Fibre Channel Data Constructs Ordered Sets and Primitives
Note that the four bytes in an Ordered Set do not provide space for addressing or routing information. Therefore, Ordered Sets that are sent outside of a frame can only be used for link management functions between two directly connected ports.
The 8b/10b encoding scheme provides 1024 transmission characters. The 8b/10b encoding scheme reserves some of these characters as special characters that differentiate control information from data. In FC, a special character called the K28.5 character is used to distinguish FC control information from data. The first character of an Ordered Set is always the K28.5 special character. The presence of the K28.5 character is what alerts the receiver to the fact that this word is an Ordered Set.
The K28.5 character is also known as the “comma character.”