Understanding USB types and the flexible USB system

by

<b>A typical four-port USB hub accepts four “a” connections.</b>” data-src=”https://s.yimg.com/ny/api/res/1.2/Wfblp3d8zo6n2zc5uD78WQ–/YXBwaWQ9aGlnaGxhbmRlcjt3PTY0MDtoPTcwMg–/https://media.zenfs.com/en/howstuffworks_992/0bf2f964458a9 4946963c6a084e80a5f”/><b></div> </div> </div> <div class=
A typical four-port USB hub accepts four “A” connections.

A hub typically has four new ports, but can have many more. You connect the hub to your computer and then connect your devices (or other hubs) to the hub. By linking hubs together, you can build up dozens of available USB ports on one computer.

Thanks to the USB standard, devices can draw their power from their USB connection. A high-power device, such as a printer or scanner, gets its own power, but low-power devices, such as mice and digital cameras, get their power from the bus for simplicity. The power (up to 500 milliamps at 5 volts for USB 2.0 and 900 milliamps for USB 3.0) comes from the computer.

If you have a lot of powered devices (such as printers and scanners), you don’t need a powered hub; none of the devices connected to the hub require additional power, so the computer can handle this.

If you have a lot of unpowered devices, such as mice and cameras, you probably need a powered hub. The hub has its own transformer and supplies power to the bus so that the devices do not overload the computer’s power supply.

The USB process

When the host boots up, it polls all devices connected to the bus and assigns each device an address. This process is enumeration; devices are also enumerated when they connect to the bus. The host also discovers from each device what type of data transfer it wants to perform:

  • Interrupt: A device such as a mouse or a keyboard, which sends very little data, would choose interrupt mode.

  • Bulk: A device such as a printer, which receives data in one large packet, uses bulk transfer mode. The printer receives a block of data (in 64 byte blocks) and verifies it to ensure it is correct.

  • Isochronous: A streaming device (such as a speaker) uses isochronous mode. Data flows between the device and the host in real time, without error correction.

The host can also send commands or request parameters with control packets.

As the devices are enumerated, the host keeps track of the total bandwidth requested by all isochronous and interrupt devices. They can consume up to 90 percent of the available 480 Mbps bandwidth (USB 3.0 increases that speed to 4.8 gigabits per second).

After 90 percent is used, the host denies access to other isochronous or interrupt devices. Control packets and bulk transfer packets use the remaining bandwidth (at least 10 percent).

The Universal Serial Bus divides the available bandwidth into frames, and the host manages the frames. Frames contain 1,500 bytes and a new frame starts every millisecond.

During a frame, isochronous and interrupt devices are given a slot, ensuring they get the bandwidth they need. Bulk and control transfers use the remaining space. The technical links at the end of the article have lots of details if you want to learn more.

USB functions

The Universal Serial Bus has the following features:

  • The computer acts as a host.

  • Up to 127 devices can connect to the host, directly or via USB hubs.

  • Individual USB cables can be as long as 5 meters; Hubs allow devices to be up to 100 feet (six cables) away from the host.

  • With USB 2.0, the bus has a maximum data rate of 480 megabits per second (10 times the speed of USB 1.0).

  • A USB 2.0 cable has two wires for power (+5 volts and ground) and a twisted pair of wires to transfer the data. The USB 3.0 standard adds four additional wires for data transfer. While USB 2.0 can only transmit data in one direction at a time (downstream or upstream), USB 3.0 can transmit data in both directions simultaneously.

  • The computer can supply up to 500 milliamps of current at 5 volts on the power wires. A USB 3.0 cable can supply up to 900 milliamps of current.

  • Low-power devices (such as mice) can draw their power directly from the bus. High power devices (such as printers) have their own power supplies and draw minimal power from the bus. Hubs can have their own power supplies to power devices connected to the hub.

  • USB devices are hot-swappable, meaning you can connect and disconnect them from the bus at any time. A USB 3.0 cable is compatible with USB 2.0 ports. You won’t get the same data transfer speed as with a USB 3.0 port, but data and power will still be transferred over the cable.

  • Many USB devices can be put into sleep mode by the host computer when the computer enters a power saving mode.

The devices connected to a USB port rely on the cable to carry power and data.

Next generations of USB technology

USB 2.0

The standard for USB version 2.0 was released in April 2000 and served as an upgrade for USB 1.1.

