Universal Serial Bus

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Universal Serial Bus (USB) is a serial bus standard to interface devices. It was originally designed for computers, but its popularity has prompted it to also become commonplace on video game consoles, PDAs, portable DVD and media players, cellphones; and even devices such as televisions, home stereo equipment (e.g., digital audio players), car stereos and portable memory devices.

The radio spectrum based USB implementation is known as Wireless USB.

Overview

USB was devised as a major component in the transition towards a legacy-free PC. The intention was to let go of all older serial and parallel ports on personal computers since these were not properly standardized and required a multitude of device drivers to be developed and maintained.

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 recognizes it, and loads the device driver it needs.

USB can connect peripherals such as mouse devices, keyboards, PDAs, 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 that had previously been in wide use; USB simplifies connecting several printers to one computer. As of 2004 there were about 1 billion USB devices in the world.

Standardization

The design of USB is standardized by the USB Implementers Forum (USB-IF), an industry standards body incorporating leading companies from the computer and electronics industries. Notable members have included Apple Computer, Hewlett-Packard, NEC, Microsoft, Intel, and Agere.

As of 2006 the USB specification is at version 2.0 (with revisions). Hewlett-Packard, Intel, Lucent, Microsoft, NEC, and Philips jointly led the initiative to develop a higher data transfer rate than the 1.1 specification. The USB 2.0 specification was released in April 2000 and was standardized by the USB-IF at the end of 2001. Previous notable releases of the specification were 0.9, 1.0, and 1.1. Equipment conforming with any version of the standard will also work with devices designed to any previous specification (known as: backward compatibility).

Smaller USB plugs and receptacles, called Mini-A and Mini-B, are also available, as specified by the On-The-Go Supplement to the USB 2.0 Specification. As of 2006, the specification is at revision 1.2.

A new smaller variant of USB, Micro-USB, was announced by the USB Implementers Forum on January 4, 2007 ^[1]

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 each individual physical 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). The pipes are synonymous to byte streams such as in the pipelines of Unix. Pipes are connections from the host controller to a logical entity on the device named an endpoint The term endpoint is also occasionally used to refer to the entire pipe.

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 into the host controller and 16 out of the 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 up to 512 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.

Host controllers

The hardware that contains the host controller and the root hub has an interface geared toward the programmer which is called Host Controller Device (HCD) and is defined by the hardware implementer. In practice, these are hardware registers (ports) in the computer.

At version 1.0 and 1.1 there were two competing HCD implementations. Compaq's Open Host Controller Interface (OHCI) was adopted as the standard by the USB-IF. However, Intel subsequently created a specification they called the Universal Host Controller Interface (UHCI) and insisted other implementers pay to license and implement UHCI. VIA Technologies licensed the UHCI standard from Intel; all other chipset implementers use OHCI. The main difference between OHCI and UHCI is the fact that UHCI is more software-driven than OHCI is, making UHCI slightly more processor-intensive but cheaper to implement (excluding the license fees). The dueling implementations forced operating system vendors and hardware vendors to develop and test on both implementations which increased cost. During the design phase of USB 2.0 the USB-IF insisted on only one implementation. The USB 2.0 HCD implementation is called the Extended Host Controller Interface (EHCI). Only EHCI can support hi-speed transfers. Each EHCI controller contains four virtual HCD implementations to support Full Speed and Low Speed devices. The virtual HCD on Intel and VIA EHCI controllers are UHCI. All other vendors use virtual OHCI controllers.

On Microsoft Windows platforms, one can tell whether a USB port is version 2.0 by opening the Device Manager and checking for the word "Enhanced" in its description; only USB 2.0 drivers will contain the word "Enhanced." On Linux systems, the lsusb -v command will list all USB devices, and USB controllers will be named OHCI, UHCI or EHCI respectively, which is also the case in the Mac OS X system profiler. On BSD systems, dmesg will show the detailed information hierarchy.

Device classes

Devices that attach to the bus can be full-custom devices requiring a full-custom device driver to be used, or may belong to a device class. These classes define an expected behavior in terms of device and interface descriptors so that the same device driver may be used for any device that claims to be a member of a certain class. An operating system is supposed to implement all device classes so as to provide generic drivers for any USB device.

