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disparity is evaluated after the first 6 bits of each transmission char-
acter and after the last 4 bits. It can be positive (more 1s than 0s) or
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Fibre Channel
Fibre Channel
111
negative (more 0s than 1s). It is desirable to try to equalize the num-
ber of 1s and 0s over time. Every byte to be transmitted is encoded
into one of two 10-bit representations depending on current running
disparities.
Every byte to be transmitted is first converted into a 10-bit trans-
mission character. There are many more possible 10-bit transmission
characters than are needed to map to particular bytes. Most remaining
10-bit encodings are not defined, and only one is used. This is the spe-
cial K28.5 transmission character that contains the comma
a 7-bit string that cannot occur in any data transmission character.The
K28.5 transmission character is used as a special control character.
Transmission Word
All information in Fibre Channel is transmitted in groups of four
transmission characters called transmission words. Some trans-
mission words have K28.5 as the first transmission character. These
are special transmission characters called ordered sets. Some
ordered sets mark the beginnings and ends of frames (frame delim-
iters). Others convey information in between frames in the form of
primitive signals (a single ordered set) and primitive sequences (a
stream of the same ordered set). Examples of ordered sets are SOF,
EOF, idle, receiver ready (R_RDY), LIP, ARB, OPN, CLS, and several
others.
Frame
Fibre Channel defines a variable-length frame consisting of 36 bytes
of overhead and up to 2,112 bytes of payload for a total maximum
size of 2,148 bytes. The total size of the frame must be an even mul-
tiple of 4 bytes so that partial transmission words are not sent.
Between 0 and 3 pad bytes are appended to the end of the payload to
satisfy this rule. A SOF delimiter and EOF delimiter mark the
beginning and end of each Fibre Channel frame. The CRC is the
same 32-bit CRC used in FDDI.
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Fibre Channel
Chapter 5
112
Sequence
A Fibre Channel sequence is a series of one or more related frames
transmitted unidirectionally from one port to another. All frames
must be part of a sequence. Frames within the same sequence have
the same SEQ_ID field in the header. The SEQ_CNT field identifies
individual frames within a sequence. For each frame transmitted in
a sequence, SEQ_CNT is incremented by 1. This provides a means
for the recipient to arrange the frames in the order in which they
were transmitted and to verify that all expected frames have been
received. Multiple sequences to multiple ports may be active at
a time.
Exchange
A Fibre Channel exchange is a series of one or more nonconcurrent
sequences between two ports. The sequences may be in either direc-
tion. All sequences (and therefore all frames) must be part of an
exchange. The originator of the exchange assigns the OX_ID field.
The responder assigns the RX_ID field. As another perspective, one
can use the following analogy:
frame word
sequence sentence
exchange conversation
Of course, one main difference is that a Fibre Channel device can
speak more than one sentence and hold more than one conversation
at a time. Information technology (IT) managers, system integrators,
and Value Added Resellers (VARs) quickly discover that Fibre Chan-
nel is built on the concepts and protocols they know well. Fibre
Channel delivers the same types of functions, only faster, easier,
more scalable, and much more reliable than SCSI and legacy
networks.
Fibre Channel systems expand the flexibility of IT organizations
with their inherent ability to run SCSI and IP protocols on the same
network. These networks bring new levels of capability and perfor-
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Fibre Channel
Fibre Channel
113
mance. Fibre Channel systems are built without restrictions. Virtu-
ally any topology that an IT organization requires is possible. The
basic building blocks are point-to-point dedicated bandwidth, loop-
shared bandwidth, and switched-scaled bandwidth. Switches and
hubs are stackable. Fibre Channel networks and storage are built
from products that are very familiar to IT professionals. Fibre Chan-
nel is what has made SANs a reality, and future developments on the
interface likely will bring more features and faster speeds.
Summary
Fibre Channel attempts to combine the benefits of channel and net-
work technologies. A channel is a closed, direct, structured, and pre-
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