USB has been popular in the market for its simplicity, maturity and plug-and-play features. However, the 480 Mbps speed of USB 2.0 was not sufficient to support new generation storage and video. Therefore, the time was ripe for migration to a faster standard; this has led to the development of the new USB 3.0 protocol. The challenge that arises for developers is how to leverage USB 3.0’s full potential.
This article will explore the impact on hardware and software design to implement USB 3.0 with particular focus on handheld products. First, we will compare the capabilities of USB 2.0 and USB 3.0 and the impact of the transition on the components that interact with the USB 3.0 module.
In a common scenario, on the device side, the processor is connected to USB, storage, and peripherals directly. Keeping this architecture in mind, the impact on processor due to the transition from High-Speed to SuperSpeed is summarized in Table 1 below.
Table 1. USB 3.0 versus USB 2.0
Data rate comparisons
The basic difference between USB 2.0 and USB 3.0 is bandwidth. The theoretical bandwidth provided by USB2.0 is 480Mbps. In reality, the maximum throughput received is about 320Mbps (40MBps), which is roughly two third of the theoretical value. With USB3.0, the raw throughput is 4.8Gbps.
If we use the same proportion rate, then the expected data speed is 3.2Gbps (400MBps). However, many developers expect to be able to provide even higher throughput. Figure 1 below shows the data rate difference between USB 2.0 and USB 3.0 for a Buffalo external storage disk for different transfer sizes. It should be noted that the USB 3.0 data rate is restricted by the storage device; otherwise a data rate of 400 Mbps can easily be achieved.
Figure 1: USB 2.0 and USB 3.0 data rate differences (To view larger image, click here.)
It can be seen that as the transfer size of a single request increases, the data rate increases in tandem. That is because as the transfer size increases per request, the number of requests and hence interrupts that the MSC device has to handle decreases, resulting in better overall performance.
After a 64KB transfer size, the data rate attains saturation because the Windows driver does not request more than 64KB data in a single SCSI request. This data shows the importance and effect of interrupts on the overall system performance.
This high data rate increases the interrupt rate and data request rate which can load the processor significantly. While the core is busy processing USB-related real-time requests, latencies increase and users see applications slow down, which is not at all a desirable result.
Data flow considerations
Unlike the USB 2.0 standard, where data is queued in one direction at a time, USB 3.0 supports simultaneous reading and writing. That is because USB 2.0 is a half-duplex protocol while USB 3.0 is a full-duplex protocol.
Full-duplex communications is achieved by adding more connections to support simultaneous data transfers. It also comes at the cost of increasing software complexity twofold as well.
With USB 2.0, the processor is involved in only one transaction at a time, and the data structures and request handling are simpler. But with the arrival of full duplex USB 3.0, the data structure will now require double the information. Again the USB software module needs to be able to handle the concurrency in data handling.