Understanding USB Types and the Flexible USB System

A hub usually has four new ports but there could be many more. You plug the hub into your computer, and then you plug your devices (or other hubs) into the hub. By chaining hubs together, you can build many of the available USB ports on one computer.

The USB standard allows devices to draw their power from their USB connection. A high power device such as a printer or scanner will have its own power supply, 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 many self-powered devices (such as printers and scanners), you don’t need a powered hub; none of the devices that connect to the hub need extra power, so the computer can handle it.

If you have a lot of unpowered devices like 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 supply.

The USB Process

When the host comes up, it queries each device connected to the bus and assigns an address to each one. This process is counting; devices are also counted when they connect to the bus. The host also finds out from each device what kind of data transfer it wants to make:

• Put in: A device such as a mouse or keyboard, which is sending very little data, would select the interrupt mode.

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

• Isochronous: A streaming device (such as a speaker) uses the isosynchronous mode. Data flows between the device and the host in real time, and there is no error correction.

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

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

After 90 percent is exhausted, the host denies access to any other asynchronous device or interrupt. Control packets and packets for bulk transfers use any remaining bandwidth (at least 10 percent).

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

During a frame, isochronous and interrupt devices obtain a slot to ensure the bandwidth they require. Bulk and control transfers use whatever space is left. There is a lot of detail in the technical links at the end of the article if you want to learn more.

Features USB

The Universal Serial Bus has the following features:

• The computer acts as a host.

• Up to 127 devices can be connected to the host, either directly or via USB hubs.

• Individual USB cables can run up to 5 meters; with hubs, devices can be up to 30 meters (even six cables) away from the host.

• With USB 2.0, the bus has a maximum data rate of 480 megawatts 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 carry the data. The USB 3.0 standard adds four more wires for data transmission. While USB 2.0 can only send data in one direction at a time (downstream or upstream), USB 3.0 can transmit data in both directions at the same time.

• On the power wires, the computer can supply up to 500 milliamps of power at 5 volts. A USB 3.0 cable can provide up to 900 milliamps of power.

• 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 provide power to devices connected to the hub.

• USB devices are hot-swappable, meaning you can plug them into the bus and unplug them 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 a USB 3.0 port, but data and power will still transfer through the cable.

• The host computer can put many USB devices to sleep when the computer goes into power saving mode.

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

Subsequent Generations of USB Technology

USB 2.0

The USB version 2.0 standard came out in April 2000 and served as an upgrade to USB 1.1.

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

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

The deployment of USB 2.0 allowed PC industry leaders to continue developing PC peripherals to complement existing high-performance computers.

In addition to improving functionality and encouraging innovation, USB 2.0 increases the productivity of user applications and allows the user to run multiple computer applications at the same time or many high-performance peripherals at the same time.

USB 3.0

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

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

USB-C

USB-C technology, introduced as the new standard in 2014, revolutionized connectivity with its multi-functional and high-performance capabilities. USB-C offers a significant upgrade over USB 3.0, with data transfer speeds of up to 10 gigabits per second, more than double the speed of its predecessor.

This advancement supports the growing demand for fast data transfer, making it ideal for tasks such as ultra-high-definition video or transferring large data files quickly.

The reversible design of USB-C cables and the USB-C port simplifies connections and improves user convenience, while its support for various protocols – including power delivery – enables faster charging and better functionality over a wide range of devices, from smartphones to laptops and beyond.

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

Lots More Information

Sources

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

• All usb. msgstr “SuperSpeed ​​USB 3.0 FAQ.” 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 Lightweight Peak Technology Could Kill 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. msgstr “Host Control Interface (xHCI) specification for USB 3.0.” (January 31, 2011) http://www.intel.com/technology/usb/xhcispec.htm

• Kingsley-Hughes, Adrian. msgstr “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

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Original article: Understanding USB Types and the Flexible USB System

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