Device classes are decided upon by the Device Working Group of the USB Implementers Forum. If the class is to be set for the entire device, the number is assigned to the bDeviceClass field of the device descriptor, and if it is to be set for a single interface on a device, it is assigned to the bInterfaceClass field of the interface descriptor. Both of these are a single byte each, so a maximum of 254 different device classes are possible (values 0x00 and 0xFF are reserved). If bDeviceClass is set to 0x00, the operating system will look at bInterfaceClass of each interface to determine the device class. Each class also optionally supports a SubClass and Protocol subdefinition. These can be used as the main device classes are continuously revised.

The most used device classes (grouped by assigned class ID) are:

0x00 

Reserved value - used in the device descriptor to signify that the interface descriptor holds the device class identifier for each interface.

0x01 

USB audio device class, sound card-like devices.

0x02 

USB communications device class used for modems, network cards, ISDN connections, Fax.

0x03 

USB human interface device class ("HID"), keyboards, mice, etc.

0x06 

Still image capture device class, identical to the Picture Transfer Protocol as used across USB

0x07 

USB printer device class, printer-like devices.

0x08 

USB mass storage device class used for flash drives, portable hard drives, memory card readers, digital cameras, digital audio players etc. This device class presents the device as a block device (almost always used to store a file system).

0x09 

USB hubs.

0x0E 

USB video device class, webcam-like devices, motion image capture devices.

0xE0 

Wireless controllers, for example Bluetooth dongles.

0xFF 

Custom device class - used to establish that a device or interface does not support any standard device class and requires custom drivers.

USB signaling

Pin numbers (looking at socket):

USB signals are transmitted on a twisted pair of data cables, labelled D+ and D−. These collectively use half-duplex differential signaling to combat the effects of electromagnetic noise on longer lines. D+ and D− usually operate together; they are not separate simplex connections. Transmitted signal levels are 0.0–0.3 volts for low and 2.8–3.6 volts for high.

Transfer speed

USB supports three data rates:

• A Low Speed rate of up to 1.5 Mbit/s (187.5 kB/s) that is mostly used for Human Interface Devices (HID) such as keyboards, mice, and joysticks.
• A Full Speed rate of up to 12 Mbit/s (1.5 MB/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 up to 480 Mbit/s (60 MB/s).

Hubs, even Hi-Speed hubs, serving a number of non-hi-speed devices, are likely to divide up a total bandwidth of 12 Mbit/s for such devices, which will slow them down unless the hub has transaction translator for each port. ^[3]

Though Hi-Speed devices are commonly referred to as "USB 2.0" and advertised as "up to 480 Mbit/s", not all USB 2.0 devices are Hi-Speed. Hi-speed devices typically only operate at half of the full theoretical (60 MB/s) data throughput rate. The maximum rate currently (2006) attained with real devices is about half, 30 MB/s.^[4] Most hi-speed USB devices typically operate at much slower speeds, often about 3 MB/s overall, sometimes up to 10-20 MB/s. The USB-IF certifies devices and provides licenses to use special marketing logos for either "Basic-Speed" (low and full) or Hi-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 are intended to 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.^[5]

Data encoding

The USB standard uses the NRZI system to encode data, and uses bit stuffing for logic 1 transmission five bits long (put logic 0 after five bits of logic 1; the receiver ignores a 0 following five logic 1). The NRZI (non-return to zero, inverted) encoding method does not change the signal for transmission of a logic 0, but it inverts the signal level for transmission of each logic 1.

Mini-USB signaling

Most of the pins of a Mini-USB connector are the same as those in a standard USB connector, except pin 4. Pin 4 is called "ID" and, in the Mini-A connector, is connected to ground, but in the Mini-B is not connected. This causes a device supporting USB On-The-Go (with a Mini-AB socket) to initially act as host when connected to a USB Mini-A connector (the "A" end of a Mini-A–Mini-B cable). The Mini-A connector also has an additional piece of plastic inside to prevent insertion into a slave-only (B-only) device.

USB connectors

There are several types of USB connectors, and some have been added as the specification has progressed. From the original USB specification:

• Series "A" plug
• Series "A" receptacle
• Series "B" plug
• Series "B" receptacle

Added in USB 2.0 specification:

• Mini-B plug
• Mini-B receptacle

Added in the On-The-Go Supplement to the USB 2.0 Specification:

• Mini-A plug (white)
• Mini-A receptacle (white)
• Mini-AB receptacle (grey)

Adapters, also from the On-The-Go Supplement to the USB 2.0 Specification (Note that no other adapters are allowed.):

• Mini-A receptacle to Standard-A plug
• Standard-A receptacle to Mini-A plug

Cables have only plugs, and hosts and devices have only receptacles. Hosts have type-A receptacles; devices, if they have receptacles, have type-B. Type-A plugs only mate with type-A receptacles, and type-B with type-B. The On-the-Go supplement allows a product to be either host or device, with a Mini-AB receptacle that accepts either a Mini-A plug or a Mini-B plug. Mini-A, Mini-B, and Mini-AB connectors are identified easily by color. The plastic inside Mini-A plugs and receptacles is always white, that in Mini-B connectors black, and that in Mini-AB receptacles grey.

