Q-Logic 5000 manual Extended Credit Distances and Cable Lengths

3 – Planning Performance

S

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 3-2describes the distances that are possible for a port with 30 credits and the minimum cable lengths.

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

You can configure recipient and donor ports using the Set Config Port CLI command.

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