USB On-The-Go (USB OTG or just OTG) is a specification first used in late 2001 that allows USB devices, such as tablets or smartphones, to act as a host, allowing other USB devices, such as USB flash drives, digital cameras, mouse or keyboards, to be attached to them. Use of USB OTG allows those devices to switch back and forth between the roles of host and device. A mobile phone may read from removable media as the host device, but present itself as a USB Mass Storage Device when connected to a host computer. USB OTG introduces the concept of a device performing both Host and Peripheral roles whenever two USB devices are connected and one of them is a USB OTG device, they establish a communication link. The device controlling the link is called the Host, while the other is called the Peripheral. USB OTG defines two roles for devices: OTG A-device and OTG B-device, specifying which side supplies power to the link, and which initially is the host. The OTG A-device is a power supplier, and an OTG B-device is a power consumer. In the default link configuration, the A-device acts as a USB host with the B-device acting as a USB peripheral. The host and peripheral modes may be exchanged later by using Host Negotiation Protocol (HNP). The wiring for the ID pin defines the initial role of each device. Overview Standard USB uses a Master/Slave architecture; a host acts as the Host device for the entire bus, and a USB device acts as a Peripheral. If implementing standard USB, devices must assume one role or the other, with computers generally set up as hosts, while (for example) printers normally function as a Peripheral. In the absence of USB OTG, cell phones often implemented Peripheral functionality to allow easy transfer of data to and from computers. Such phones could not readily be connected to printers as they also implemented the peripheral role. USB OTG directly addresses this issue. When a device is plugged into the USB bus, the host device sets up communications with the device and handles service provisioning (the host's software enables or does the needed data-handling such as file managing or other desired kind of data communication or function). That allows the devices to be greatly simplified compared to the host; for example, a mouse contains very little logic and relies on the host