USB is a host-centric bus where the host / terminal stem started all transactions. The first package / markers (tokens) produced by the host beginning to clarify whether the packets that follow it will be read or written and what is the purpose of the device and endpoint. Subsequent packets are data packets, followed by a report on whether the handshaking packet data or marker has been well received or the endpoint fails to receive the data well.
Each process consists of a transaction on USB:
* Package token / marker signal (Header that describes the data that follows)
* Options packet data (including the level of charge) and
* The status of the package (to acknowledge / notification of the results of the transaction and for error correction)