3.3 Hubs

Hewlett-Packard's implementation of FC-AL for a private loop includes a hub. FC-AL hubs connect devices to the loop. It is a simple way to connect participants in a private loop. There are two types of hubs:

• Passive, which only reacts to ports being inserted into or removed from a loop

• Active, which are able to do configuration changes dynamically, based on some controlling protocol.

Some hubs can sense or manage configuration changes in the loop, including:

- knowing when NL_Ports are added

- knowing when NL_Ports are removed

- knowing when address changes occur for an entire set of NL_Ports

- switching NL_Ports into or our of a loop

Hubs also provide port bypass circuits to "heal" a loop when a device is removed or fails This allows for less disruption in operations. Hubs help solve the problems of cabling devices and keeping track of which loop a device is on. With central cabling by way of the FC-AL hub, it is easy to add and remove devices from arbitrated loops .

There are some definite advantages to using Fibre Channel hub:

• Extremely fast solution for connecting peripherals and hosts (nodes)

• Up to 124 NL_Ports per loop

• Loop topology eliminates wiring clutter

• FC-AL hubs enhance subsystem availability

• FC-AL hubs provide port bypass circuits to permit "hot" repair