Introduction

In any serial communications protocol, the only thing the receiver sees is a long stream of signals denoting transmitted bits. Some of these bits are included in transmitted data, while some are part of control information, such as is used for marking the ends of Frames or for notifying the receiver that the transmitter is shutting down. The protocol must specify a way of distinguishing between data and the various types of control information.

In Fibre Channel, these functions are carried out using Ordered Sets. These are four-character Transmission Words that all start with the K28.5 Special Character. K28.5, as described in the Transmission, Reception, and Error Detection section, on page 80, are distinguished by a series of 7 bits, termed a comma, which will not occur in any valid Data Transmission Character or combination of Transmission Characters. Besides maintaining byte and word synchronization, detection of the comma notifies the receiver that the transmitter is sending control information, rather than data information.

The three Transmission Characters following the K28.5 character in the 4-byte word indicate what control information is being sent. Currently, there are defined the following types of Ordered Sets:

• Frame delimiters mark the boundaries of Frames and describe Frame contents.

  • Start of Frame delimiters, including SOFc1, SOFi1, SOFn1, SOFi2, SOFn2, SOFi3, SOFn3, SOFf, SOFc4, SOFi4, and SOFn4 mark the start of various types of Frames.

  • End of Frame delimiters, including EOFn, EOFt, EOFdt