USB defines a single connector type for attaching all USB peripherals to the host system. This chapter introduces the physical aspects of USB connectors and cables. The chapter also discusses the role of host software in detecting and reporting power related problems. USB employs NRZI encoding and differential signaling to transfer information across USB cables. These transfer types and the process used to initiate and perform them are described in this chapter.
Every transfer broadcast over the USB consists of a combination of packets.
These packets are combined to define individual transactions that are performed as part of a larger transfer. Each transaction type is defined, along with the individual packets that comprise them. CRC and other error checking is performed to verify data delivery and if errors occur retries of the failed transmission are performed. This chapter discusses the various sources of errors and the error detection mechanisms used by USB to identify them, and the error recovery that is performed to overcome them. USB devices support power conservation by entering a suspended state.