SuperSpeed USB (USB 3.0) has been getting a lot of attention now as products become available in the market. The most obvious benefit is the more than 10 times increase in speed over USB 2.0 high-speed; 480 Mbps to 5 Gbps - but there are several others. 1) Foremost, after the speed increase, was improving the power efficiency of the bus; 2) next was maintaining backwards compatibility; and 3) was improving the data transfer efficiency itself. There are multiple aspects of the new specification that were developed to address reducing the overall power footprint of new USB devices including:
The first change to help reduce power consumption was to eliminate device polling. In USB 2.0, the host controller continuously polls each of the devices in the tree to check if they have data that they need to send to the host system. Device polling means that all devices must be fully "alive" and capable of transmitting data at all times, and that each is "always" burning power by transmitting NAKs to the inquiries of the host system when they do not have data to transmit. Finally, the host is continuously consuming power asking the devices if they have any data, which in most cases they do not. The next change was changing the packet transfer from broadcast to directed. When a USB 2.0 host has data to send to a device, it broadcasts the data on each of its ports. Each hub in the tree must also re-broadcast the packets on each of its downstream ports. Lastly, each of the devices on the bus must process the data (consuming power) to determine whether they are the intended target of the transfer. In SuperSpeed USB, the protocol was changed to direct the packets to only the intended target. This requires a little more intelligence in the host. It must know specifically where in the tree each device is, including what hub port (or ports, if multiple hubs are between it and the host) from which it is downstream. This reduces the overall power consumption in that only the specific downstream host and hub ports to which the device is connected must transmit data, and only the target must process the data. The third power reduction-focused change was to define two intermediate idle states. In USB 2.0, there are two states: ACTIVE and SUSPEND. In SuperSpeed USB, there are also FAST EXIT IDLE (U1) and SLOW EXIT IDLE (U2), in addition to ACTIVE (U0) and SUSPEND (U3). This allows devices to lower their power consumption when they are not transmitting or receiving data. In the FAST EXIT IDLE mode, the link goes idle but the clocking on the device stays on. While in SLOW EXIT IDLE, both the link and clocking are turned off, which requires a longer time to re-train the link before data can be transmitted. The ACTIVE and SUSPEND modes remain the same in both USB 2.0 and USB 3.0. The 10-times speed increase also enables lower overall power consumption. No, I am not saying that a 5-Gbps transceiver requires less power to transmit data than a 480-Mbps transceiver. I agree with all who have claimed that a 5-Gbps transceiver requires between two and five times the peak current than that of USB 2.0 transceiver. However, what I am talking about is lowering the overall power footprint, not just the peak current that is consumed at a small slice in time when the transmitter is active. When you factor in the approximately 10 times decrease in the transmitter's actual active time, the total power required to transmit a fixed amount of data (for instance, moving a file from the PC to a flash drive) is between 20 percent (two times peak and 1/10 the time) and 50 percent (5 times peak and 1/10 the time) the total power required for transmitting that same amount of data over USB 2.0. When you combine the bus usage efficiency (no broadcast packets and eliminate polling), the improved IDLE power states, and the lower average transmit power, SuperSpeed USB consumes approximately one-third (or less!) the power of USB 2.0! The next key attribute that was a focus during development was maintaining backwards compatibility with what Brian O'Rourke of In-Stat has called "the most successful PC interface of all time". It was determined during the development effort that the existing cable and connector solutions were not going to be adequate to reliably transfer data at 5 Gbps. The developers determined that the signaling would need to be done over separate conductors, versus those used for USB 2.0. They choose to use a full duplex differential signaling method based on the PCI Express electrical specification. At the same time, it was decided to not make any changes to the existing USB 2.0 signaling. This required adding at least two new differential pairs, in addition to the existing USB 2.0 differential pair and VBUS and GND. When you include the ground shield for the two new SuperSpeed differential pairs, this brings the total conductors to nine in the cable and nine contacts in the connector. So what then does backwards compatibility really mean? If we approach this from the end-user perspective, it means that ALL my existing products that are compliant to the specification will seamlessly connect to and work with all new products supporting the new specification! This means that the existing cables (i.e., plugs) must be able to be inserted into the appropriate new receptacle. The reverse is also true, that the new cables must be able to be inserted into the old receptacles - again, where appropriate. The fourth key value is improving overall bus usage efficiency. I have already touched on the first aspect of this: the elimination of polling. Additionally, the full duplex architecture of SuperSpeed USB allows for concurrent bi-directional data flow as opposed to the half duplex USB 2.0 architecture. In summary, the USB 3.0 Promoter's Group had four key values in mind as they developed the specification. Decreasing the power required to transfer data, maintaining backwards compatibility, increasing the bandwidth utilization efficiency, and of course the over 10 times increase in the raw bit rate. These values will enable increase, both in the Sync-and-Go experience of the consumer as well as extend the battery life for these content-rich consumer products, and provide the needed headroom to do the same for flash-based products for the next five years. To see the video that demonstrates the speed of SuperSpeed USB3 versus High-Speed USB 2.0, visit http://www.ti.com/usb3video-ca.
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SuperSpeed USB (USB 3.0)
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