Distance Extension Through Bb_Credit - McDATA StorageWorks 2/140 - Director Switch Planning Manual

Distance Extension
Through BB_Credit
• SAN routing requirements - If a single logical Fibre Channel
fabric (created through a stretched E_Port connection) is
unacceptable because of the potential for disruptive fabric
rebuilds, include one or more SAN routers in the extended-
distance link.
Token-based buffer-to-buffer flow control governs transmission of
data and link control frames in a Fibre Channel switched fabric. To
manage flow control, Fiber channel fabric ports (F_Ports) or node
ports (N_Ports) for the Intrepid 10000 Director are assigned a variable
number of BB_Credits. The credits are typically user-defined or set to
a default value. A frame cannot be transmitted through a fabric
unless accounted for by a buffer credit.
During fabric port login, two communicating Fibre Channel ports
exchange available BB_Credit information. When a data or link
control frame is transmitted, the BB_Credit count for the port is
decremented. When the transmitting port receives a corresponding
receive-ready (R_RDY) link control frame, the BB_Credit count for
the port is incremented.
Longwave laser transceivers and a sufficient allocation of BB_Credits
are required to support long-distance transmission of Fibre Channel
data frames (up to 35 km). Installation of repeaters or DWDM
equipment is required to support data transmission in excess of
35 km. In either case (repeated or unrepeated) sufficient BB_Credits
are required.
BB_Credits required to support a specific transmission distance are
also a function of data rate. The faster the data rate, the shorter the
distance supported. Approximate relationships between data rate,
BB_Credits, and transmission distance are:
• At 1.0625 gigabits per second (Gbps), one BB_Credit supports a
two km transmission distance (1:2 ratio).
• At 2.1250 Gbps, one BB_Credit supports a one km transmission
distance (1:1 ratio).
• At 10.2000 Gbps, six BB_Credits support a one km transmission
distance (6:1 ratio).
Implementing SAN Internetworking Solutions
Implementing SAN Internetworking Solutions
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