Transforming an I/O Operation to FC frames

A read or write I/O operation (Figure 2-28) between an initiator and a target undergoes a series of transformations before being transmitted on a Fibre Channel link. 启动程序和目标程序之间的读取或写入 I/O 操作（图 2-28）在光纤通道链路上传输之前要经过一系列转换。

Figure 2-28 An I/O operation in Fibre Channel

An I/O operation is mapped to a Fibre Channel Exchange. The initiator assigns and identifies an exchange using a unique OX_ID. Likewise, the target assigns and identifies the exchange using a unique RX_ID. Upper levels handoff data to the FC-2 level in the form of information units (IU). At FC-2 level, an IU is mapped to a sequence that is identified by a unique Sequence ID (SEQ_ID). A sequence, based on its size, is divided into one or more Data Fields that are identified by a unique Sequence Count (SEQ_CNT). A Data Field can carry up to 2112 bytes which is the maximum payload size of a single Fibre Channel frame (Figure 2-25). I/O 操作映射到光纤通道交换。启动程序使用唯一的 OX_ID 分配和识别交换。同样，目标也使用唯一的 RX_ID 分配和识别交换。上层以信息单元（IU）的形式向 FC-2 层传送数据。在 FC-2 层，一个 IU 映射到一个序列，该序列由一个唯一的序列 ID (SEQ_ID) 标识。序列根据其大小分为一个或多个数据字段，这些数据字段由唯一的序列计数 (SEQ_CNT) 标识。一个数据字段最多可包含 2112 字节，这是单个光纤通道帧的最大有效载荷大小（图 2-25）。

Each Data Field is encapsulated within an optional extended header, mandatory header, and CRC to make a Fibre Channel frame. Then, the SOF and EOF delimiters are added to clearly define the frame boundaries. Further, inter-frame fill words are added in the form of primitive signals. Finally, a stream of bits is ready to be transmitted on the wire. 每个数据字段都封装在可选的扩展报头、必选报头和 CRC 中，组成一个光纤通道帧。然后，添加 SOF 和 EOF 定界符，以明确界定帧的边界。然后，以原始信号的形式添加帧间填充字。最后，比特流就可以在线路上传输了。

Encoding the Frames and Special Func