This part of ISO/IEC 9314 specifies a hybrid ring control (HRC) protocol which provides a mode of operation in which both packet switched and isochronous data are transmitted within the same special frame structure, called a cycle. HRC is designed to operate with the existing media access control (MAC), physical layer (PHY), and physical medium dependent (PMD) layers of the FDDI protocol. The HRC is composed of the hybrid multiplexer (H-MUX) and the isochronous media access control (IMAC) protocols. The H-MUX integrates packet and isochronous data into cycles which it transmits onto and receives from the medium using the services of the physical layer. The l-MAC provides separate transmission channels for the transfer of user isochronous data streams. The format, clocking and synchronization of cycles, and the operation and interfaces of the H-MUX and l-MAC are defined by this part of ISO/IEC 9314. These interfaces include the interface to the FDDI station management (SMT) protocol. The HRC is designed to support various transmission rates, from 100 Mbps upwards, in increments of 6,144 Mbps. All transmission rate dependent parameters defined in this part of ISO/IEC 9314 assume a transmission rate of 100 Mbps. Stations composed of FDDI and HRC entities are referred to as FDDI-II stations. The FDDI packet MAC (P-MAC) and the HRC components, and their architectural relationship to LLC and a circuit switching Multiplexer (CS-MUX) are illustrated in figure 1. This figure does not imply an implementation configuration FDDI-II networks consist of FDDI-II stations. Interoperability between FDDI and FDDI-II stations on the same network is provided in HRC basic mode, which only supports packet transmission. The set of FDDI standards, ISO/IEC 9314, specifies the interfaces, functions, and operations necessary to ensure interoperability between conforming FDDI implementations. This part of ISO/IEC 9314 specifies a hybrid ring control protocol: HRC. Conforming implementations may employ any design technique that does not violate interoperability.