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While the main goal of USB 3.0 is to increase available bandwidth, the new standard also sets its sights on average power consumption. USB 2.0 permanently polls for available devices, requiring power. In contrast, USB 3.0 was designed around four different link states, named U0-U3. The link state U0 represents an active connect, while link state U3 puts the device into suspend mode.
If a connection is idle, link state U1 will disable send and receive functions. Link state U2 goes one step further by switching off the internal clock. Connected devices hence can switch to U1 link state immediately after finishing transfers, which is expected to introduce measurable average power savings when compared to USB 2.0.
Apart from the different link states, USB 3.0 also supports higher amperage than USB 2.0. The threshold was 500 mA in the case of USB 2.0, and is being shifted to 900 mA. In addition, the amperage while handshaking the connection was increased from 100 mA with USB 2.0 to 150 mA with USB 3.0. Both stats are particularly important for portable hard drives, which often times require slightly more amperage. Previously, they had to be operated using y-cables to get power from two different USB ports while using one port for data, even though this violated the USB 2.0 specification.
New Cables, But Still Compatible
Let’s talk about cables a bit more. USB 3.0 doesn't employ fibre optics, as this would be too expensive for the mainstream. Hence, it still relies on copper cabling. But now there will be nine (instead of only four) wires. Data transfer is performed through four out of the five additional wires in differential mode (SDP--Shielded Differential Pair). One pair takes care of sending and the other handles receiving data. This is similar to Serial ATA and provides the full bandwidth in both the upstream and downstream directions. The fifth wire is the ground wire.
USB 3.0 is backward-compatible with USB 2.0, making the connectors appear as if they were the same as the conventional connector type A. The USB 2.0 pins remain where they are, while the five new pins were placed deep inside the connector cover. This means that you have to fully insert a USB 3.0 cable into a USB 3.0 port in order to make sure you're running in USB 3.0 mode by making the additional connections. Otherwise, you’ll fallback to USB 2.0 speed. The USB-IF recommends that all manufacturers color-code the inside of the connector cover using color “Pantone 300C.”
This is similar for the type B USB connector, although the differences are more visible here. The almost-square connectors for USB 3.0 can clearly be identified through the five additional contacts.
Mobile Device Connectors
There will be a very noticeable change for mobile devices, though. While the old Micro-B USB 2.0 connector is 6.86 millimeters wide, the USB 3.0 Micro-B type for cell phones, smartphones or media players will be as wide as 12.25 millimeters. Again, the connectors were designed in a way to maintain USB 2.0 compatibility.
Finally, cable length is going to change. While five meters were allowed for USB 2.0 connections, USB 3.0 will only support a maximum length of three meters.