ISO/IEC 9314的这一部分为光纤分布式数据接口（FDDI）规定了介质访问控制（MAC），即数据链路层（DLL）的中间子层。FDDI（ISO/IEC 9314）使用光纤或其他传输介质在信息处理系统、子系统和外围设备之间提供高带宽（100 Mbit/s）的通用互连。FDDI可以被配置为支持至少80Mbit/s(10Mbyte/s)的持续数据传输速率。FDDI为分布在许多公里范围内的许多节点提供连接。FDDI的某些默认参数值（例如定时器设置）是基于多达1 000个传输链路或多达200公里的总光纤路径长度（通常分别对应于500个节点和100公里的双光纤电缆）来计算的；然而，通过增加这些参数值，FDDI协议可以支持更大的网络。 如图1所示，ISO/IEC 9314由 a)物理层(PL)，其被分成两个子层: 1)物理介质相关(PMD)，其在FDDI网络中的节点之间提供数字基带点对点通信。PMD提供从节点到节点传输适当编码的数字比特流所需的所有服务。PMD定义并描述了光纤驱动器和接收器、介质相关代码要求、电缆、连接器、功率预算、光旁路规定以及物理硬件相关特性。它指定了符合FDDI附件的互连点。最初的PMD标准ISO/IEC 9314-3定义了多个-光纤模式。正在开发附加的PMD子层标准，用于连接到单模光纤和SONET。 2)物理层协议(PHY)，其提供PMD和数据链路层之间的连接。PHY与上游码位数据流建立时钟同步，并将该输入码位流解码为等效符号流以供更高层使用。PHY提供数据和控制指示符符号和码位之间的编码和解码、介质调节和初始化、输入和输出码位时钟的同步以及向或从更高层传输信息所需的八位组边界的描绘。要在介质上传输的信息由PHY使用组传输码进行编码。b)数据链路层(DLL)，其被分成两个或更多个子层: 1)可选的混合环控制(HRC)，其在共享的FDDI介质上提供分组和电路交换数据的复用。HRC包括两个内部组件，混合多路复用器(H-MUX)和等时MAC(I-MAC)。H-MUX保持同步的125 μ s周期结构并复用分组和电路交换数据流，I-MAC提供对电路交换信道的访问。 2)介质访问控制(MAC)，其提供对介质的公平和确定性访问、地址识别以及帧校验序列的生成和验证。它的主要功能是分组数据的传送，包括帧生成、重复和移除。MAC的定义包含在ISO/IEC 9314的这一部分中。3）可选的逻辑链路控制（LLC），它为MAC和网络层之间任何所需的分组数据适配服务提供公共协议。FDDI未指定LLC。 4)可选的电路交换复用器(CS-MUX)，它为I-MAC和网络层之间任何所需的电路数据适配服务提供公共协议。FDDI未指定CS-MUX。 c)站管理(SMT)，其在节点级提供必要的控制以管理在各个FDDI层中进行的过程，使得节点可以在环上协同工作。SMT提供诸如控制配置管理、故障隔离和恢复以及调度策略等服务。 本文包含的MAC定义被设计成尽可能独立于物理介质和操作速度。ISO/IEC 8802-5中采用的处理令牌环MAC操作的概念已经被修改以适应更高的FDDI速度，同时保留一组类似的服务和设施。 ISO/IEC 9314规定了确保符合FDDI实现之间的互操作性所必需的接口、功能和操作。ISO/IEC 9314的这一部分提供了功能描述。一致性实现可以采用不违反互操作性的任何设计技术。如果不使用混合模式操作的附加能力（如本文所定义），则符合ISO/IEC 9314这一部分的实现也应与符合ISO 9314-2的实现互操作。除了ISO/IEC 9314的这一部分之外，鼓励实施者参考ISO 9314-2。

This part of ISO/IEC 9314 specifies the Media Access Control (MAC), the middle sublayer of the Data Link Layer (DLL), for Fibre Distributed Data Interface (FDDI).
FDDI (ISO/IEC 9314) provides a high-bandwidth (100 Mbit/s), general-purpose interconnection among information processing systems, subsystems and peripheral equipment, using fibre optics or other transmission media. 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 many kilometres in extent. Certain default parameter values for FDDI (e.g. timer settings) are calculated on the basis of up to 1 000 transmission links or up to 200 km total fibre path length (typically corresponding to 500 nodes and 100 km of dual fibre cable, respectively); however, the FDDI protocols can support much larger networks by increasing these parameter values. As shown in figure 1, ISO/IEC 9314 consists of a) A Physical Layer (PL), which is divided into two sublayers:

1) A Physical Medium Dependent (PMD), which provides the digital baseband pointto-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 fibre-optic 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 initial PMD standard, ISO/IEC 9314-3, defines attachment to multi-mode fibre. Additional PMD sublayer standards are being developed for attachment to single-mode fibre
and SONET.

2) A Physical Layer Protocol (PHY), 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), 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), 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. The definition of MAC is contained in this part of ISO/IEC 9314.

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. The MAC definition contained herein is designed to be as independent as possible from both the physical medium and the speed of operation. Concepts employed in ISO/IEC 8802-5, dealing with Token Ring MAC operation have been modified to accommodate the higher FDDI speeds, while retaining a similar set of services and facilities. ISO/IEC 9314 specifies the interfaces, functions, and operations necessary to ensure interoperability between conforming FDDI implementations. This part of ISO/IEC 9314 provides a functional description. Conforming implementations may employ any design technique that does not violate interoperability. Implementations that conform to this part of ISO/IEC 9314 shall also be interoperable with implementations that conform to ISO 9314-2 if the additional capability of hybrid mode operation (as defined in this document) is not being used. Implementers are encouraged to consult ISO 9314-2 in addition to this part of ISO/IEC 9314.