As shown in Figures 7.2 and 7.3, Frames can be classified into Data Frames, including Device Data, Basic and Extended Link Data, Video Data, and FC-4 Link Data, and into Link Control Frames, including ACKs, the BSY and RJT Link Responses, and the Link Credit Reset (LCR) Link Command. The rules of transmission of Data Frames and corresponding response Frames (Link Control Frames) are equally applicable to each type of Data Frame.

Data Frames contain Payloads of between 0 and 2,112 bytes, with data, control, header, or status information from an ULP or FC-4 level. Each Frame is transmitted as a unit from a source N_Port, through a Fabric or Arbitrated Loop, to the destination N_Port indicated in the Frame Header.

Link Control Frames are returned to the source N_Port for Frames transmitted in Class 1 and Class 2 Sequences. Link Control Frames are divided into three types. Acknowledgments, or ACKs, have normal Frame Headers, with no Payload. ACKs acknowledge successful receipt and storage of Data Frames at the destination N_Port. There are three types of ACKs. An ACK_1 acknowledges one Data Frame, an ACK_N acknowledges N Data Frames, and an ACK_0 can be used to acknowledge all the Data Frames of a Sequence. The three ACK forms provide increasingly efficient acknowledgment traffic, with correspondingly less control over Data Frame flow control. Each ACK Frame contains Sequence Qualifier Header field values that are identical to those in the Data Frames being acknowledged, and the...