HP StorageWorks 2/140 FW 08.01.00 McDATA Products in a SAN Environment Plannin - Page 191
Distance Extension Through BB_Credit, SAN routing requirements
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Distance Extension Through BB_Credit Implementing SAN Internetworking Solutions 4 • 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 extendeddistance 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 4-49