USB 3.0

 

USB 3.0 is the third major version of the Universal Serial Bus (USB) standard for computer connectivity. Among other improvements, USB 3.0 adds a new transfer mode called "SuperSpeed" (SS), capable of transferring data at up to 5 Gbit/s (625 MB/s), which is more than ten times as fast as the 480 Mbit/s (60 MB/s) high speed of USB 2.0. Beside different connectors used on USB 3.0 cables, they are also distinguishable from their 2.0 counterparts by either the blue color of the ports or the SS initials on the plugs.
A successor standard named USB 3.1 was released in July 2013, providing transfer rates up to 10 Gbit/s (1.25 GB/s, called "SuperSpeed+"), which effectively put it on par with the first version of Thunderbolt.

Differences in comparison to USB 2.0

The USB 3.0 specification is similar to USB 2.0 but with many improvements and an alternative implementation. Earlier USB concepts like endpoints and four transfer types (bulk, control, isochronous and interrupt) are preserved but the protocol and electrical interface are different. The specification defines a physically separate channel to carry USB 3.0 traffic. The changes in this specification make improvements in the following areas:

• Transfer speed – added a new transfer type called SuperSpeed or SS, 5 Gbit/s (electrically, it is more similar to PCI Express 2.0 and SATA than USB 2.0)^[2]
• Increased bandwidth – instead of one-way communication, USB 3.0 uses two unidirectional data paths: one to receive data and the other to transmit
• Power management – U0 through U3 link power management states are defined
• Improved bus utilization – a new feature is added (using packets NRDY and ERDY) to let a device asynchronously notify the host of its readiness (no need for polling)
• Support to rotating media – bulk protocol is updated with a new feature called Stream Protocol that allows a large number of logical streams within an Endpoint

USB 3.0 has transmission speeds of up to 5 Gbit/s, which is ten times faster than USB 2.0 (480 Mbit/s) before taking into account that USB 3.0 is full duplex whereas USB 2.0 is half duplex, giving USB 3.0 the potential total bandwidth if utilized both ways to twenty times that of USB 2.0.

Architecture and features

Close-up of a Standard-A USB 3.0 connector, showing its front row of four pins for the USB 1.x/2.0 backwards compatibility, and a second row of five pins for the new USB 3.0 connectivity.

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 unicasting and the limited multicasting of packets, combined with asynchronous notifications, enables links that are not actively passing packets to be put into reduced power states, allowing for 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.^[4]
All data is sent as a stream of eight bits (one-byte segments) that are scrambled and then converted into a 10-bit format, what is known as the 8b/10b encoding. This helps to reduce electromagnetic interference (EMI). The inverse process is carried out at the receiving end. Scrambling is implemented using a free running linear feedback shift register (LFSR). The LFSR is reset whenever a COM symbol is sent or received.^[4]
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)