3.5 Ordered Sets

To move data across the network, Fibre Channel uses a concise command syntax referred to as ordered sets. These transmission words are composed of four encoded bytes, or 40 bits, the first byte of which is the K28.5 special character. Ordered set recognition logic triggers on the K28.5 character, and the three following data bytes define the function provided by the ordered set.

Ordered sets fall into three general categories: start of frame, end of frame, and primitive signals. One subset is used to indicate the start and end of data frames and the class of service used to ship the frame. The start of frame (SOF) delimiter K28.5 D21.5 D22.2 D22.2, for example, is used in a frame header to initiate a connectionless level of service. Following the SOF ordered set, as many as 2,112 bytes of data would compose the body of the frame. An end of frame (EOF) ordered set for example, K28.5 D21.4 D21.6 D21.6 would indicate that the transmission of this frame was complete. Fibre Channel defines 11 different types of SOF ordered sets, and 8 EOF words. The specific SOF or EOF used informs the recipient as to what further action (such as an acknowledgment), if any, is required.

A group of ordered sets known as primitive signals is used to indicate actions or events on the transport. The IDLE primitive, for example, has the notation K28.5 D21.4 D21.5 D21.5 and is used as a fill word between data frames and during periods of inactivity. The Close primitive (CLS) is used in arbitrated loop configurations to indicate the end of a transaction. Only a single occurrence of a primitive signal will trigger a response. If a primitive signal is missed or is corrupted in transport (for example, by excessive jitter on a link), logic on the end nodes should time out on the transaction and recover via higher-level protocols.

Primitive sequences are a group of ordered sets used to indicate or initiate state changes on the transport. Unlike primitive signals, primitive sequences require that at least three consecutive words be recognized before any action is taken. A Fibre Channel disk drive, for example, would issue a sequence of loop initialization primitives (LIPs) when first inserted into an arbitrated loop topology. Downstream nodes would not respond to the primitive sequence until three consecutive LIPs had been received.

Although most frame delimiting ordered sets are common to all Fibre Channel topologies, supersets of primitive signals and primitive sequences have been defined specifically for arbitrated loop and Fibre Channel fabric operation. A shared topology such as arbitrated loop requires special commands for gaining exclusive control of the transport. A dedicated connection to a fabric, on the other hand, assumes that the transport is always available and so requires no negotiation for access. The ordered sets unique to each topology are discussed in Chapter 4.