1. Ordered Sets Ordered Sets Introduction
Fibre Channel (FC) defines a few simple control signals to perform frequently required control functions, such as marking the start and end of frames and maintaining link synchronization. These simple control signals are called Ordered Sets. This lesson describes the role of Ordered Sets in FC, and the purpose of each Ordered Set.
Importance
Ordered Sets are special FC data units that are used in critical control functions, including flow control. You need to be able to recognize the Ordered Sets and understand the function of each Ordered Set before you can learn about functions such as flow control. In addition, being able to quickly recognize Ordered Sets will allow you to effectively use protocol traces to identify and resolve problems on a SAN.

2. Lesson Objective
Upon completion of this lesson, you will be able to describe the role of Ordered Sets in Fibre Channel and recognize some of the most commonly used Ordered Sets.
Performance Objective
Upon completion of this lesson, you will be able to describe the role of Ordered Sets in FC and recognize some of the most commonly used Ordered Sets.
Enabling Objectives
Define an Ordered Set
State the purpose of the K28.5 character
Identify the three types of Ordered Sets
State the purpose of Frame Delimiters
State the purpose of Primitive Signals
State the purpose of Primitive Sequences

3. Outline What are Ordered Sets? The K28.5 Character
Types of Ordered Sets
What are Frame Delimiters?
What are Primitive Signals?
What are Primitive Sequences?
Prerequisites
All previous lessons in Curriculum Unit 2, Module 1.

4. What are Ordered Sets? An Ordered Set is a command signal that:
Is a single word
Can be sent outside of a frame
Is not addressable
What are Ordered Sets?
Objective
Define an Ordered Set
Introduction
This section defines the concept of an Ordered Set.
Definition
When a node transmits data to another node, FC packages that data into frames. Frames are used to transport both data and FC control signals. However, there are a few simple command signals that ports must use frequently to perform basic link synchronization and flow control functions. Wrapping an entire frame around each one of these command signals would waste bandwidth and increase latency.
In FC, the smallest unit of data is the word, which is a 4-byte unit of data. All of the data that flows across an FC network—frame headers, control signals, and even the application data that is being sent from node to node—consists of a series of 4-byte words. FC designates special words to be used as simple command signals. These signals are called Ordered Sets.
Facts
Ordered Sets do not have to be encapsulated in a frame. Note, however, 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.
Example
The preceding graphic shows an example of an Ordered Set, displayed in green, that occurs outside of a frame.
Frames
Ordered Set

5. The K28.5 Character Frames Ordered Set Byte Byte Data Word Ordered Set
Objective
State the purpose of the K28.5 character
Introduction
This section explains the relevance of the K28.5 character to Ordered Sets.
Fact
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.
An Ordered Set consists of a single transmission word that starts with the K28.5 special character. When a port receives the K28.5 character, it knows that the three bytes that follow are part of an Ordered Set. The second byte identifies the Ordered Set, and the third and fourth bytes identify subtypes of that Ordered Set.
The K28.5 character is also known as the “comma character.”
Byte
Byte
K28.5
Data Word
Ordered Set

6. Types of Ordered Sets Transmission Word Ordered Set Data Word
K28.5, Dxx.y, Dxx.y, Dxx.y
Data Word
Dxx.y, Dxx.y, Dxx.y, Dxx.y
Primitive Signal
Types of Ordered Sets
Objective
Identify the three types of Ordered Sets
Introduction
This section introduces the three types of Ordered Sets.
Facts
There are three types of Ordered Sets:
Frame Delimiters are used to mark the beginning and end of frames.
Primitive Signals are used to initiate, synchronize, and terminate communication sessions, and to maintain synchronization when no other information is being transmitted on the link. The two types of Primitive Signals are fill words and control signals.
Primitive Sequences are similar to Primitive Signals, but are transmitted repeatedly until a response is received. They are used for link and loop initialization.
Frame Delimiter
Fill Word
Control Signal
Primitive Sequence
Start-of-Frame
End-of-Frame
Idle
Arbitrate
Receiver Ready
Virtual Circuit Ready
Close
Open
Dynamic Half-Duplex
Mark
Synchronize
Non-Operational State
Offline State
Link Reset
Link Reset Response
Loop Initialization
Loop Port Bypass Loop Port Enable

