FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availability Planning Guide (AA-RS2DC-TE, June 2003)

Planning Considerations for Fibre Channel Topologies
68 SAN High Availability Planning Guide
Hubs are cascaded through H_Port-to-H_Port connections (one port per switch or
hub). Server S
1
communicates with device D
1
through a loop that includes
H_Ports on all three hubs and NL_Ports on the remaining 18 devices.
Figure 28: 20-Device private arbitrated loop
Although connection of additional devices to a loop does not impact switch
bandwidth (1.0625 Gbps), it does adversely impact overall loop performance
because part of the bandwidth is dedicated to overhead instead of information
transmission.
Loop performance is a complex function of several factors, including the:
■ Loop round-trip time — The time required for a frame to travel completely
around a loop is a function of the propagation delay associated with each
H_Port and NL_Port, and the time required to travel through the fiber-optic or
copper transmission medium. The addition of ports (through cascaded hubs),
devices, and cabling increases the round-trip time.
S1
Hub Hub
D1
H_Ports
H_Ports
Loop Switch
Loop Switch