The Physical Layer defines how adjacent processing elements electrically connect to each other. The physical layer specifications describe the electrical requirements for connecting endpoints and switches together.
The Parallel Physical specification was deprecated in 10/2013 and is no longer used.
The Serial Physical specification outlines the requirement for devices utilizing an electrical serial connection medium. The Serial RapidIO (SRIO) specification defines a full duplex Serial Physical Layer interface (link) between devices. The links are comprised of one or more lanes, each lane being a pair of unidirectional serial signaling paths with one path in each direction. Further, it allows ganging of up to sixteen serial lanes for applications requiring higher link performance. It also defines a protocol for link management and packet transport over a link.