USB3 Speed Implications
A new version of the ubiquitous Universal Serial Bus (USB) technology will make data rates more than ten times as fast by the addition of fiber optic links alongside the traditional copper wires. The USB 2.0 version that we have now has a maximum data rate of 480 megabits per second so the new USB would be 4.8 gigabits per second. So how would this fast data rate be used and what would be the disadvantages of using fiber optics. One this that is immediately obvious is that USB hard drives would be faster and have greater applications.
But let us check how much easier a USB hard drive would be by checking the calculations. A one Terabyte drive could be copied in almost one and a half seconds. This is assuming that all of the components in your system were fast enough. But this exactly the point. The hard drive is a mechanical device with a spin rate and average seek time and would introduce a data rate reduction from this fact alone.
Also hardware interfaces like a SATA hard drive attached to USB3 port is limited to 150 megabits per second. The value here being the burst speed not a sustained transfer rate. And realistically since most hard drives have only 8 MB of cache or so this would be a short burst of less than a second. Consider on the other hand that Flash memory is slow to write, yet very fast to read. Thats why Windows uses it for ReadyBoost caching. There is extremely low latency but not enough bandwidth to sustain high levels of I/O.
The 4.8 gigabit per second speed of USB3 is the bus speed, that is shared by all devices connected to the same host. So one disk won’t saturate the bus, but if you plug in a bunch of them the bandwidth won’t seem so incredibly massive anymore. There is also bandwidth reserved by isochronous devices. Maybe to utilize the speed with a and external drive, you’d need a RAID external enclosure.
Another thing to consider is the introduction of fiber into the link is how fragile it would make the cable. Would you be able to carry it in your pocket and not fracture? Glass fiber is actually very flexible. You can bend it in any way you want, it won’t break. You can cut it, but that takes considerable force. And if you break the fiber, you’ll break the copper wires as well. The weakest point may just be the connectors. Getting dirt into the connector would probably ruin it.
One problem with the old USB is in the protocol. And, this had to be changed in USB3. USB2 is synchronous that means that data every packet received must be acknowledged in a return packet before the next data packet can be transmitted. This back and forth for each data packet means a lot of wasted time where the channel is essentially idle. Interestingly, sometimes using a shorter cable can make a noticeable improvement.
Firewire for example works well for video streaming from a DV cam because it has very little overhead. Even though USB2 supposedly does 480Mbps, it really can’t do DV because there’s too much overhead. USB3 had to get rid of the CPU dependency and overhead issues. This because Firewire has both synchronous and asynchronous modes. In its async mode, a bunch of packets can be transmitted before any acknowledgment back is required. That’s bad if you have problems with the cables, since it will result in a lot of retransmits, but bad firewire cables are the exception, not the rule. So async is almost always way more efficient than synch.
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