Fibre Channel Primer

46 Fibre Channel: Connection to the Future
with a second node or switch port. Only one pair may communicate
on the loop at one time. Any class of service may be used; however,
most loop applications use Class 3. When control is released, another
arbitration takes place. Fairness is provided by guaranteeing equal
access to all ports.
The loop is self-configuring and may operate with or without a
switch present. A loop node or switch port self-discovers its
environment and works properly (without manual intervention) with
other nodes and switches in the Fibre Channel system.
An isolated loop not connected to a switch is called a Private
Loop. If the loop is linked into a Fibre Channel switch via a port
called an FL_Port, the loop is called a Public Loop.
Nodes automatically relay frames unless that node has
established a temporary connection with another node. Loop nodes
monitor the inbound line for primitive signals addressed to that node.
After a primitive signal is received directing a temporary point-to-
point connection with another node, all frames received are intended
for that node and are directed internal to the node. This mechanism
provides a very simple and effective means of reliable loop
communications.
Hubs
Loops can be wired node to node, but if a node fails or is not
powered on, the loop is out of operation. This is overcome with the
use of a hub. A hub uses a port-bypass circuit (PBC) to detect whether
an active node is present. The PBC either opens the loop to insert the
active node or closes the loop, ensuring the loop is operational. Hubs
provide the ability to hot-plug nodes in and out of the loop.
Hubs provide a physical star wiring environment similar to the
concept of structured wiring for LANs. Hubs can be stacked to build
loops of up to 127 nodes.
Figure 6.9 Fibre Channel hubs provide reliable loop connectivity
Hub
Copper or
optical links.
Stacked up
to 127 ports.