There is a limited set of cables allowed by the USB specification. Cables fall into two categories — "detachable" and "captive". For purposes of the specification, "captive" includes any cable with a custom connector on the device end. Any captive cable has only a type-A plug, either Standard-A or Mini-A. Any detachable USB cable has one type-A connector (either Standard-A or Mini-A) and one type-B connector (either Standard-B or Mini-B).

Detachable USB cable types:

• Standard-A plug to Standard-B plug
• Standard-A plug to Mini-B plug
• Mini-A plug to Standard-B plug
• Mini-A plug to Mini-B plug

Any cable with a receptacle or with two "A" or two "B" connectors is, by definition, not USB.^[6] However, many cable manufacturers make and sell USB-compatible (yet not strictly conforming) extension cables with a Standard-A plug on one end and Standard-A receptacle on one end. Cables with two type A or even two type B plugs are available from more specialist suppliers.

Note that only "full-speed" and "hi-speed" devices use detachable cables. Compliant "Low-speed" devices only use captive cables, because the low-speed specification does not allow for the electrical characteristics of standard detachable USB cables.

The Mini-A, Mini-B, and Mini-AB connectors are used for smaller devices such as PDAs, mobile phones or digital cameras. The Series "A" plug is approximately 4 by 12 mm, the Series "B" approximately 7 by 8 mm, and the Mini-A and Mini-B plugs approximately 3 by 7 mm.

The connectors which the USB committee specified were designed to support a number of USB's underlying goals, and to reflect lessons learned from the varied menagerie of connectors then in service. In particular:

• The connectors are designed to be robust. Many previous connector designs were fragile, with pins or other delicate components prone to bending or breaking, even with the application of only very modest force. The electrical contacts in a USB connector are protected by an adjacent plastic tongue, and the entire connecting assembly is further protected by an enclosing metal sheath. As a result USB connectors can safely be handled, inserted, and removed, even by a small child. The encasing sheath and the tough moulded plug body mean that a connector can be dropped, stepped upon, even crushed or struck, all without damage; a considerable degree of force is needed to significantly damage a USB connector.
• It is difficult to incorrectly attach a USB connector. Connectors cannot be plugged-in upside down, and it is clear from the appearance and kinesthetic sensation of making a connection when the plug and socket are correctly mated. However, it is not obvious at a glance to the inexperienced user (or to a user without sight of the installation) which way round a connector goes, so it is often necessary to try both ways.
• The connectors are particularly cheap to manufacture.
• The connectors enforce the directed topology of a USB network. USB does not support cyclical networks, so the connectors from incompatible USB devices are themselves incompatible. Unlike other communications systems (e.g. RJ-45 cabling) gender-changers are almost never used, making it difficult to create a cyclic USB network.
• A moderate insertion/removal force is specified. USB cables and small USB devices are held in place by the gripping force from the receptacle (without the need for the screws, clips, or thumbturns other connectors require). The force needed to make or break a connection is modest, allowing connections to be made in awkward circumstances or by those with motor disabilities.
• The connector construction always ensures that the external sheath on the plug contacts with its counterpart in the receptacle before the four connectors within are connected. This sheath is typically connected to the system ground, allowing otherwise damaging static charges to be safely discharged by this route (rather than via delicate electronic components). This means of enclosure also means that there is a (moderate) degree of protection from electromagnetic interference afforded to the USB signal while it travels through the mated connector pair (this is the only location when the otherwise twisted data pair must travel a distance in parallel). In addition, the power and common connections are made after the system ground but before the data connections. This type of staged make-break timing allows for safe hot-swapping and has long been common practice in the design of connectors in the aerospace industry.
• The USB standard specifies relatively low tolerances for compliant USB connectors, intending to minimize incompatibilities in connectors produced by different vendors (a goal that has been very successfully achieved). Unlike most other connector standards, the USB spec also defines limits to the size of a connecting device in the area around its plug. This was done to avoid circumstances where a device complied with the connector specification but its large size blocked adjacent ports. Compliant devices must either fit within the size restrictions or support a compliant extension cable which does.

