SuperSpeed USB Frequently Asked Questions

1. What is SuperSpeed USB?
   • SuperSpeed USB is the next evolution of the most successful PC-centric interconnect of all time. SuperSpeed USB utilizes dual simplex data transmission at a bit rate of 5 Gbps. While USB 2.0 high-speed (and even USB full-speed, 12 Mbps and low-speed, 1.5 Mbps) are more than adequate for many applications, there are many applications for which the USB connection is becoming the bottleneck. The main application focus is to enhance the Sync-n-Go experience of end-users with their content-rich consumer peripheral devices. The 5 Gbps data rate should provide headroom for the next five years.
2. What is the difference between USB 3.0 and SuperSpeed USB?
   • Nothing! USB 3.0 is the title of the specification that was developed by the Promoter's Group. Management of the specification is under the domain of the USB-IF. The USB-IF has trademarked USB 3.0 under the SuperSpeed USB brand as part of the compliance program.
3. How is SuperSpeed USB (USB 3.0) different from USB 2.0?
   • The most obvious change in SuperSpeed USB versus USB 2.0 high-speed is the over 10X speed increase from 480 Mbps to 5 Gbps.
     In addition to the increase in speed, the physical layer electrical signaling has changed from the simple two-wire system to a dual-simplex data path. This is done over a completely different set of connections than the existing USB 2.0 two-wire interface, which remains untouched.
     A key change is the elimination of polling. In USB 2.0, the host continuously polls all connected peripheral devices to see if they have data they need to send to the host controller. This means that all devices must be "on" at all times. In SuperSpeed USB, this polling is replaced by asynchronous notification. The host waits until a higher level application (such as file manager) tells it that there is a peripheral that has data it needs to send to the host. The host then contacts that specific peripheral telling it that the host is ready to accept that data and asks if it is ready to send the data. When both ends of the link are ready, the data is transferred.
     In addition to eliminating polling, SuperSpeed USB eliminated the broadcast nature of the USB 2.0 bus protocol. SuperSpeed USB uses directed data transfer from/to the host and only the target function (and obviously any hubs in the path). This again enables power savings by only requiring the device for which the data is intended to turn on its transceiver. The other peripherals on the bus do not need to burn power determining that the data request is not directed towards them.
     The amount of power that is available on the bus also was changed from 5V and 500mA in USB 2.0 to 5V, and 900mA for SuperSpeed USB. The unconfigured (what a device can pull from the upstream connection during the enumeration process) current limit was also raised from 100mA to 150mA.
4. Is SuperSpeed USB backwards compatible to USB 2.0?
   • One of the key elements that the USB 3.0 Promoter's Group focused on was maintaining backwards compatibility with the over 10 billion USB 2.0 systems and peripherals already in the market. It was mandatory that any changes in the electrical signaling scheme NOT drive the need for a new form factor plug or receptacle on the host, and if possible, on the peripheral as well. The USB 3.0 A-side receptacle (what you find on PCs) are the same exact footprint as those in USB 2.0. The five new contacts were added in such a way that they would not interfere with the insertion of a USB 2.0 A-plug into a USB 3.0 A-receptacle, as well as when a USB 3.0 A-plug was inserted into a USB 2.0 receptacle. On the B-side (the receptacle you see on peripherals), it was not quite as simple to make this happen. A USB 3.0 B-receptacle will accept either a USB 3.0 B-plug or a USB 2.0 B-plug. However, a USB 3.0 B-plug will not "fit" in a USB 2.0 B-receptacle. But then again, it really would do the consumer no good for this to work anyway since a peripheral with a 2.0 B-plug is only capable of transmitting at USB 2.0 speeds. Therefore, it has no way to take advantage of the benefits of USB 3.0 and an existing USB 2.0 cable is all that would be needed for this.
5. How much more power will it take to transmit data in SuperSpeed USB than it did in USB 2.0?
   • Another key change was to focus on improving the power efficiency of the bus. This is ideal for extending the battery life for portable devices, whether hosts (notebook PCs) or peripherals. The specification defines excellent power characteristics, especially for idle links. Both upstream and downstream ports can initiate lower power states of the link. There is also local power management control using multiple link power states defined to further improve the power use efficiency. As mentioned above, eliminating polling and broadcasting also has reduced the overall power requirements. If you consider a simple file transfer, this focus on bus efficiencies combined with the over 10X speed increase more than makes up for the actual higher power that the transmitter uses (?). The result is that SuperSpeed USB will only use one-third the amount of power that USB 2.0 high-speed requires to transmit the same amount of data.