7. What are Frame Delimiters?
Frame Delimiters mark the beginning and end of frames
SOF
Start of Frame
EOF
End of Frame
What are Frame Delimiters?
Objective
State the purpose of Frame Delimiters
Introduction
This section explains the type of Ordered Set called Frame Delimiters.
Definition
Frame Delimiters are Ordered Sets that are used to mark the beginning and the end of each frame.
Facts
There are two Frame Delimiters:
Start-of-Frame (SOF) is used to mark the beginning of a frame and also indicates whether this is the first frame in a sequence and which class of service is in operation.
End-of-Frame (EOF) is used to mark the end of a frame and also indicates whether this is the last frame in the sequence and which class of service is in operation. EOF has both negative disparity and positive disparity variants to ensure that every frame ends with negative running disparity.
Example
The preceding diagram shows an example of the SOF and EOF frame delimiters that mark the beginning and end of a frame. S O F E O F
Header
Payload
CRC
Frame

8. What are Frame Delimiters? (cont.)
SOFix
SOF Initiate Class x
SOFnx
SOF Normal Class x
EOFn
EOF Normal
EOFt
EOF Terminate
EOFa
EOF Abort
Facts
There are multiple subtypes of both SOF and EOF. These subtypes are used to convey information about the status of the frame. For example:
SOFix (SOF Initiate) precedes the first frame of a sequence; x is a number that indicates the Class of Service.
SOFnx (SOF Normal) precedes frames other than the first frame of a sequence; x is a number that indicates the Class of Service.
EOFn (EOF Normal) concludes a frame that is not the last frame of a sequence.
EOFt (EOF Terminate) concludes a frame that is the last frame of a sequence.
EOFa (EOF Abort) concludes a frame that was aborted abnormally during transmission.
EOFt
SOFn3
EOFn
SOFn3
EOFn
SOFi3
Class 3 Sequence

9. Frame Delimiters (cont.)
Example
The image illustrates a captured analyzer trace that displays a single Class 2 frame and its SOF and EOF Frame Delimiters.
Note to Developer
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10. What are Primitive Signals?
Fill words maintain link synchronization
IDLE
Idle—sent between frames
ARB
Arbitrate for loop (FC-AL)
What are Primitive Signals?
Objective
State the purpose of Primitive Signals
Introduction
This section explains the type of Ordered Set called Primitive Signals.
Definition
Primitive Signals are Ordered Sets that are used to signal events at the transmitting port. Primitive Signals are used for flow control and clock synchronization. On arbitrated loops, Primitive Signals are also used for virtual circuit management.
Example
The preceding graphic illustrates an FC sequence consisting of a frame preceded and followed by IDLE fill words.
Facts: Fill Words
To maintain synchronization, the FC-1 layer generates continuous transmissions over each link. This minimizes latency and reduces the rate of errors on the link. These continuous transmissions consist of fill words, which are a type of Primitive Signal. There are two fill words:
The IDLE signal assists with clock synchronization and retiming. At least six idles are transmitted between frames, and additional idles may be added between frames to assist in synchronization and retiming. Idles are sent continuously when no frames are being transmitted to maintain synchronization.
The ARB (Arbitrate) signal is used only on arbitrated loops. ARB is part of the mechanism that loop ports use to decide which port has control over the loop at any particular time. S O F E O F
IDLE
IDLE
Header
Payload
CRC
IDLE
IDLE
Frame