However, the physical layer is changed in some examples. For example, the IBM UltraPort is a proprietary USB connector located on the top of IBM's laptop LCDs. It uses a different mechanical connector while preserving the USB signaling and protocol. Other manufacturers of small items also developed their own small form factor connector, and a wide variety of these have appeared. For specification purposes, these devices were treated as having a captive cable.

An extension to USB called USB On-The-Go allows a single port to act as either a host or a device - chosen by which end of the cable plugs into the socket on the unit. Even after the cable is hooked up and the units are talking, the two units may "swap" ends under program control. This facility targets units such as PDAs where the USB link might connect to a PC's host port as a device in one instance, yet connect as a host itself to a keyboard and mouse device in another instance. USB On-The-Go has therefore defined two small form factor connectors, the Mini-A and Mini-B, and a universal socket (Mini-AB), which should stop the proliferation of proprietary designs.

Wireless USB is a standard being developed to extend the USB standard while maintaining backwards compatibility with USB 1.1 and USB 2.0 on the protocol level.

The maximum length of a USB cable is 5 meters; greater lengths require hubs [2]. USB Connections can be extended to 50 m over CAT5 or up to 10 km over fiber by using special USB Extender products developed by various manufacturers.

Power supply

Standard

The USB specification provides a 5 V (volts) supply on a single wire from which connected USB devices may draw power. The specification provides for no more than 5.25 V and no less than 4.35 V between the +ve and -ve bus power lines.

Initially, a device is only allowed to draw 100 mA. It may request more current from the upstream device in units of 100 mA up to a maximum of 500 mA. In practice, most ports will deliver the full 500 mA or more before shutting down power, even if the device hasn't requested it or even identified itself. If a (compliant) device requires more power than is available, then it cannot operate until the user changes the network (either by rearranging USB connections or by adding external power) to supply the power required.

If a bus-powered hub is used, the devices downstream may only use a total of four units — 400 mA — of current. This limits compliant bus-powered hubs to 4 ports, among other things. Equipment requiring more than 500 mA, hubs with more than 4 ports and hubs with downstream devices using more than four 100 mA units total must provide their own power. The host operating system typically keeps track of the power requirements of the USB network and may warn the computer's operator when a given segment requires more power than is available.

Non-standard

A number of devices use this power supply without participating in a proper USB network. The typical example is a USB-powered reading light; fans, mug heaters, battery chargers (particularly for mobile telephones) and even miniature vacuum cleaners are also available. In most cases, these items contain no digitally-based circuitry, and thus are not proper USB devices at all. This can cause problems with some computers—the USB specification requires that devices connect in a low-power mode (100 mA maximum) and state how much current they need, before switching, with the host's permission, into high-power mode.

Some USB devices draw more power than is permitted by the specification for a single port. This is a common requirement of external hard and optical disc drives and other devices with motors or lamps. Such devices can be used with an external power supply of adequate rating; some external hubs may, in practice, supply sufficient power. For portable devices where external power is not available, but not more than 1 A is required at 5 V, devices may have connectors to allow the use of two USB cables, doubling available power but reducing the number of USB ports available to other devices. Amongst others, a number of peripherals for IBM laptops (now made by Lenovo) are designed to use dual USB connections.

USB-powered devices attempting to draw large currents without requesting the power will not work with certain USB controllers, and will either disrupt other devices on the bus or fail to work themselves (or both). Those problems with the abuse of the USB power supply have inspired a number of April Fool hoaxes, like the introduction of a USB-powered George Foreman iGrill^[7] and a desktop USB Fondue Set.^[8]

USB compared to other standards

Storage

USB implements connections to storage devices using a set of standards called the USB mass-storage device class (referred to as MSC or UMS). This was initially intended for traditional magnetic and optical drives, but has been extended to support a wide variety of devices. USB is not intended to be a primary bus for a computer's internal storage: buses such as ATA (IDE), Serial ATA (SATA), and SCSI fulfill that role.

