3 – Planning
Performance
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3.3.1
Distance
Consider the physical distribution of devices and switches in the fabric. Choose
SFP transceivers that are compatible with the cable type, distance, Fibre Channel
revision level, and the device host bus adapter. Refer to Appendix A for more
information about cable types and transceivers.
Each Fibre Channel port is supported by a data buffer with a 16 credit capacity;
that is, 16 maximum sized frames. For fibre optic cables, this enables full
bandwidth over the following approximate distances:
26 kilometers at 1-Gbps (0.6 credits/Km)
13 kilometers at 2-Gbps (1.2 credits/Km)
6 kilometers at 4-Gbps (2.4 credits/km)
Longer distances can be spanned at full bandwidth on 1/2/4-Gbps ports by
extending credits to G_Ports, F_Ports, and E_Ports. Each port can donate 15
credits to a pool from which a recipient port can borrow. However, 1/2/4-Gbps
ports can borrow only from other 1/2/4-Gbps ports. 10-Gbps ports cannot borrow
or donate credits. The recipient port also loses a credit in the process. For
example, you can configure a 1/2/4-Gbps recipient port to borrow 15 credits from
one donor port for a total of 30 credits (15+15=30).
Regardless of how many credits are borrowed, extending credits requires a
minimum cable length that is dependent on transmission speed. Extending credits
over short cables can result in excessive port resets. Table 3-2 describes the
distances that are possible for a port with 30 credits and the minimum cable
lengths.
You can configure recipient and donor ports using the Set Config Port CLI
command.
Table 3-2. Extended Credit Distances and Cable Lengths
Transmission Speed Range for 30 Credits Minimum Cable Length
1-Gbps 50 Km (30÷0.6) 3 Km
2-Gbps 25 Km (30÷1.2) 1.5 Km
4-Gbps 12 Km (30÷2.4) 0.75 Km
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