11. What are Primitive Signals? (cont.)
Control signals are used in flow control
R_RDY
Receiver Ready
OPN
Open Port (FC-AL)
CLS
Close Port (FC-AL)
SYN
Clock Synchronize
VC_RDY
Virtual Circuit Ready
DHD
Dynamic Half-Duplex
Facts: Control Signals
In addition to fill words, there is another type of Primitive Signal called control signals. Control signals are simple commands that are sent between ports primarily to perform flow control functions.
The preceding graphic displays an example of the Primitive Signal R_RDY. The target port sends R_RDY to inform the initiator that the target port has a free buffer.
The control signals defined by the FC specification include:
R_RDY (Receiver Ready) is sent by a port when the port is ready to receive data. R_RDY is used in buffer-to-buffer flow control.
OPN (Open Port) is sent by an initiator on an arbitrated loop when the initiator wants to open a connection with a target port.
CLS (Close Port) is sent by an initiator on an arbitrated loop when the initiator wants to close a connection with a target port.
SYN (Clock Synchronization) is used to maintain clock synchronization in point-to-point and switched fabric configurations.
VC_RDY (Virtual Circuit Ready) is used for flow control in Class 4 virtual-circuit connections.
DHD (Dynamic Half-Duplex) is transmitted to change a full-duplex transfer to a half-duplex transfer between two L_Ports on an arbitrated loop. FC FC
Frame
R_RDY
Initiator
Target

12. Primitive Signals (cont.)
Example
The image displays a sample analyzer trace taken from an FC-AL. The trace shows examples of the Primitive Signals R_RDY, IDLE, CLS, ARB, and OPN. The lower window displays a detailed view of the highlighted item in the upper window—in this case, the R_RDY Ordered Set. Note that Byte 0 of the R_RDY ordered set is the K28.5 character.
Note to Developer
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13. What are Primitive Sequences?
Primitive Sequences are used for link control:
They are transmitted repeatedly until a response is received
Ports must receive three consecutive primitives before responding
What are Primitive Sequences?
Objective
State the purpose of Primitive Sequences
Introduction
This section explains the type of Ordered Set called Primitive Sequences.
Definition
Primitive Sequences are another type of Ordered Set. Primitive Sequences are simple link-level protocols that are used for link control.
The difference between Primitive Signals and Primitive Sequences is that Primitive Signals are transmitted once, while Primitive Sequences are transmitted repeatedly until a response is received. Ports are required to receive a minimum of three consecutive occurrences of the same Primitive Sequence before acknowledging its validity and responding accordingly.

14. What are Primitive Sequences? (cont.)
NOS
Non-Operational State
OLS
Offline State LR
Link Reset
LRR
Link Reset Response
LIP
Loop Initialization Primitive
LPB
Loop Port Bypass
LPE
Loop Port Enable
Used on fabrics
Used on loops
Example
The preceding graphic shows an example of the NOS-OLS Primitive Sequence exchanged between a host N_Port and a switch F_Port.
Facts
The Primitive Sequences are:
NOS (Non-Operational State) and OLS (Offline State). NOS is sent by a fabric port that is in an offline condition. The receiving port responds with OLS to begin the link initialization process.
LR (Link Reset) and LRR (Link Reset Response). LR is sent by a fabric port that wants to initiate a link reset operation; the receiving port responds with LRR.
LIP (Loop Initialization). LIP is sent by a loop port to begin the loop initialization process.
LPB (Loop Port Bypass), and LPE (Loop Port Enable). When a port receives LPB, it places itself in bypass mode. While in bypass mode, the port retransmits all received transmission words but does not participate in loop operations or process data. LPE re-enables a bypassed port. FC
NOS
OLS
Switch
Host

15. Primitive Sequences (cont.)
Example
The image shows an analyzer trace that displays the initial bootstrap sequence between an N_Port and F_Port in a switch. The top of the trace displays the OLS, LR, LRR link initialization sequence during login. Note that the link initialization sequence is followed by a series of IDLEs to establish clock synchronization before the Fabric Login (FLOGI) command sequence begins.
Note to Developer
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16. Lesson Review Practice
Identify the Ordered Sets displayed in the screen image.
State the type of each Ordered Set and identify its function.
Note to Developer
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17. Summary
Ordered Sets are single-word control signals that are sent outside of frames
The K28.5 character is used to distinguish Ordered Sets from data words
The three types of Ordered Sets are:
Frame Delimiters: Mark the beginning and end of frames
Primitive Signals: Initiate, synchronize, and terminate communication sessions; maintain link synchronization
Primitive Sequences: Link control; transmitted repeatedly (at least 3 times) until a response is received
Summary: Ordered Sets
In this lesson, you learned how to describe the role of Ordered Sets in FC and recognize some of the most commonly used Ordered Sets, including Frame Delimiters, Primitive Signals, and Primitive Sequences.