Chapter 2 - 18. Understanding Congestion in Fibre Channel Fabrics

Effect of Primitive Signals on Data Rate

Primitive signals, such as R_RDY, do not contribute to the data rate of an FC link, and hence, they do not increase link utilization. Primitive Signals, however, are accounted in the bit rate. As explained earlier, the difference between the bit rate and data rate leaves enough space for Primitive Signals to be transmitted without contributing to the link utilization. Let’s understand it in detail. R_RDY 等原始信号不影响 FC 链路的数据传输速率，因此不会提高链路利用率。然而，原始信号会计入比特率。如前所述，比特率和数据率之间的差额为原始信号的传输留出了足够的空间，而不会增加链路利用率。让我们来详细了解一下。

An FC port is ready to send frames when it reaches an active state. But it sends frames only when an upper-layer protocol hands off data to the Fibre Channel levels. If there is no data from the upper layers, the FC port continuously transmits fill words (4 bytes), for example, Idle, at a bit rate (Figure 2-29) defined by the port speed. These fill words maintain synchronization between the sender and the receiver and keep the link ready for frame transmission. FC 端口在达到激活状态时就可以发送帧。但只有当上层协议向光纤通道层发送数据时，FC 端口才会发送帧。如果上层没有数据，FC 端口会以端口速度定义的比特率（图 2-29）持续传输填充字（4 字节），例如空闲。这些填充字可保持发送方和接收方之间的同步，并使链路为帧传输做好准备。

Figure 2-29 Fill words on a Fibre Channel link with no traffic

When an upper layer hands off data to Fibre Channel levels and as a result the FC port has frames for transmission, it replaces multiple fill words with the frame. But fill words are never completely eliminated because they make the inter-frame fill (Figure 2-30). Only a frame, which carries the Data Field, adds to the data rate on a link. 当上层将数据移交给光纤通道层时，FC 端口因此有了要传输的帧，它会将多个填充字替换为帧。但填充字永远不会被完全消除，因为它们会造成帧间填充（图 2-30）。只有携带数据字段的帧才会增加链路上的数据传输速率。

Figure 2-30 Fill word primitive signals and frame