The different A and B plugs prevent accidentally connecting two power sources. However, some of this directed topology is lost with the advent of multi-purpose USB connections (such as USB On-The-Go in smartphones, and USB-powered Wi-Fi routers), which require A-to-A, B-to-B, and sometimes Y/splitter cables.

Dan Froelich (20 May 2009). "Isochronous Protocol" (PDF). USB.org. Archived from the original (PDF) on 17 August 2014 . Retrieved 21 November 2014. USB Device Class Specifications". www.usb.org. Archived from the original on 13 August 2014 . Retrieved 4 May 2018.

USB 2.0 Specification Engineering Change Notice (ECN) #1: Mini-B connector" (PDF). 20 October 2000. Archived (PDF) from the original on 12 April 2015 . Retrieved 29 December 2014– via www.usb.org. Note that this describes the power distribution model of USB that existed before Power-Delivery (USB-PD). Before PD, USB provided up to 7.5W through Type-A and Type-B connectors, and up to 15W through USB-C. All pre-PD USB power is provided at 5V. Universal Serial Bus Device Class Specification for Device Firmware Upgrade, Version 1.0" (PDF). USB Implementers Forum. 13 May 1999. pp.7–8. Archived from the original (PDF) on 24 August 2014 . Retrieved 8 September 2014. a b c d e "USB4® Specification v2.0" (ZIP) (Version 2.0ed.). USB. 30 June 2023 . Retrieved 23 October 2023. a b c d "USB 3.1 Specification– Language Usage Guidelines from USB-IF" (PDF). Archived (PDF) from the original on 12 March 2016 – via www.usb.org.

Thunderbolt combines PCI Express and Mini DisplayPort into a new serial data interface. Original Thunderbolt implementations have two channels, each with a transfer speed of 10Gbit/s, resulting in an aggregate unidirectional bandwidth of 20Gbit/s. [115] The connectors the USB committee specifies support a number of USB's underlying goals, and reflect lessons learned from the many connectors the computer industry has used. The connector mounted on the host or device is called the receptacle, and the connector attached to the cable is called the plug. [1] The official USB specification documents also periodically define the term male to represent the plug, and female to represent the receptacle, though these uses are inconsistent with established definitions of connector gender. [2] USB2.0 provides for a maximum cable length of 5 meters (16ft 5in) for devices running at high speed (480Mbit/s). [88] The USB Device Working Group has laid out specifications for audio streaming, and specific standards have been developed and implemented for audio class uses, such as microphones, speakers, headsets, telephones, musical instruments, etc. The working group has published three versions of audio device specifications: [65] [66] USB Audio1.0, 2.0, and 3.0, referred to as "UAC" [67] or "ADC". [68]USB 3.0 Specification Now Available" (PDF) (Press release). San Jose, Calif. 17 November 2008. Archived from the original (PDF) on 31 March 2010 . Retrieved 22 June 2010– via usb.org. USB was designed to standardize the connection of peripherals to personal computers, both to exchange data with and to supply electric power. It has largely replaced interfaces such as serial ports and parallel ports and has become commonplace on a wide range of devices. Examples of peripherals that are connected via USB include computer keyboards and mice, video cameras, printers, portable media players, mobile (portable) digital telephones, disk drives, and network adapters. USB is competitive for low-cost and physically adjacent devices. However, Power over Ethernet and the MIDI plug standard have an advantage in high-end devices that may have long cables. USB can cause ground loop problems between equipment, because it connects ground references on both transceivers. By contrast, the MIDI plug standard and Ethernet have built-in isolation to 500 V or more. Hershenhoren, Razi; Reznik, Omer (October 2010). "USB2.0 Protocol Engine" (PDF). Archived from the original (PDF) on 4 August 2020 . Retrieved 30 January 2019.

The host controller directs traffic flow to devices, so no USB device can transfer any data on the bus without an explicit request from the host controller. In USB2.0, the host controller polls the bus for traffic, usually in a round-robin fashion. The throughput of each USB port is determined by the slower speed of either the USB port or the USB device connected to the port. rpms/dfu-util: USB Device Firmware Upgrade tool". fedoraproject.org. 14 May 2014. Archived from the original on 8 September 2014 . Retrieved 8 September 2014. The USB1.1 standard specifies that a standard cable can have a maximum length of 5 meters (16ft 5in) with devices operating at full speed (12Mbit/s), and a maximum length of 3 meters (9ft 10in) with devices operating at low speed (1.5Mbit/s). [86] [87] [88] USB 3.0 Promoter Group Announces USB 3.2 Update" (PDF) (Press release). Beaverton, Oregon, US. 25 July 2017. Archived (PDF) from the original on 21 September 2017 . Retrieved 27 July 2017– via www.usb.org. FireWire vs. USB2.0". USB Ware. Archived from the original on 16 March 2007 . Retrieved 19 March 2007.Use class information in the interface descriptors. This base class is defined to use in device descriptors to indicate that class information should be determined from the Interface Descriptors in the device. The D± signals used by low, full, and high speed are carried over a twisted pair (typically unshielded) to reduce noise and crosstalk. SuperSpeed uses separate transmit and receive differential pairs, which additionally require shielding (typically, shielded twisted pair but twinax is also mentioned by the specification). Thus, to support SuperSpeed data transmission, cables contain twice as many wires and are larger in diameter. [34]