USB is a communication protocol initiated by a PC (USB host) that transmits a device (for example, a set of USB speakers) and then responds. Each transmission is addressed to a specific device and addressed to a specific endpoint of the device. The IN transmission sends data to the PC. When the host initiates an IN transmission, the device must respond with the data the host needs. OUT transfers data to the device. When the host performs an OUT transfer, it sends packets that the device must capture. In the field of USB audio, IN transmission and OUT transmission can be used to transmit audio samples: one OUT transmission sends audio data from the PC to the speaker, and IN transmission is used to send audio data from the microphone to the PC.