The Buzz with Buses

Cable woes, slow ports that are few in number (usually two serial and one parallel) and tedious installations, all gave reason for the development of better technologies for connecting peripheral devices to a PC. Came the mid-90s and two such technologies–Universal Serial Bus (USB 1.0) and FireWire (IEEE-1394)–became commercially available.

But what drove the designs of FireWire and USB? In theory, transferring binary data between two machines is pretty simple. Connect two devices with a cable and let the data flow. In real life, however, this simplicity is deceptive. 

Fire on wires



In 1986, Apple began work on the design of FireWire. The Institute of Electrical and Electronic Engineers got into the act and in 1995 brought out the IEEE 1394 -1995 High Performance Serial Bus standard. It defined the media, topology and protocol for a serial data cable bus that allows general-purpose high-speed data transfer. The FireWire cable bus is platform-independent and carries digital data, thus making it a digital interface. It can connect digital devices (up to 63), such as personal computers, audio and video products, printers and scanners, digital video cameras, displays, recorders and other devices that require high-speed data transfer (currently at 800Mbps).

The FireWire way



FireWire is implemented on your PC as a port. A cable plugged in at this port carries digital information to a FireWire device, which can be another computer or a peripheral. When a host computer powers up, it queries all the devices that are connected and assigns each an address, a process called enumeration. FireWire devices are plug and play. The host will auto-detect the device and ask for a driver to complete installation. If a device is already installed, it activates it and begins speaking with it. Devices are hot-pluggable, too, which means that they can be plugged in or out while the power of the host computer is on. They come self-powered or bus-powered. In the latter case, two power conductors in the cable supply power (8 to 40 volts, 1.5 amps max) from the computer to the unpowered device. Two twisted pair sets that carry data complete a FireWire cable. They provide a direct digital link between up to 63 devices at scalable speeds of 100, 200, 400 and 800Mbps. Made of 28 AWG (American Wire Guage) wires, they span a maximum of 100m. 

Devices can be connected to a PC or to each other since FireWire allows for peer-to-peer transfer. This means that a digital camera can speak with a digital recorder without the computer being powered on.

A Universal Bus



Universal Serial Bus (USB 1.0 and subsequently 1.1), from Intel, was aimed at replacing legacy ports (serial, parallel ports) with a single ‘universal’ port. USB 1.1’s maximum speed of 12 Mbits/sec was a vast improvement over the old 115 kbps serial ports. USB provided a low cost, user-friendly way of connecting peripherals (up to 127), such as keyboards, mice, printers and scanners to a PC. Though it is not a standard, it has been widely supported across the industry. Ironically enough, the big break for USB came when Apple brought out the iMac, which had only USB ports. 

USB 1.0/1.1



Also plug and play enabled, hot pluggable and with powered cables, USB 1.0/1.1 reaches speeds of 12Mbps as compared to FireWire’s initial 400Mbps. USB 1.0/1.1 initially complimented FireWire by supporting low-speed devices like mice and keyboards. 

USB supports 127 devices by daisy chaining. This can be achieved with hubs. A hub lets you connect many devices to itself. Hubs can be external devices or integrated into peripherals such as keyboards and monitors. A hub could also power devices such as mice, keyboard and joysticks. High power consuming devices were separately powered.

USB cables are different from FireWire. A typical cable has two types of connectors. On one end is the upstream ‘A’ connector that goes to the computer. The other end has a downstream ‘B’ connector that goes to the device. Likewise, a hub would have one A connector and four B connectors. Inside the cable, two wires supply power and ground to attached peripherals, and one pair set carries data. Cables for low-speed devices do not require shields and twisted-pair signal conductors, as present in FireWire cables.

USB 2.0 



In 1999, Intel announced a new version–USB 2.0–that was to be backward compatible with USB 1.1 and offered speeds at 480 Mbps. With Intel integrating USB 2.0 with their chipsets and built-in support in Windows XP, this is likely to gain acceptance in the years to come. With USB 2.0 touting speeds of 480Mbps, FireWire had its first real competitor. 

The single distinguishing feature of USB 2.0 is a dramatic increase in speed. Initially quoted by Intel to reach speeds between 120 Mbps and 240 Mbps, USB has achieved a data rate of 480 Mbps. However, one must keep in mind that FireWire is touted to reach speeds of 1.6Gb and 3.2Gb in the near future in addition to currently scaling up to a speed of 800Mbps. 

USB 2.0, like the earlier versions is host dependent. This means that for the devices to communicate with each other, they need a host computer to be powered on. FireWire, on the other hand, supports peer-to-peer transfers. 

Consider a stream of data coming into your speakers from a CD-ROM drive. With isochronous (iso-same, chrono-time) transfers, the data is transferred in a single stream with little or no interruptions. This is achieved by reserving bandwidth on the bus, up to 80% in FireWire. The remaining 20% is for asynchronous transfer where data is transferred after acknowledgement that previous packets have been received. 

With USB and its client-hub architecture, isochronous transfers aren’t possible in their true sense. USB delivers data only from peripherals to hosts or hosts to peripherals. Therefore your USB CD-ROM will transfer the data to your computer. However, the computer will not be able to complete this transfer to its USB speakers without introducing skips or blips. With peer-to-peer transfers between any two nodes, FireWire enables isochronous transfers. 

USB 2.0 is being marketed as a more cost-effective option to FireWire. However, there is a school of thought that this is not possible across the board. USB 2.0 will be both forward and backward compatible with USB 1.1 peripherals. Backward compatibility means USB 1.1 devices can be hooked up to 2.0 hubs. These hubs will thus have to duplicate the logic, one for USB 2.0 and the other for existing USB1.1 speeds. 

Once a hub is running in two speeds it will also demand for a split in bandwidth, which isn’t good news either. Forward compatibility implies that USB 2.0 devices should be able to run on USB 1.1 hubs at lower speeds. Existing USB cables may not work under high-speeds and therefore users might also have to go in for a fresh set of cables.

The bus tolls for thee



With USB 2.0 gaining popularity, FireWire has stiffer competition. Not to be discounted lightly however is the role FireWire plays in binding together the growing number of information appliances that will merge personal computer and consumer electronics technology. 

More specialized devices with heavy payloads (such as digital video cameras, digital VTRs, DVD players and DTV receivers) will still gravitate toward FireWire (particularly when ultra-quick 1394b arrives, promising speeds eventually up to 3.2

Gbits/sec). 

This, however, does not mean that FireWire will dominate over USB or vice versa. Newer versions of USB and FireWire will overlap in middle ground and they will have to coexist there. USB will own the low and middle positions, while FireWire will occupy the middle and high ground.

Priya Ramachandra