This part of ISO/IEC 11518 specifies a physical-level, point-to-point, full-duplex, link interface for reliable, flow-controlled transmission of user data at 6 400 Mbit/s per direction, across distances of up to 1 km. A parallel copper cable interface for distances of up to 40 m is specified. Connections to a separate longer-distance optical interface are provided. Small fixed-size micropackets provide an efficient, low-latency structure for small transfers, and a component for large transfers. Specifications are included for: – automatic retransmission to correct flawed data; – the format of a small data transfer unit called a micropacket; – a message structure that includes routing information for network applications; – end-to-end, as well as link-to-link, checksums; – the timing requirements of the parallel signals; – a parallel interface using copper coaxial cable; – connections to a separate local optical interface; – a link-level protocol tuned for a maximum distance of 1 km.