USB 3.0

Abstract

USB 3.0 is the second major revision of the Universal Serial Bus (USB) standard for computer connectivity. The standard from 2008 implements a 5 Gbit/s transfer rate. In the late 1990s, the first major revision was made to the USB 1.1 specification. The revision was called USB 2.0 which added a new transfer speed called High Speed (HS – 480 Mbit/s) to the earlier speeds (Low Speed (LS) – 1.5 Mbit/s and Full Speed (FS) – 12 Mbit/s).he USB 3.0 specification uses the same concepts of USB 2.0 but with many improvements and totally different implementation. Earlier USB concepts like endpoints and four transfer types (bulk, control, isochronous and interrupt) are preserved but the protocol and electrical interface are significantly different. It is so different that 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 Super Speed or SS – 5 Gbit/s (electrically it is more similar to PCIe Gen2 than USB 2.0);

'''more 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 of 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 10 times faster than USB 2.0 (480 Mbit/s). USB 3.0 significantly reduces the time required for data transmission, reduces power consumption, and is backwards compatible with USB 2.0.

=== Architecture and features ===
In USB 3.0 dual-bus architecture is used to allow both USB 0 (HIGH Speed/LOW Speed/FULL Speed) and USB 3.0 (Super Speed) operations to take place simultaneously, thus providing backward compatibility. Connections are such that they also permit forward compatibility, that is, run USB 3 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 then 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 then, 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 a transfer mode at 5.0 Gbit/s additionally to the three existing transfer modes. The raw 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.

All data is sent as a stream of eight bits which are scrambled and then converted into 10-bit format. This helps to reduce [[electromagnetic interference]] (EMI). The inverse process is carried out at the receiving end. Scrambling is implemented using a free running [[LFSR|Linear Feedback Shift Register]] (LFSR). The LFSR is reset whenever a COM symbol is sent or received.

It is still going to be tethered to 16 feet (maximum) cables with active repeaters for extended lengths. So far, USB 3.0 still runs on copper cabling with most likely the same inherent limitations.

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USB-3.0