HP XP7 Storage - Manual (Page 40)

Cable length and switch requirements
Multimode or single-mode optical fiber cables are required on primary and secondary systems.
The type of cable and number of switches depends on the distance between primary and secondary
sites.
•
0 to 0.5 km: multimode optical shortwave fiber cables are required.
•
0.5 km to 1.5 km (1,640 to 4,920 feet): multimode shortwave fibre-channel interface cables
are required; one switch is required, two maximum.
•
1.5 km to 10 km: single optical long-wave fiber cables are required.
•
10 km to 30 km (6.2 to 18.6 miles): single-mode long-wave fibre-channel interface cables
are required; one switch is required, two maximum.
•
Greater than 30 km (18.6 miles): approved third-party channel extender products and
telecommunications lines are required.
This information is illustrated in Figure 8 (page 40) .
Additional switches
When the initiator port on the primary system sends data to the secondary system, the fibre-channel
protocol accommodates a certain number of un-acknowledged frames before the sender must stop
sending. These are known as buffer credits. As fibre-channel frames are sent out, available buffer
credits are exhausted. As acknowledgments come back, the supply of buffer credits is replenished.
Because it takes longer for acknowledgments to return as distance increases, exhausting the supply
of buffer credits becomes increasingly likely as distance increases.
Adding fibre-channel switches on either end of the replication network provides the additional
credits necessary to overcome buffer shortages due to the network latency.
Figure 8 Data path types, switches, and distances
40 Planning the data path