The New Choices for External Drive Connection

USB’s biggest weakness as a external connector is speed; or, more accurately, lack thereof. FireWire 400, while having lower maximum bandwidth than USB 2.0, is usually about 50% faster than USB. The reason is down to USB’s reliance on drivers and CPU processing, while FireWire is peer-to-peer. FireWire 800 doubles bandwidth and moves to full, instead of half, duplex (two way, instead of one way at a time) communication. eSATA buries them all, offering more bandwidth than even a Solid State Drive can use.

USB 3.0 turns things up to 5Gb/s, as opposed USB 2.0’s 480Mbits. It introduces 8b/10b signaling though, which brings the bandwidth available to your data to 400MB/s. Still a lot better than 2.0’s 60MB/s. USB 3.0 also one ups 2.0 with full duplex traffic.

Another technical tweak to be happy about is no more polling, a component of the aforementioned CPU load. Polling also increases power consumption of connected devices. USB 3.0 will also try to reduce power consumption with better control of devices (like automatically turning them off when not in use). While reducing the power used by an idle device is good, having more power available when a device needs it is also a plus.

3.0 covers that with more juice on tap. Both versions carry 5 volts, but the new one raises the power on each port from 100 mA to 150 mA. It also increases the number of ports that can shift power to one that needs more from 5 to 6. That makes a total of 4.5 watts: which should mean not having to plug in (and use up) two USB connectors for drives, or, faster charging of mobile devices.

Even if we assume processing overhead knocks the wind out of USB 3.0’s sails, it has the full-duplex advantage. It should manage to come close enough to eSATA to make the difference to a HDD user irrelevant, since either would be more than enough. Factor in that USB is pushing its power advantage and, if its processing is efficient enough to deliver speeds needed by the SSD market, it is hard to get behind eSATA going forward.

But eSATA has two big moves to make that could put it right back on top.

SATA-IO, or the Serial Advanced Technology International Organization isn’t taking the threat to their external storage supremacy lying down. They have two new specs in the works.

One is SATA 6Gb/s; which will, predictably, carry 6Gbytes per second, or about 568 MB/s after 8b/10b overhead. USB 3.0 is backwards compatible in the sense that the plugs will fit in the ports, but limits speed to the oldest of the cables or devices. SATA 6 (SATA 3 means 3Gb/s, there is no such thing as SATA 2 or II. Don’t blame SATA-IO: they saw this coming and tried to get people onboard with calling versions by their speed (like we do with FireWire and did with PATA). I was saying: SATA 6) will use identical cable and ports. Replacing cables isn’t that expensive relative to a new drive, but keeping track of them it a pain and the cost actually adds up quite a bit. This is even more significant when it comes to the ports built in to your PC enclosure, which will always be USB 2.0. A SATA port should behave according to the speed of the drive or controller (usually on the motherboard).

SATA 6 will hang on to its lower processor overhead and software requirement advantages. But it also hangs on to half duplex, while USB 3.0 goes full. It will be impossible to see which of these is fastest in practical terms until equipment is available to test. The point is: they should both have enough speed for most people’s (hard disk, not solid state disk, buyer’s) needs. Since USB can deliver power and obviate a separate power cable, and USB ports are just about everywhere, it comes out on top.

That is where power over SATA, or eSATAp, comes in. There aren’t many details on this yet, but there is an initiative to deliver at least enough power for a 2.5” HDD through SATA. Presumably, this will require a new connector. But it could be made backwards compatible with the existing one: compatible in the sense that it would fit, but not take advantage of the improvement. We examine this in detail, and bring FireWire into the mix, in the next article