USB 3.0, the latest version of USB (Universal Serial Bus), provides better speed and more efficient power management than USB 2.0. USB 3.0 is backward compatible with USB 2.0 devices; however, data transfer speeds are limited to USB 2.0 levels when these devices inter-operate.
edit What is USB 3.0 and USB 2.0?
Universal Serial Bus (USB) is an industry standard developed in the mid-1990s that defines the cables, connectors and communications protocols used in a bus for connection, communication and power supply between computers and electronic devices. Now even devices like smartphones, PDAs and video game consoles are connected to the computers with USB ports allowing recharging and communication thereby replacing the requirement of adapters and power chargers.
USB3.0 was released in November 2008, almost eight years after the release of USB 2.0.
edit USB 3.0 Highlights and Benefits over USB 2.0
- Transfer rates - USB 2.0 offers transfer rates of 480 Mbps and USB 3.0 offers transfer rates of 4.8 Gbps - that's 10 times faster.
- Addition of another physical bus - The amount of wires has been doubled, from 4 to 8. Additional wires require more space in both the cables and connectors, so there are new types of connectors.
- Power consumption - USB 2.0 provides up to 500 mA whereas USB 3.0 provides up to 900 mA. The USB 3 devices will provide more power when needed and conserve power when the device is connected but idling.
- More bandwidth - instead of one-way communication, USB 3.0 uses two unidirectional data paths, one to receive data and the other to transmit while USB 2.0 can only handle only one direction of data at any time.
- Improved bus utilization - a new feature has been added (using packets NRDY and ERDY) to let a device asynchronously notify the host of its readiness.
When data is being transferred through USB 3.0 Devices, cables and connectors transaction is initiated by the host making a request followed by a response from the device. The device either accepts the request or rejects it. If accepted then device sends data or accepts data from the host. If there is lack of buffer space or data, it responds with a Not Ready (NRDY) signal to tell the host that it is not able to process the request. When the device is ready then, it will send an Endpoint Ready (ERDY) to the host which will then reschedule the transaction.
edit Physical Differences
USB 3.0 Connectors are different from USB 2.0 Connectors and the 3.0 connectors are usually colored blue on the inside in order to distinguish them from the 2.0 connectors.
edit Backward Compatible
USB 3.0 is compatible with USB 2.0. However, the USB 3.0 product will perform at the same level as a USB 2.0 product, so speed and power benefits will not be fully realized.
USB 3.0 receptacles are electrically compatible with USB Standard 2.0 device plugs if they physically match. USB 3.0 type-A plugs and receptacles are completely backward compatible, and USB 3.0 type-B receptacles will accept USB 2.0 and earlier plugs. However, USB 3.0 type-B plugs will not fit into USB 2.0 and earlier receptacles.
This means that USB 3.0 cables cannot be used with USB 2.0 and USB 1.1 peripherals, although USB 2.0 cables can be used with USB 3.0 devices, if at USB 2.0 speeds.
Here is a good informational video explaining the various features of USB 3.0 vs USB 2.0:
The following videos review the speed of USB 2.0 vs USB 3.0 products:
For a similar product, the USB 3.0 version is generally more expensive than it's USB 2.0 version.
You can check the current prices on Amazon for a few USB 2.0 vs USB 3.0 enabled devices: