This part of ISO/IEC 9314 specifies the requirements for the Fibre Distributed Data Interface (FDDI); token ring low-cost fibre physical layer medium dependent (LCF-PMD). FDDI provides a high-bandwidth (100 Mbit/s), general-purpose interconnection among computers and peripheral equipment using fibre optics as the primary transmission medium. FDDI can be configured to support a sustained data transfer rate of at least 80 Mbit/s (10 Mbyte/s). FDDI provides connectivity for many nodes distributed over distances of several kilometers in extent. Default values for FDDI are calculated on the basis of 1 000 physical links and a total fibre path length of 200 km (typically corresponding to 500 nodes and 100 km of dual fibre cable). FDDI consists of: a) a Physical Layer (PL), which is divided into two sublayers 1) A Physical Layer, Medium Dependent (PMD) sublayer (ISO/IEC 9314-3), with several alternative medium choices, which provides the digital baseband point-to-point communication between nodes in the FDDI network. The PMD provides all services necessary to transport a suitably coded digital bit stream from node to node. The PMD defines and characterizes the medium drivers and receivers, medium-dependent code requirements, cables, connectors, power budgets, optical bypass provisions, and physical-hardware-related characteristics. It specifies the point of interconnectability for conforming FDDI attachments. The original PMD standard (ISO/IEC 9314-3), called PMD, defines attachment to multimode fibre up to 2 km, while this LCF-PMD, optically interoperable with the original PMD, defines low-cost attachments to multi-mode fibre up to 500 m. Additional PMD sublayer standards are for attachment to single mode fibre (SMF-PMD), and twistedpair up to 100 m (TP-PMD); 2) A Physical Layer Protocol (PHY) sublayer (ISO/IEC 9314-1), and its enhancement, (PHY-2), which provides connection between the PMD and the Data Link Layer. PHY establishes clock synchronization with the upstream code-bit data stream and decodes this incoming code-bit stream into an equivalent symbol stream for use by the higher layers. PHY provides encoding and decoding between data and control indicator symbols and code bits, medium conditioning and initializing, the synchronization of incoming and outgoing code-bit clocks, and the delineation of octet boundaries as required for the transmission of information to or from higher layers. Information to be transmitted on the medium is encoded by the PHY using a group transmission code; b) a Data Link Layer (DLL), which is divided into two or more sublayers 1) An optional Hybrid Ring Control (HRC) (ISO/IEC 9314-5), which provides multiplexing of packet and circuit switched data on the shared FDDI medium. HRC comprises two internal components, a Hybrid Multiplexer (H-MUX) and an Isochronous MAC (I-MAC). H-MUX maintains a synchronous 125 μs cycle structure and multiplexes the packet and circuit switched data streams, and I-MAC provides access to circuit switched channels; 2) A Media Access Control (MAC) (ISO/IEC 9314-2), and its enhancement (MAC-2), which provides fair and deterministic access to the medium, address recognition, and generation and verification of frame check sequences. Its primary function is the delivery of packet data, including frame generation, repetition, and removal; 3) An optional Logical Link Control (LLC), which provides a common protocol for any required packet data adaptation services between MAC and the Network Layer. LLC is not specified by FDDI; 4) An optional Circuit Switching Multiplexer (CS-MUX), which provides a common protocol for any required circuit data adaptation services between I-MAC and the Network Layer. CS-MUX is not specified by FDDI; c) a Station Management (SMT), which provides the control necessary at the node level to manage the processes under way in the various FDDI layers such that a node may work cooperatively on a ring. SMT provides services such as control of configuration management, fault isolation and recovery, and scheduling policies. FDDI LCF-PMD is a supporting document to FDDI PHY and FDDI PHY-2 which should be read in conjunction with it. The FDDI SMT document should be read for information pertaining to supported FDDI node and network configurations. The original FDDI PMD should be read for issues relating to FDDI LCF-PMD to FDDI PMD optical interoperability. ISO/IEC 9314 specifies the interfaces, functions, and operations necessary to ensure interoperability between conforming FDDI implementations. This standard provides a functional description. Conforming implementations may employ any design technique that does not violate interoperability.