USB 3.0 Speeds Up Performance

The USB connector has been one of the greatest success stories in the history of computing, with more than 2 billion USB-connected devices sold to date. But in an age of terabyte hard drives, the once-cool throughput of 480 megabits per second that a USB 2.0 device can realistically provide just doesn't cut it any longer.

What is it?

 USB 3.0 (aka "SuperSpeed USB") promises to increase performance by a factor of 10, pushing the theoretical maximum throughput of the connector all the way up to 4.8 gigabits per second, or processing roughly the equivalent of an entire CD-R disc every second. USB 3.0 devices will use a slightly different connector, but USB 3.0 ports are expected to be backward-compatible with current USB plugs, and vice versa. USB 3.0 should also greatly enhance the power efficiency of USB devices, while increasing the juice (nearly one full amp, up from 0.1 amps) available to them. That means faster charging times for your iPod--and probably even more bizarre USB-connected gear like the toy rocket launchers and beverage coolers that have been festooning people's desks.

When is it coming? 

The USB 3.0 spec is nearly finished, with consumer gear now predicted to come in 2010. Meanwhile, a host of competing high-speed plugs--DisplayPort, eSATA, and HDMI--will soon become commonplace on PCs, driven largely by the onset of high-def video. Even FireWire is looking at an imminent upgrade of up to 3.2 gbps performance. The port proliferation may make for a baffling landscape on the back of a new PC, but you will at least have plenty of high-performance options for hooking up peripherals.

Architecture and features

In USB 3.0, dual-bus architecture is used to allow both USB 2.0 (Full Speed, Low Speed, or High Speed) and USB 3.0 (SuperSpeed) operations to take place simultaneously, thus providing backward compatibility. Connections are such that they also permit forward compatibility, that is, running USB 3.0 devices on USB 2.0 ports. The structural topology is the same, consisting of a tiered star topology with a root hub at level 0 and hubs at lower levels to provide bus connectivity to devices.

Data transfer and synchronization

The SuperSpeed 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, the device sends data or accepts data from the host. If the endpoint is halted, the device shall respond with a STALL handshake. 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, it will send an Endpoint Ready (ERDY) to the host which will then reschedule the transaction.

The use of unicast and the limited amount of multicast packets, combined with asynchronous notifications, enables links that are not actively passing packets to be put into reduced power states, which allows better power management.

Data encoding

The "SuperSpeed" bus provides for a transfer mode at a nominal rate of 5.0 Gbit/s, in addition to the three existing transfer modes. Accounting for the encoding overhead, the raw data throughput is 4 Gbit/s, and the specification considers it reasonable to achieve 3.2 Gbit/s (0.4 GB/s or 400 MB/s) or more in practice.

All data is sent as a stream of eight-bit (one-byte) segments that are scrambled and converted into 10-bit symbols via 8b/10b encoding; this helps the receiver to decode correctly even in the presence of electromagnetic interference (EMI). Scrambling is implemented using a free-running linear feedback shift register (LFSR). The LFSR is reset whenever a COM symbol is sent or received.

Unlike previous standards, the USB 3.0 standard does not directly specify a maximum cable length, requiring only that all cables meet an electrical specification: for copper cabling with AWG 26 wires, the maximum practical length is 3 meters (9.8 ft).

Power and charging

As with earlier versions of USB, USB 3.0 provides power at 5 volts nominal. The available current for low-power (one unit load) SuperSpeed devices is 150 mA, an increase from the 100 mA defined in USB 2.0. For high-power SuperSpeed devices, the limit is six unit loads or 900 mA (4.5 watts), almost twice USB 2.0's 500 mA.

USB 3.0 ports may also implement other USB specifications for increased power, including the USB Battery Charging Specification for up to 1.5 A or 7.5 watts, or the USB Power Delivery specification for up to 100 watts.