USB 2.0 provided additional bandwidth for multimedia and storage applications and has a data transfer speed 40 times faster than USB 1.1. To allow for a smooth transition for both consumers and manufacturers, USB 2.0 included full forward and backward compatibility with original USB devices and works with cables and connectors made for original USB.

USB 2.0 supports three speed modes (1.5, 12, and 480 megabits per second) and supports low-bandwidth devices such as keyboards and mice, as well as high-bandwidth devices such as high-resolution webcams, scanners, printers, and storage systems with high capacity .

The deployment of USB 2.0 has allowed PC industry leaders to move forward with the development of PC peripherals to complement existing high-performance PCs.

In addition to improving functionality and encouraging innovation, USB 2.0 increases user application productivity and allows the user to run multiple PC applications simultaneously or multiple high-performance peripherals simultaneously.

USB 3.0

The USB 3.0 standard became official on November 17, 2008 [source: Everything USB]. USB 3.0 offers speeds 10 times faster than USB 2.0 at 4.8 gigabits per second.

It was intended for applications such as transferring high-definition video footage or backing up an entire hard drive to an external drive. As hard drive capacity grew, so did the need for a high-speed data transfer method.

USB-C

Introduced as the new standard in 2014, USB-C technology revolutionized connectivity with its versatile and powerful capabilities. USB-C offers a significant upgrade over USB 3.0, with data transfer speeds of up to 10 gigabits per second, more than doubling the speed of its predecessor.

These advancements support the growing demand for high-speed data transfer, making it ideal for tasks such as quickly transferring ultra-high-definition video or large data files.

The reversible design of USB-C cables and the USB-C port simplifies connections and increases ease of use, while the support of various protocols – including power delivery – enables faster charging and greater functionality on a wide range of devices, from smartphones to laptops and beyond.

We updated this article using AI technology, then made sure it was fact-checked and edited by a HowStuffWorks editor.

Much more information

Sources

  • Chan, Norman. “Everything you need to know about USB 3.0, plus photos of the first spliced ​​cables.” Maximum PC. August 18, 2008. (January 28, 2011) http://www.maximumpc.com/article/features/everything_you_need_know_about_usb_30_plus_first_spliced_cable_photos

  • Europe. “Harmonization of a charging option of a regular mobile phone charger – frequently asked questions.” June 29, 2009. (January 31, 2011) http://europa.eu/rapid/pressReleasesAction.do?reference=MEMO/09/301

  • Everything USB. “SuperSpeed ​​USB 3.0 Frequently Asked Questions.” February 1, 2011. (February 1, 2011) http://www.everythingusb.com/superspeed-usb.html

  • Faas, Ryan. “Will Light Peak Leave USB 3.0 in the Dust?” IT world. November 5, 2010. (January 31, 2011) http://www.itworld.com/hardware/126694/will-light-peak-leave-usb-30-dust

  • Ganapati, Paul. “Intel’s Light Peak Technology Could Destroy USB 3.0.” Wired. April 15, 2010. (January 28, 2011) http://www.wired.com/gadgetlab/2010/04/intels-light-peak-technology-could-kill-usb-30/

  • Intel. “Extensible Host Controller Interface (xHCI) Specification for USB 3.0.” (January 31, 2011) http://www.intel.com/technology/usb/xhcispec.htm

  • Kingsley-Hughes, Adrian. “Is Intel Delaying USB 3.0 to Give Light Peak an Advantage?” ZDNet. June 3, 2010. (January 31, 2011) http://www.zdnet.com/blog/hardware/is-intel-delaying-usb-30-to-give-light-peak-an-advantage/8493

  • Knupffer, Nick. “USB 3.0 for the Masses – Dispelling the Myths.” Intel. June 11, 2008. (January 31, 2011) http://blogs.intel.com/technology/2008/06/usb_30_for_the_masses_dispelli.php

  • Lilly, Paul. “Intel’s Light Peak Technology vs. USB 3.0.” Tested. April 15, 2010. (January 28, 2011) http://www.tested.com/news/intels-light-peak-technology-vs-usb-30/160/

  • Perenson, Melissa J. “USB 3.0 has finally arrived.” Computer world. January 10, 2010. (January 28, 2011) http://www.pcworld.com/article/186566/usb_30_finally_arrives.html

  • Universal Serial Bus. “USB 3.0 Specification.” (January 28, 2011) http://www.usb.org/developers/docs/

Original article: Understanding USB types and the flexible USB system

Copyright © 2024 HowStuffWorks, a division of InfoSpace Holdings, LLC, a System1 company

Leave a Comment