2.3 FC-1: The Transmission Protocol Level

The FC-1 level defines the transmission protocol including the 8B/10B encode/decode scheme, byte synchronization, and character-level error control. This protocol uses the 8B/10B encoding scheme that encodes 8-bit bytes into 10-bit transmission characters . The 8B/10B encoding was developed by IBM and was determined to be the best for the expected error rate of the system.

The 8B/10B code has outstanding line characteristics including long transmission distances and very good error-detection capability. The 8B/10B code finds errors that a parity check cannot detect. Parity does not find even numbers of bit errors, only odd numbers . But 8B/10B finds almost all errors. Fibre Channel also employs a Cyclic Redundancy Check (CRC) on transmitted data. This also assists with error detection.

To assist with transmission the 8B/10B code uses 12 special characters. However, we are only concerned with one, the 28.5 special character. At present, it is the only special character used by Fibre Channel in the 8B/10B code.

2.3.1 8B/10B Encoding

The format of the 8B/10B character is Ann.m , where:

• A is equal to "D" for data or "K" for a special character

• nn is the decimal value of the lower five bits of a byte (bits EDCBA)

• "." is a period

• m is the decimal value of the upper three bits of a byte (bits HGF)

Figure 2-9 shows the translation of the HEX number 45.

Figure 2-9. 8B/10B Encoding

The 8B/10B encoded bytes have a property know as "disparity," which can be positive, negative, or neutral. An 8B/10B byte has negative disparity if there are more binary ones in the byte than binary zeroes. Conversely, the byte has positive disparity if there are more binary zeroes than ones. Neutral disparity is when the number of binary ones equals the number of binary zeroes.

2.3.2 K28.5 Special Character Encoding

The K28.5 special character has the following components :

• K stands for special character

• 28 is the decimal value of bits EDCBA -- 11100

• "." is a period

• 5 is the decimal value of bits HGF -- 101

Figure 2-10 shows the translation of the HEX number BC.

Figure 2-10. 8B/10B Translation

2.3.3 FC-1 Transmission Word

A transmission word is composed of four transmission characters. Remember, the 8B/10B code encoded an 8-bit byte into a 10-bit character. Therefore, a transmission word is a 40-bit group of four 10B transmission characters. A transmission word can be one of two kinds:

• data ” the first transmission character is an encoded data byte

• ordered set ” the forth transmission character is the K28.5 special character

If the transmission word is data, each of the four transmission characters is an encoded data byte.

If the transmission word is an ordered set, the first byte is a K28.5 transmission character. The other three transmission characters are normal encoded data bytes. Ordered sets permit control functions to be imbedded in the bit stream. One simple use of ordered sets is to determine at a receiver where word boundaries are. If all transmission words are data transmission words, the receiver has only a 2.5% chance of getting it right (that is, one in 40 bits).

2.3.4 FC-1: Ordered Set

An ordered set is a transmission word beginning with a special character, as previously discussed, the K28.5 character. Because this special character is present, this transmission word has a special control function meaning. There are three possible meanings:

• Frame delimiter ” This defines what class of service is required. (Classes of service will be explained later in this chapter.) The frame contains a start of frame (SOF) and an end of frame (EOF) delimiter.

• Primitive signals ” There are two kinds of primitive signals:

  - The ordered set may be a primitive signal used for buffer-to-buffer flow control.

  - There is an ordered set for idle primitives. Idles are words that fill the space between frames . In Fibre Channel, the transmitter must continuously send something over the media. This helps preserve bit, byte, and word synchronization, and permits faster communication.

• Primitive sequences ” A set of three identical ordered sets for link control. These are used for notification of link failures and loss of synchronization.