The Evolution of the Universal Serial Bus

December 8, 2004
Portability is becoming a driving force in both consumer and professional media systems. The implementation of videoconferencing, videophones, Webcams and transportable hard-disk devices has given rise to widespread adoption of the Universal Serial Bus (USB).

The first USB standard was introduced in 1995 by Intel, Compaq, Microsoft and several other IT computer companies. USB 1.x, an external bus standard, supports data transfer rates of 12 Mbps and is capable of supporting up to 127 peripheral devices. The early USB implementation has generated USB 1.0 and USB 1.1. With the exception of the newest USB revision, 2.0 OTG ("On-The-Go") support, USB is essentially asymmetric and supports the hot-plugging of all its peripherals. Peripheral devises may also rely on the power delivered through the bus (bus-power).

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The earlier specification, USB Version 1.1, supports speeds of 1.5 Mbps to 12 Mbps, and is suitable for low-speed devices, such as HIDs (mice and keyboards up through medium-speed devices, such as 10 Mbps adapters.

In 2001, a team from Compaq, HP, Intel, Lucent, Microsoft, NEC and Philips introduced USB Revision 2.0. It added a higher speed of 480 Mbps, retaining the legacy low-speed USB 1.1. USB 2.0 is suited for multiple large transfers for devices such as MP3 jukeboxes and Webcams-as USB video. While the design data rate for USB is 480 Mbps, the device's signal at 12 Mbps for highspeed and 1.5 Mbps for low-speed devices uses a 1.5 Mbps subchannel. Adapters are available that allow USB to interoperate with EIDE, SCSI-2 and Ethernet.


By definition, an "A-device" attaches to the A-end of the cable and is a downstream port; the "B-device" attaches to the B-end of the cable and is an upstream port. The Type A USB connector is recognized by its flat slot characteristic, generally found on the host end; the Type B connector is more square and generally found on the peripheral end.

USB 2.0 did not require that cables be changed; however, lower-quality cables that barely handle USB 1.1 should not be expected to handle USB 2.0. New "mini-B" cables can help eliminate the need for proprietary connectors on many small USB peripherals, yet some report problems with long USB cables, especially with adapters.

USB 2.0 hubs support high-speed devices, whereas the older USB 1.1 hubs did not. Connection of USB 2.0 peripherals to USB 1.1 hosts and hubs will work at 12 Mbps but not at 480 Mbps. The high-speed hubs all include transaction translator support that helps prevent full- and low-speed devices from wasting USB bandwidth.

To achieve high-speed performance, an updated host controller that supports USB 2.0 high speed through the Enhanced Host Controller Interface (EHCI) standard is required. The EHCI specification is a licensed contributor's agreement. It includes a description of the hardware/software interface between system software and the host controller hardware. The specification is intended for hardware component designers, system builders and software developers.

It addresses such issues as system power management, robust solutions to legacy USB 1.1 host controllers, minimization of hardware complexities and support for 32- and 64-bit addressing.

USB can be either of bus-power (Bdevices that draw power from the USB cable) or self-power. Power levels are characterized as either high or low. Highpower B-devices consume more than 100 mA operating current; the downstream ports of high-power A-devices can source 500 mA sustained and can provide at least 4.75 V on the bus. Low-power Bdevices consume 100 mA or less operating current; and a low-power A-device is a downstream port that is guaranteed to source only 100 mA sustained.

Self-powered hubs and all root ports must offer high-power downstream ports. The USB 2.0 On-The-Go specification allows devices to be either high or low power.


USB 2.0 OTG, a supplement to USB 2.0, supports point-to-point peer-style hookups for USB devices typically not used with a PC host, such as cell phones with direct-to-printer support, keyboard-to-PDA, digital cameras, MP3-to-MP3 players or other devices that rely on the new USB mini-connectors and support battery-friendly low-power operation. The supplement in OTG defines a new dual-role type of peripheral that can become a low-power USB host when you connect it to another peripheral.

Most OTG-capable peripherals will have special mini-AB sockets that accept either the mini-B peripheral jack at one end of a USB cable or the mini-A host jack at its other end.

The concept is that if you use the peripheral end, they start out as a peripheral; use the host end, they start out like a USB host. Devices take on roles, but because of the dual-role capability, and with an optional OTG Host Negotiation Protocol (HNP), the wrong device-which might have been initialized as the host-can be switched without forcing the cables to be switched around.


Disk interface technologies support concurrent requests to different devices, as does USB. These requests are split into a start phase that then can disconnect and later reconnect for a complete phase. This sort of time-sharing scheme allows the bus to be used for other purposes, such as for slower I/O completes, similar to when a disk drive is seeking and reading several sectors; or as in USB, for 1.5 Mbps mouse-motion events.

When a USB 2.0 host is talking to a USB 1.1 device, it uses a companion controller, which makes it behave as a USB 1.1 host.

In a USB 2.0 hub, a transaction translator is employed. Hosts talk to the translator at high speed, with the translator performing a full- or low-speed I/O to the USB 1.1 device. The translator buffers data in either direction and later returns the results to the host at high speed. Only a single translator is permitted in a hub, or one per port. However, with just one translator, a hub can support as much bandwidth as the USB 1.1 host permits.

This split transaction technology allows applications, such as full-speed Webcams and disk-drive interfaces, each using 10 Mbps of I/O, to operate at the same time.


IEEE-1394, also known as FireWire, is a bus technology with bandwidths of between 400 and 1,000 Mbps. IEEE-1394 can handle up to 63 units on the same bus and is also hot-swappable.

While one might think these two serial busses seem similar, IEEE-1394 and USB are intended to fulfill different market and cost requirements. USB is considerably less expensive than IEEE-1394, the latter having the potential to move more data in a given amount of time. IEEE-1394 has a more complex protocol and signaling rate and is best-suited high-bandwidth entertainment applications.


From a compatibility perspective, the USB Implementers Forum Inc. (USB-IF), established in 1995, supplies a certified logo for products that demonstrate significant compliance to the USB specification.

The USB-IF does not endorse specific products, but those sporting the logo are good choices to minimize the risk of compatibility problems.

USB is most appropriate computer peripherals such as mass storage, audio/video, scanners, printers and keyboards.

Still, with the proliferation of small portable disk drives for data exchange, USB interfaces may someday alter the complexion of those small form-factor imagecapturing and storage devices for both news and field acquisition.

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