what is usb?

Universal Serial Bus (USB) is an industry standard developed in the mid-1990s that defines the cables, connectors and communication protocol used in a bus for connection, communication, and power supply between computers and electronic devices.

there are four basic kinds or sizes related to the USB connectors and types of established connections:

• the older "standard" size, in its USB 1.1/2.0 and USB 3.0 variants (for example, on USB flash drives)
• the "mini" size (primarily for the B connector end, such as on many cameras)
• the "micro" size, in its USB 1.1/2.0 and USB 3.0 variants (for example, on most modern cellphones)
• the versatile "USB on-the-go" scheme, in both mini and micro sizes.

what is in usb 2.0?

• Mini-A and Mini-B Connector ECN: Released in October 2000.
  Specifications for Mini-A and B plug and receptacle. Also receptacle that accepts both plugs for On-The-Go. These should not be confused with Micro-B plug and receptacle.
• Errata as of December 2000: Released in December 2000
• Pull-up/Pull-down Resistors ECN: Released in May 2002
• Errata as of May 2002: Released in May 2002
• Interface Associations ECN: Released in May 2003.
  New standard descriptor was added that allows associating multiple interfaces with a single device function.
• Rounded Chamfer ECN: Released in October 2003.
  A recommended, compatible change to Mini-B plugs that results in longer lasting connectors.
• Unicode ECN: Released in February 2005.
  This ECN specifies that strings are encoded using utf-16le. USB 2.0 specified unicode, but did not specify the encoding.
• Inter-Chip USB Supplement: Released in March 2006
• On-The-Go Supplement 1.3: Released in December 2006.
  usb on-the-go makes it possible for two USB devices to communicate with each other without requiring a separate USB host. In practice, one of the USB devices acts as a host for the other device.
• Battery Charging Specification 1.1: Released in March 2007 (Updated 15 Apr 2009).
  Adds support for dedicated chargers (power supplies with USB connectors), host chargers (USB hosts that can act as chargers) and the No Dead Battery provision, which allows devices to temporarily draw 100mA current after they have been attached. If a USB device is connected to dedicated charger, maximum current drawn by the device may be as high as 1.8A. (Note that this document is not distributed with USB 2.0 specification package only USB 3.0 and USB On-The-Go.)
• Micro-USB Cables and Connectors Specification 1.01: Released in April 2007.
• Link Power Management Addendum ECN: Released in July 2007.
  This adds "sleep", a new power state between enabled and suspended states. Device in this state is not required to reduce its power consumption. However, switching between enabled and sleep states is much faster than switching between enabled and suspended states, which allows devices to sleep while idle.
• Battery Charging Specification 1.2:[20] Released in December 2010.
  Several changes and increasing limits including allowing 1.5 A on charging ports for unconfigured devices, allowing High Speed communication while having a current up to 1.5 A and allowing a maximum current of 5 A.

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Low Speed

This was intended for cheap, low data rate devices like mice. The low speed captive cable is thinner and more flexible than that required for full and high speed.

Full Speed

This was originally specified for all other devices.

High Speed

The high speed additions to the specification were introduced in USB 2.0 as a response to the higher speed of Firewire.

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Architecture

The USB is based on a so-called 'tiered star topology' in which there is a single host controller and up to 127 'slave' devices. The host controller is connected to a hub, integrated within the PC, which allows a number of attachment points (often loosely referred to as ports). A further hub may be plugged into each of these attachment points, and so on. However there are limitations on this expansion.

As stated above a maximum of 127 devices (including hubs) may be connected. This is because the address field in a packet is 7 bits long, and the address 0 cannot be used as it has special significance. (In most systems the bus would be running out of bandwidth, or other resources, long before the 127 devices was reached.)