However, USB has one important advantage in making it possible to install and remove devices without opening the computer case, making it useful for external drives. Today a number of manufacturers offer external portable USB hard drives, or empty enclosures for drives, that offer performance comparable to internal drives. These external drives usually contain a translating device that interfaces a drive of conventional technology (IDE, ATA, SATA, ATAPI, or even SCSI) to a USB port. Functionally, the drive appears to the user just like another internal drive. Other competing standards that allow for external connectivity are eSATA and Firewire.

Human-interface devices (HIDs)

As of 2006, most PCs and motherboards have at least two USB ports, but still retain PS/2 keyboard and mouse connectors. AT keyboard connectors are less frequently found. Motherboards for non-portable PCs usually have a number of USB 2.0 hi-speed ports, some available at the back of the computer case, others requiring USB sockets on the front or rear of the computer to be connected via a cable to a header on the motherboard. Joysticks, keypads, tablets and other human-interface devices are also progressively migrating from MIDI, PC game port, and PS/2 connectors to USB. Mice and keyboards are frequently fitted with USB connectors, but are generally supplied with a small USB-to-PS/2 adaptor so that they can be used with either USB or PS/2 ports. These adaptors only make use out of the fact that such HID interfaces are equipped with controllers that are capable of serving both the USB and the PS/2 protocol. Hence, there is no logic inside these adaptors.

Apple computers have exclusively used USB for all wired mice and keyboards since January 1999. Apple wireless mice and keyboards have always used the Bluetooth standard.

FireWire

USB was originally seen as a complement to FireWire (IEEE 1394), which was designed as a high-speed serial bus which could efficiently interconnect peripherals such as hard disks, audio interfaces, and video equipment. USB originally operated at a far lower data rate and used much simpler hardware, and was suitable for small peripherals such as keyboards and mice.

About the time that the 1394a standard was reaching completion, Apple threatened to charge $1.00 per port to license Apple's intellectual property (IP) relating to 1394a (Apple had previously not charged any royalty for their IP in 1394). This fee was considered by many of the USB Core companies to be excessive so they started work on updating the USB standard to offer data rates that were competitive with 1394a. Even though the 1394 IP license fee was eventually set at $0.25 per system (a price set by a group of companies owning the "essential patents" contained in 1394), the work on USB 2.0 continued. Intel chose to use USB 2.0 in their chipsets rather than to require additional connectors to support 1394 as well as USB. Lack of 1394 support on Intel’s chipset virtually assured that 1394 would have no significant market penetration in the commercial PC market.

USB 2.0 Hi-Speed reached a performance level sufficient for consumer equipment while retaining compatibility with older devices. An example of how the popularity of USB displaced FireWire in a commercial device is the Apple iPod. It was originally released with a FireWire connector, which was eventually modified to allow for both USB and FireWire connections when the product was released for Windows. 3rd generation iPods used USB and Firewire for data transfer and only allows a FireWire connection to charge the battery from the main adapter. The iPod does charge via both cables when connected to the host computer. Starting from the 4th generation, iPods use USB for data transfer and both USB and Firewire for charging.

Today, USB Hi-Speed is used in many consumer products. FireWire, however, retains its popularity in areas involving content creation, such as video and audio production.

Technical differences

The most significant technical differences between FireWire and USB include the following:

• USB networks use a tiered-star topology, while FireWire networks use a repeater-based topology.
• USB uses a "speak-when-spoken-to" protocol; peripherals cannot communicate with the host unless the host specifically requests communication. A FireWire device can communicate with any other node at any time, subject to network conditions.
• A USB network relies on a single host at the top of the tree to control the network. In a FireWire network, any capable node can control the network.

These and other differences reflect the differing design goals of the two buses: USB was designed for simplicity and low cost, while FireWire was designed for high performance, particularly in time-sensitive applications such as audio and video.

USB 2.0 Hi-Speed versus FireWire

The signaling rate of USB 2.0 Hi-Speed mode is 480 Mb/s, while the signaling rate of FireWire 400 (IEEE 1394a) is 393.216 Mb/s.^[9] USB requires more host processing power than FireWire due to the need for the host to provide the arbitration and scheduling of transactions. USB transfer rates are theoretically higher than FireWire due to the need for FireWire devices to arbitrate for bus access. A single FireWire device may achieve a transfer rate for FireWire 400 as high as 41 MB/s, while for USB 2.0 the rate can theoretically be 55 MB/s (for a single device). In a multi-device environment FireWire rapidly loses ground to USB: FireWire's mixed speed networks and long connection chains dramatically affect its performance.^{[citation needed]}

The peer-to-peer nature of FireWire requires devices to arbitrate, which means a FireWire bus must wait until a given signal has propagated to all devices on the bus. The more devices on the bus, the lower is its peak performance. Conversely, for USB the maximum timing model is fixed and is limited only by the host-device branch (not the entire network). Furthermore, the host-centric nature of USB allows the host to allocate more bandwidth to high priority devices instead of forcing them to compete for bandwidth as in FireWire.

