SATA 6Gb/s and USB 3 – Asus Style
SuperSpeed USB 3
Hi-Speed USB 2.0 has served us well. Its 480Mbit/s capability was such an improvement over the 12Mbit/s of USB 1.1 that it was really incredible. But time has gone on, system capabilities have increased, the number of USB devices is now unbelievable, and we have outgrown the data transfer rate of USB 2.0. Uses of USB has also grown far beyond the initial intentions for USB, such as recharging batteries of USB devices, multimedia players, and cell phones.
SuperSpeed USB 3 is the next generation of USB data transfer, with a capability of 4.8Gbit/s. Before you think that this is only 10 times faster than USB 2.0, be aware that earlier versions of USB utilized half-duplex signaling, that is, data could travel only one direction at a time, so USB devices and the system were either sending or receiving data, and there was always data waiting to be sent. USB 3 is full-duplex, allowing USB devices and the system to both send and receive data at the same time. Additionally, USB 3 does not have the maximum cable length limitations of USB 2.0 and earlier, which was 5m. USB 3 cables can be any length, though the 4.8Gbit/s maximum speed is attained at a max of 3m.
Another benefit of USB 3 is the power management capabilities. USB 2.0 has a maximum power draw per port of 500mA. If you have a Blackberry that gives you an error message when trying to charge it via a USB cable, you see that devices have outgrown USB 2.0’s power capabilities. USB 3 has nearly doubled that with a maximum power draw per port of 900mA.
Mechanical hard drives barely have the capability of saturating the 1.5Gb/s capability of SATA 1.0, much less the 3.0Gb/s capability of SATA 2.0. But SSDs have changed all of that, they already are capable of saturating SATA 3Gb/s’ max net read speed of 250MB/s. The latest revision of Serial ATA is SATA 6Gb/s. With a theoretical transfer rate of 750MB/s, it will more than handle the capability of SSDs for some time to come.
The problem with adding USB 3 (and SATA 6Gb/s) to a motherboard is that there currently aren’t any extra buses to carry the data to the CPU. Both can be sent via PCI-E, but there are only 16 PCI-E lanes available in the Intel P55 Express chipset for normal data transfer. As mentioned earlier, the popular method of adding USB 3 to a motherboard is to utilize some of the 16 PCI-E lanes that would normally be used by the video card(s) to carry data to the CPU. Adding SATA 6Gb/s is done by one of a couple of methods, either using an existing PCI 1.1 lane, or by PCI-E lane switching. As a result, it will either compensate half the PCI Express 2.0 bandwidth from the CPU, or restrict the performance of the SATA6Gb/s to half of its speed, which in other words, means forcing the SATA 6Gb/s controller to operate under close to equivalent performance to a SATA2 controller.
The problem is compounded when adding both USB 3 and SATA 6Gb/s to the existing PCI-E system. The typical solution is to entry level lane switching, which eats up half of the 16 existing lanes of PCI-E bandwidth. In effect, you give up the ability of SLI or CrossfireX to have the capability of either full speed USB 3 or SATA 6Gb/s (not both).
Asus is the first company to add an additional PCI-E bridge chip to the motherboard, they call the PLX bridge, to provide the additional bandwidth needed to accommodate USB 3 and SATA 6Gb/s. This is the obvious solution, not unlike the additional SATA 2.0 brige chips already being used to add SATA, PATA, and eSATA ports above the storage capabilities available from the chipset. I’m sure that eventually everyone else will come on board, but USB 3 and SATA 6Gb/s without compensation are available from Asus now, on the P7P55D-E Pro and other motherboards.
Asus’ PLX bridge uses a lane of PCI-E 2.0 to provide adequate bandwidth to the NEC USB 3 controller. It takes four lanes of PCI 1.1, running into two lanes of PCI-E 2.0, to provide the bandwidth necessary for SATA 6Gb/s. It also provides four extra lanes of PCI-E for other possible uses.