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Consumers that only a few years ago were first using a PC are now flourishing in a digital home that includes one or more PCs, and a multitude of other connected devices ranging from digital camcorders and still cameras to home servers. The connected home has exposed consumers to a broad variety of networks and device interconnects including USB, FireWire, Wi-Fi (IEEE 802.11), eSATA, and others. With so many choices in how to connect devices, invariably the choices overlap to some extent. Design engineers need to try and make the best choice for the connectivity options integrated into a new product that will maximize the end-user experience. Both quantitative factors such as performance and qualitative factors such as ease of use affect that user experience. To illustrate the tradeoffs, we evaluated interfaces that might be used to connect external storage devices to a PC or even a DVR or set-top box.
There are a fairly short list of options in data-storage interface choices. Inside a PC, or a DVR, the disk drive is almost certainly connected via SATA (Serial ATA) these days. You may still find the older ATA, now sometimes called PATA or Parallel ATA, in some systems, but new designs almost exclusively use the serial version of the standard. In external storage devices, designers have far more choice. Dedicated home servers that link directly to a network obviously require either an Ethernet or Wi-Fi connection. For simple external disk drives, designers can choose between USB, FireWire, and eSATA (External SATA).
Figure 1: LaCie d2 Quadra external hard drive
(Click on image to enlarge)
As you may guess, eSATA is an external version of the SATA interface. Originally, SATA and eSATA featured 1.5-Gbps maximum data rates. The second-generation of SATA and eSATA products support 3-Gbps data rates. Some people mistakenly refer to these 3-Gbps products as SATA-2 or SATA-II. That terminology is technically incorrect. We will simply specify the data rate to avoid confusion in this article. The industry group that promulgates SATA technology is working on a 6-Gbps version.
Today, USB is the most ubiquitous of the external connections on a PC. USB connects mouse, printer, scanner, and other peripherals on most PCs. And of course USB is the standard connection for the Flash drives that have eliminated floppy disks. USB operates at a maximum of 480 Mbps, and the industry is looking in the next couple of years to increase the maximum data rate an order of magnitude.
FireWire or IEEE 1394 has a long history as the interconnect of choice for digital camcorders and can be found on most PC and notebooks. FireWire can also be found on most set-top boxes and on many HDTV sets. From day one, FireWire was designed as much to connect media-centric devices as computing-centric devices. The original version of FireWire moved data at a maximum speed of 100 Mbps. But even back then FireWire was generally faster than USB because USB is a polled interface managed my the host processor in a PC while FireWire is a peer-to-peer interconnect. Over the years FireWire evolved in 200-, 400, and 800-Mbps flavors and the 1394 Trade Association is pushing forward with 1.6- and 3.2-Gbps versions coming soon " the 1.6-Gbps flavor will arrive this year.
The terminology around FireWire can be confusing as the trade association defined subsequent standards using alpha suffixes such as 1394, 1394a, and 1394b. Those evolutionary steps did provide the speed increases but also many other feature additions. So to avoid confusion we will skip that a and b terminology and refer to FireWire 400 or 800 to denote the speed of the interconnect being discussed.
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