Despite all this and despite USB's theoretically higher speed, in real life benchmarks the actual speed of FireWire hard drives nearly always beats USB 2.0 hard drives by a significant margin.^[10] In addition to this, some operating systems take a conservative approach to scheduling transactions and limit the number of transfers per frame, reducing the maximum transfers from, say, the theoretical 13 per frame to 10 or 9.

In 2002, FireWire was updated with the IEEE 1394b specification. This provides a new datarate called S800, which operates at 786.432 Mb/s, and a new arbitration scheme which scales better to higher data rates. S800 requires a new physical layer, but S800 nodes can be connected to existing FireWire 1394a ports, just as USB high-speed nodes will operate with older full-speed hosts. However, unlike USB Hi-Speed systems, which can change the speeds on each branch, data are sent to or received from a 1394a device at this device's speed (or less if there are even slower nodes in between) and using legacy arbitration. IEEE 1394b also provides rates up to approximately 3.2 Gb/s; however, the higher rates use special physical layers which are incompatible with 1394a devices.

Version history

USB

• USB 0.7: Released in November 1994.
• USB 0.8: Released in December 1994.
• USB 0.9: Released in April 1995.
• USB 0.99: Released in August 1995.
• USB 1.0 Release Candidate: Released in November 1995.
• USB 1.0: Released in January 1996.
  Specified data rates of 1.5 Mbps (Low-Speed) and 12 Mbps (Full-Speed). Did not anticipate or pass-through monitors. Few such devices actually made it to market.
• USB 1.1: Released in September 1998.
  Fixed problems identified in 1.0, mostly relating to hubs. Earliest revision to be widely adopted.
• USB 2.0: Released in April 2000.
  Added higher maximum speed of 480 Mbps (now called Hi-Speed). Current revision. Allowed Low-Speed and Full-Speed to be designated as 2.0 compliant (a major confusion factor to the market).
• USB 2.0: Revised in December 2002.
  Added specification clarifications.

USB On-The-Go Supplement

• USB On-The-Go Supplement 1.0: Released in December 2001.
• USB On-The-Go Supplement 1.0a: Released in June 2003.
• USB On-The-Go Supplement 1.2: Released in April 2006. This is the current revision.

Extensions to USB

The PictBridge standard allows for interconnecting consumer imaging devices. It typically uses USB as the underlying communication layer.

Microsoft's original Xbox game console uses standard USB 1.1 signaling in its controllers, but features a proprietary connector rather than the standard USB connector. With the introduction of the newer Xbox 360 model, Microsoft switched to the standard USB 2.0 connector. Similarly, IBM UltraPort uses standard USB signaling, but via a proprietary connection format. Powered USB uses standard USB signaling with the addition of extra power lines for point-of-sale terminals.

The USB Implementers Forum is working on a wireless networking standard based on the USB protocol. Wireless USB is intended as a cable-replacement technology, and will use Ultra-Wideband wireless technology for data rates of up to 480 Mbit/s. Wireless USB is well suited to wireless connection of PC centric devices, just as Bluetooth is now widely used for mobile phone centric personal networks (at much lower data rates). See http://www.usb.org/developers/wusb/ for more details.

References

See also

• U3
• USB Flash Drive
• USB Hub
• USB Implementers Forum
• USB streaming
• CEA-936-A
• ACCESS.bus
• FireWire
• PCMCIA
• Peripheral Component Interconnect (PCI)
• Serial cable (for RS-232)
• Wi-Fi
• List of device bandwidths

External links

• Home of USB Implementers Forum, Inc., including the USB 2.0 specification
• USB FOR DOS
• Universal Host Controller Interface (UHCI)
• What is USB?PDF - Short, simple description of USB with good pictures of cable breakdown and plugs
• Challenges of Migrating to Wireless USB - Article showing the differences between USB and Wireless USB from a technical point of view
• A Mac USB 2.0 vs. FireWire comparison Speed comparison using Apple drivers for USB 2.0
• Focus on Universal Serial Bus hardware design

This article is part of a series on computer expansion buses.