Fibre Channel Primer

30 Fibre Channel: Connection to the Future
3. Sequence. Fibre Channel places no limits on the size of
transfers between applications (in LANs, the software is sensitive to
the maximum frame or packet size that can be transmitted). Frame
sizes are transparent to software using Fibre Channel because an
upper-layer protocol data structure, called a Sequence, is the unit of
transfer (see Chapter 5 for a discussion of the upper layers). A
Sequence is composed of one or more related frames for a single
operation, flowing in the same direction on the link. It is the FC-2
layer’s responsibility to break a Sequence into the frame size that has
been negotiated between the communicating ports and between the
ports and the fabric. For example, a unit of data generated by an
upper-layer protocol is disassembled into a single Sequence of one
or more frames. The Sequence is also the recovery boundary in Fibre
Channel. When an error is detected, Fibre Channel identifies the
Sequence in error and allows that Sequence (and subsequent
Figure 4.5 Fibre Channel Frame/Sequence/Exchange structure
64 Bytes
Optional
Header
4 Bytes
Start
of
Frame
4 Bytes
End
of
Frame
4 Bytes
CRC
Error
Check
24 Bytes
Header
2112 Byte Payload
2048 Byte Payload
Frame
Frame
Frame
Frame
S
E
Q
U
E
N
C
E
Frame
Frame
Frame
Frame
S
E
Q
U
E
N
C
E
E
X
C
H
A
N
G
E