Fibre Channel Primer

40 Fibre Channel: Connection to the Future
Class 2: Acknowledged Connectionless Service
Class 2 is a connectionless service, independently switching each
frame and providing guaranteed delivery with an acknowledgment
of receipt. The path between two interconnected devices is not
dedicated. The switch multiplexes traffic from N_Ports and NL_Ports
without dedicating a path through the switch.
Class 2 credit-based flow control eliminates congestion that is
found in many connectionless networks. If the destination port is
congested, a “Busy” is sent to the originating N_Port. The N_Port
will then re-send the message. This way, no data is arbitrarily discarded
just because the switch is busy at the time.
The very low frame latency in Class 2 makes it ideal for shorter
data transfers like those in most business applications.
Figure 6.3 Class 2 service provides low-latency guaranteed delivery
Class 3: Unacknowledged Connectionless Service
Class 3 is a connectionless service, similar to Class 2, but no
confirmation of receipt is given. This unacknowledged transfer is used
for multicasts and broadcasts on networks and for storage interface
on Fibre Channel loops. The loop establishes a logical point-to-point
connection and reliably moves data to and from storage.
Class 3 Arbitrated Loop transfers are also used for IP networks.
Some applications use logical point-to-point connections without
using a network layer protocol, taking advantage of Fibre Channel’s
reliable data delivery.
Storage
Backup
Server
Switch
Workstations
JBOD
Hub