USB
USB(Universal Serial Bus) is a serial bus standard to interface devices. It was designed for computers such as PCs and the Apple Macintosh, but its popularity has prompted it to also become commonplace on video game consoles, PDAs, cellphones; and even devices such as televisions and home stereo equipment (e.g., mp3 players), and portable memory devices.
The radio spectrum-based USB implementation is known as Wireless USB.
Overview
A USB system has an asymmetric design, consisting of a host controller and multiple daisy-chained devices. Additional USB hubs may be included in the chain, allowing branching into a tree structure, subject to a limit of 5 levels of branching per controller. No more than 127 devices, including the bus devices, may be connected to a single host controller. Modern computers often have several host controllers, allowing a very large number of USB devices to be connected. USB cables do not need to be terminated.
Because of the capability of daisy-chaining USB devices, early USB announcements predicted that each USB device would include a USB port to allow for long chains of devices. In this model, computers would not need many USB ports, and computers shipped at this time typically had only two. However, for economical and technical reasons, daisy chaining never became widespread. To reduce the necessity of USB hubs, computers now come with a large number of USB ports, typically six. Most modern desktop computers have up to half of their total complement of USB ports on the front panel, to facilitate temporary connection of portable devices.
USB was designed to allow peripherals to be connected without the need to plug expansion cards into the computer's ISA, EISA, or PCI bus, and to improve plug-and-play capabilities by allowing devices to be hot-swapped (connected or disconnected without powering down or rebooting the computer). When a device is first connected, the host enumerates and recognises it, and loads the device driver it needs.
USB can connect peripherals such as mouse devices, keyboards, gamepads and joysticks, scanners, digital cameras, printers, external storage, networking components, etc. For many devices such as scanners and digital cameras, USB has become the standard connection method. USB is also used extensively to connect non-networked printers, replacing the parallel ports which were widely used; USB simplifies connecting several printers to one computer. As of 2004 there were about 1 billion USB devices in the world. As of 2005, the only large classes of peripherals that cannot use USB, because they need a higher data rate than USB can provide, are displays and monitors, and high-quality digital video components.
Technical Details
USB connects several devices to a host controller through a chain of hubs. In USB terminology devices are referred to as functions, because in theory what we know as a device may actually host several functions, such as a router that is a Secure Digital Card reader at the same time. The hubs are special purpose devices that are not officially considered functions. There always exists one hub known as the root hub, which is attached directly to the host controller.
These devices/functions (and hubs) have associated pipes (logical channels) which are connections from the host controller to a logical entity on the device named an endpoint. The pipes are synonymous to byte streams such as in the pipelines of Unix, however the term endpoint is also (sloppily) used to mean the entire pipe, even in the standard USB documentation.
These endpoints (and their respective pipes) are numbered 0-15 in each direction, so a device/function can have up to 32 active pipes, 16 inward and 16 outward. (The OUT direction shall be interpreted out of the host controller and the IN direction is into the host controller.)
Each endpoint can transfer data in one direction only, either into or out of the device/function, so each pipe is uni-directional. Endpoint 0 is however reserved for the bus management in both directions and thus takes up two of the 32 endpoints — all USB devices are required to implement endpoint 0, so there is always an inward and an outward pipe numbered 0 on any given device.
In these pipes, data is transferred in packets of varying length. Each pipe has a maximum packet length, typically 2n bytes, so a USB packet will often contain something on the order of 8, 16, 32, 64, 128, 256, 512 or 1024 bytes.
The pipes are also divided into four different categories by way of their transfer type:
- control transfers - typically used for short, simple commands to the device, and a status response, used e.g. by the bus control pipe number 0
- isochronous transfers - at some guaranteed speed (often but not necessarily as fast as possible) but with possible data loss, e.g. realtime audio or video
- interrupt transfers - devices that need guaranteed quick responses (bounded latency), e.g. pointing devices and keyboards
- bulk transfers - large sporadic transfers using all remaining available bandwidth (but with no guarantees on bandwidth or latency), e.g. file transfers
When a device (function) or hub is attached to the host controller through any hub on the bus, it is given a unique 7 bit address on the bus by the host controller.
The host controller then polls the bus for traffic, usually in a round-robin fashion, so no device can transfer any data on the bus without explicit request from the host controller. The interrupt transfers on corresponding endpoints do not actually interrupt any traffic on the bus: they are just scheduled to be queried more often and in between any other large transfers, thus "interrupt traffic" on a USB bus is really only high-priority traffic.
To access an endpoint, a hierarchical configuration must be obtained. The device connected to the bus has one (and only one) device descriptor which in turn has one or more configuration descriptors. These configurations often correspond to states, e.g. active vs. low power mode. Each configuration descriptor in turn has one or more interface descriptors, which describe certain aspects of the device, so that it may be used for different purposes: for example, a camera may have both audio and video interfaces. These interface descriptors in turn have one default interface setting and possibly more alternate interface settings which in turn have endpoint descriptors, as outlined above. An endpoint may however be reused among several interfaces and alternate interface settings.
Transfer speed
USB supports three data rates.
- A Low Speed rate of 1.5 Mbit/s (183 KiB/s) that is mostly used for Human Interface Devices (HID) such as keyboards, mice, and joysticks.
- A Full Speed rate of 12 Mbit/s (1.5 MiB/s). Full Speed was the fastest rate before the USB 2.0 specification and many devices fall back to Full Speed. Full Speed devices divide the USB bandwidth between them in a first-come first-served basis and it is not uncommon to run out of bandwidth with several isochronous devices. All USB Hubs support Full Speed.
- A Hi-Speed rate of 480 Mbit/s (57 MiB/s).
Though Hi-Speed devices are commonly referred to as "USB 2.0", not all USB 2.0 devices are Hi-Speed. A USB device should specify the speed it will use by correct labeling on the box it came in or sometimes on the device itself. The USB-IF certifies devices and provides licenses to use special marketing logos for either "Basic-Speed" (low and full) or High-Speed after passing a compliancy test and paying a licensing fee. All devices are tested according to the latest spec, so recently-compliant Low Speed devices are also 2.0.
Hi-Speed devices should fall back to the slower data rate of Full Speed when plugged into a Full Speed hub. Hi-Speed hubs have a special function called the Transaction Translator that segregates Full Speed and Low Speed bus traffic from Hi-Speed traffic. The Transaction Translator in a Hi-Speed hub (or possibly each port depending on the electrical design) will function as a completely separate Full Speed bus to Full Speed and Low Speed devices attached to it. This segregation is for bandwidth only; bus rules about power and hub depth still apply.
A hub will have one or more Transaction Translators and there is no standard way to determine the number of transaction translators a hub may have. All low and full speed devices connected to one transaction translator will share the low/full speed bandwidth. This means that hubs can have dramatically different performance depending upon the number of transaction translators and the devices plugged into their ports. e.g. a hi-speed 7 port hub with only 1 transaction translator with 7 low/full speed devices plugged in, will act no differently than a USB 1.1 hub and all devices compete for the same low/full speed bandwidth. If the hub were to have a transaction translator for each of the seven ports, then each device would have all the full/low speed bandwidth available to it and would only compete for the hi-speed bandwidth, which is much greater.
Summary : USB (universal serial bus) is device used for saving data while making it portable to connect it to any device for its access. USB's can be used for video games,mp3's, stereo equipments ,television etc. With USB's one can save ringtones to transfer them on mobile sets. Other ringtones such as music ringtones and funny ringtones can also be transferred on the USB device for making the data portable.
