turns until all have had a chance to use the bus. After each packet gets sent, a brief time called the sub-action gap elapses, after which another device can send its packet. If no devices start to transmit when the sub-action gap ends, all devices wait a bit longer, stretching the time to an arbitration reset gap. After that time elapses, a new fairness interval begins, and all devices get to send one more packet. The cycle continues.

To handle devices that need a constant stream of data for real-time display, such as video or audio signals, FireWire uses a special isochronous mode. Every 125 microseconds, one device in the FireWire that needs isochronous data sends out a special timing packet that signals that isochronous devices can transmit. Each takes a turn in order of its priority, leaving a brief isochronous gap delay between their packets. When the isochronous gap delay stretches out to the sub-action gap length, the devices using ordinary asynchronous transfers take over until the end of the 125-microsecond cycle when the next isochronous period begins.

The scheme guarantees that video and audio gear can move its data in real time with a minimum of buffer memory. (Audio devices require only a byte of buffer; video may need as many as six bytes.) The 125-microsecond period matches the sampling rate used by digital telephone systems to help digital telephone services.

The new 1394b standard also brings a new arbitration system called Bus Owner/Supervisor/Selector (BOSS). Under this scheme, a device takes control of the bus as the BOSS by being the last device to acknowledge the receipt of a packet sent to it (rather than broadcast over the entire tree) or by receiving a specific grant of control. The BOSS takes full command of the tree, even selecting the next node to be the BOSS.

Connectors

In the original FireWire version only a single, small, six-pin connector was defined for all purposes. Each cable had an identical connector on each end, and all FireWire ports were the same. The contacts were arranged in two parallel rows on opposite sides inside the metal-and-plastic shield of the connector. The asymmetrical "D" shape of the active end of the connector ensured that you plugged it in properly. Figure 11.5 shows this connector.

The revised 1394a standard added a miniaturized connector. To keep it compact, the design omitted the two power contacts. This design is favored for personal electronic devices such as camcorders. Figure 11.6 shows this connector.

The 1394b standard will add two more connectors to the FireWire arsenal, each with eight contacts. The Beta connector is meant for systems that use only the new 1394b signaling system and do not understand the earlier versions of the standard. In addition, the new standard defines a bilingual connector, one that speaks both the old and new FireWire standards. The new designs are keyed so that while both beta and bilingual connectors will fit a bilingual port, only a beta connector will fit a beta-only port. Figure 11.7 shows this keying system.

Cabling

In current form, FireWire uses ordinary copper wires in a special cable design. Two variations are allowed—one with solely four signal wires and one with the four signal wires and two power wires. In both implementations, data travels across two shielded twisted pairs of AWG 28 gauge wire, with a nominal impedance of 110 ohms. In the six-wire version, two AWG 22 gauge wires additionally carry power at 8 to 33 volts with up to 1.5 amperes to power a number of peripherals. Another shield will cover the entire collection of conductors.

The upcoming IEEE 1394b standard also allows for two forms of fiber optical connection—glass and plastic—as well as ordinary Category 5 twisted-pair network cable. The maximum length of plastic fiber optical connections is 50 meters (about 160 feet); for glass optical fiber, the maximum length is 100 meters (about 320 feet). Either style of optical connection can operate at speeds of 100 or 200Mbps. Category 5 wire allows connections of up to 100 meters but only at the lowest data rate sanctioned by the standard, which is 100Mbps.

All FireWire cables are crossover cables. That is, the signals that appear on pins 1 and 2 at one end of the cable "cross over" to pins 3 and 4 at the other end. This permits all FireWire ports to be wired the same and serve both as inputs and outputs. The same connector can be used at each end of the cable, and no keying is necessary, as is the case with USB.

The FireWire wiring scheme depends on each of the devices that are connected together to relay signals to the others. Pulling the plug to one device could potentially knock down the entire connection system. To avoid such difficulties and dependencies, FireWire uses its power connections to keep in operation the interface circuitry in otherwise inactive devices. These power lines could also supply enough current to run entire devices. No device may draw more than three watts from the FireWire bus, although a single device may supply up to 40 watts. The FireWire circuitry itself in each interface requires only about two milliwatts.

The FireWire wiring standard allows for up to 16 hops of 4.5 meters (about 15 feet) each. As with current communications ports, the standard allows you to connect and disconnect peripherals without switching off power to them. You can daisy-chain FireWire devices or branch the cable between them. When you make changes, the network of connected devices will automatically reconfigure itself to reflect the alterations.