Fibre Channel Primer

36 Fibre Channel: Connection to the Future
Fibre Channel switches relieve each individual port of the
responsibility for station management. Each node only has to manage
a simple point-to-point connection between itself and the switch.
No complex routing algorithms are used. A node simply does
the equivalent of dialing a phone number by entering an identification
number for the destination node in the header preceding the payload
data. If it is an invalid number, the switch rejects it. If for some reason
the switch cannot process the data, it responds with a “Busy” signal
and the node tries again.
Think of a Fibre Channel switch as similar to the telephone
system. To reach another number, you simply pick up the phone and
dial. The telephone system takes responsibility for finding the number
dialed and making the connection. Fibre Channel delivers the same
low-cost, highly reliable, scalable connectivity and service.
Fibre Channel switches appear as single interconnecting entities
to the nodes. Switch ports are the access points for physically
connecting nodes. Communication between nodes and a switch or
node-to-node is performed according to the Fibre Channel standard.
During initialization, a node logs in, automatically discovers the type
of topology to which it is connected, and operates accordingly.
A switch accepts the responsibility for routing frames according
to the class of service selected. The only consideration needed is the
point-to-point attachment of each node to the switch. Distance
limitations are resolved by choosing the media and components
needed to meet the application’s requirements.
Loop and Hub Topologies. A Fibre Channel loop topology
connects up to 127 ports. Loop topologies provide resilient
interconnection through their use of port bypass circuits (PBCs). PBCs
are installed either into the backplane of a disk enclosure or an external
device called a hub. The PBC automatically detects the presence of a
node and inserts it into the loop. Similarly, it detects a node that has
failed or been powered off and removes it from the loop. This is
accomplished automatically without manual intervention.