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Appendix A A-7
Fibre Channel Overview
Working With Fibre Channel
Fibre Channel
Overview
The Fibre Channel protocol will work when devices are connected or
disconnected while data is being transferred because of this additional
information. Error detection and recovery at all levels of the protocol are
also provided.
A fairness algorithm is built into the FCAL protocol so that all devices
have equal access to the system. To keep the phase lock loops
synchronized, Fibre Channel continually communicates, even between
frames, supporting device arbitration for access to the loop. Fibre
Channel devices also use this communication to report on buffer size
available for communication. If there is nothing to report, the Fibre
Channel protocol fills up the space between frames with idle characters.
Characters must be present on the bus at all times to keep the high
frequency circuits working correctly, reconstructing data clocks and
detecting data on the bus. Even the data encoding is arranged so that
there is never a continuous string of ones or zeros.
The lower levels of Fibre Channel protocol is handled in the hardware
and, to a minor extent, by the low level HBA drivers. System and
application software does not need to be aware of any low level protocol
operations. Additional features exist in the protocol that more advanced
applications can use.
Fibre Channel addressing introduces three major advantages:
• Provides large number of addresses
• Detects address conflicts
• Automatically reassigns new addresses when conflicts occur
With Fibre Channel, the data network can be distributed and very large.
The ANSI Fibre Channel committee developed a method that devices
must use to check and report addresses before data can be sent or
received. ANSI also added the ability to semi-dynamically assign address
devices on the loop. Addresses are only checked and conflicts resolved
when a Fibre Channel device is added or removed. During normal
operations (i.e. no devices being connected or disconnected), device
addresses are not changed. When a Fibre Channel bus configuration
changes, each device on the bus verifies and reports its address.