FC-2 defines three classes of service

• Class 1 - Connection oriented (virtual circuit) in which two Nodes must establish a connection before transmitting data to ensure maximum bandwidth. Used for sustained, high-bandwidth apps and guarantees correct ordering of frames. Connections must be torn down, as well.
• Class 2 - Acknowledged connectionless service used when ACKs are required but the size of data doesn't warrant the time required to setup a connection.
• Class 3 - connectionless datagram service
• Mixed mode is an option of Class 1 in which the QoS facilitator delivers Class 2 and 3 frames when sufficient bandwidth is available.

The frame is the basic unit of information interchange and is created using 4 byte words (MSB in byte 0 and LSB in byte 3).

Frame # of words Frame Component # of bytes 1 Start of Frame 4 6 Header 24 0 - 16 Optional Header 0 - 64 0 - 512 Payload 0 - 2048 1 CRC 4 1 End of Frame 4

Optional Header Word Byte 0 Byte 1 Byte 2 Byte 3 0 R_CTL D_ID 1 Reserved S_ID 2 TYPE F_CTRL 3 SEQ_ID DF_CTL SEQ_CNT 4 OX_ID RX_ID 5 PARAMETER

Notes:

• R_CTL = routing control flags (FT = Frame Type)
  • FT-0 link control: Ack, link_response, link_command
  • FT-1 data: FC-4 Device_Data, FC-4 Vide_Data, and Link_data (link services)
• D_ID = Destination ID - the WWN of the dest N_port
• S_ID = Source ID - the WWN of the source N_port
• TYPE of FC-4 upper layer protocol being used:
  • CP IPI-3 peer, master, or slave
  • IS8802
  • IPI-3 peer, master, or slave
  • HiPPI-FP
  • SCSI FCP
  • Fibre Channel Services
  • Futurebus
  • ATM
  • SBCCS channel or control unit
• F_CTRL - Frame control flags:
  • Abort sequence condition
  • Invalidate X_ID
  • Exchange context
  • Pause/continue sequence
  • Fill bytes
  • Sequence context
  • First sequence
  • Chained sequence
  • Last sequence
  • End sequence
  • New X_ID assigned
  • Sequence initiative
• SEQ_ID - the sequence ID contains an unsigned binary number identifying the sequence of which the frame is a part. the SEQ_ID is assigned by the sequence initiator and is unique for a specific D_ID and S_ID pair, while the sequence is open.
• DF_CTL - the data field control flags identify optional headers (network, association, or device) in the data field and frame header extension.
  • Network header consists of two 8-byte fields that are used for communication between and FC port and a non-FC network port, e.g. ethernet port
  • Association header consists of a 32-byte field that contains a process associator and an operation associator.
  • Device header is a 16-, 32-, or 64-byte field that is entirely defined by the desired FC-4 protocol mapping documents, SCSI, TCP/IP, or HiPPI, for example.
• SEQ_CNT - the sequence count contains an unsigned binary number identifying the order of frame transmission within a single sequence or multiple consecutive sequences for the same exchange. For example, 0 is the first frame, 9 is the tenth frame, and so on. The sequence count of each subsequent data frame is incremented by one.
• OX_ID - the originator exchange ID field identifies the exchange ID assigned by the originator of the exchange. This field is also used by the originator exchange status block to track the progress of a series of sequences that comprise an exchange.
• RX_ID - the responder exchange ID field is assigned by the responder to provide a unique, locally meaningful identifier at the responder for an exchange established by an originator and identified by an OX_ID.
• PARAMETER - this field is a relative offset for data or link control information.

There are two types of frames: data and Link control frames.

Data frames include

• Link data frames - used to abort sequences/exchanges, login, logout, read status and timeout values, determine basic accepts and rejects
• Device data frames - execute SCSI commands and transfer data
• Video data frames - video data

Link control frames include

• Acknowledge (ACK) frames
• Link response (busy and reject) frames
• Link command frames

Class 1 service is circuit switched and creates a dedicated circuit and bandwidth between two nodes on the fabric. When the connection is no longer needed, the circuit is torn down.

The upper layer protocols allow for connectionless communication to occur when resources are available. Mixed mode traffic is handled by sub-fabric processing.

When only a fraction of the bandwidth is requested, a virtual circuit is established. The remaining resources are available and managed by the QoS facilitator as a class 4 service.

Class 6 is multicast and reliable.

Fibre Channel Level 4 Protocol Mapping - SCSI

required Fibre Channel protocol (FCP) frames required for SCSI mapping:

• FCP command frame - normally sent as a single-frame sequence at the start of an exchange. The FCP_CMND contains all the necessary information to start the SCSI I/O process.
• Transfer ready frame - the FCP_XFER_RDY frame specifies the relative offset and length of the data sequence to follow.
• Response frame - the RCP_RSP frame defines the data structure containing the SCSI status byte, SCSI autosense data, and the FCP response code. This frame signals the completion of a SCSI command or task management function.
• Data frame - when FCP_DATA frames are transmitted across FC links they take on the following format:
  SOF
  HEADER: 01 dd dd dd   00 ss ss ss   08 xx 00 0x   id 00 cc cc   OX_ID RX_ID   00 00 00 00  
  PAYLOAD: 128 to 2048 bytes
  CRC   EOF

  ---

  FC-AL : Fibre Channel - Arbitrated Loop

  • supports up to 127 nodes - 127 NL_ports (normal loop) and 1 FL_port (fabric loop)
  • uses almost all the rules of normal FC-PH
  • can be used in combination with fabric - when used with a fabric, FC-AL is a public loop. It is mutually exclusive with point-to-point connections.
  • operates as a public or private loop
  • provides access fairness - FC-AL uses the honor system: a port relinquishes the loop and does not arbitrate again until all other arbitrating ports have had a turn on the loop.
  • requires external bypass circuitry
    • comes in the form of LRC (Loop Resiliency Circuit)
    • permits device removal and insertion without recabling
    • must be implemented in backplane environments3
  • is usually implemented in dual loops
  • uses a repeater function with each node - no buffering; each transmission word is received, examined, and repeated if necessary.
  • self-discovers connection to loop or fabric
  • typically implements only Class 3 transfers
  • requires arbitration
  • includes new primitive signals (8) and sequences (7)
  • allows all devices to share bus bandwidth