Brocade Fabric OS Administrators Guide (Supporting Fabric OS v7.3.0) - Manual (Page 103)

destination switch before resuming I/O. The primitive is dependent on whether you have R_RDYs
enabled on your switch using the portCfgISLMode command. When a device logs in to a fabric, it
typically requests from two to sixteen buffer credits from the switch, depending on the device type,
driver version, and configuration. This determines the maximum number of frames the port can transmit
before receiving an acknowledgement from the receiving device.
For more information on how to set the buffer-to-buffer credits on an extended link, refer to Buffer-to-
Buffer Credits and Credit Recovery on page 117.
Congestion versus over-subscription
Congestion occurs when a channel is bottlenecked and fully utilized. This kind of bottleneck is a
congestion bottleneck. You should be aware that "over-subscription" does not have the same meaning
as "congestion". Over-subscription refers only to the potential for congestion; an over-subscribed link
may go through a lifetime of normal operation and never be congested. The term over-subscription is
not to be used in place of congestion, which is the actual contention for bandwidth by devices through
an ISL.
Virtual channels
Virtual channels create multiple logical data paths across a single physical link or connection. They are
allocated their own network resources such as queues and buffer-to-buffer credits. Virtual channel
technology is the fundamental building block used to construct Adaptive Networking services. For more
information on Adaptive Networking services, refer to
Optimizing Fabric Behavior on page 375.
Virtual channels are divided into three priority groups. P1 is the highest priority, which is used for Class
F, F_RJT, and ACK traffic. P2 is the next highest priority, which is used for data frames. The data virtual
channels can be further prioritized to provide higher levels of Quality of Service. P3 is the lowest priority
and is used for broadcast and multicast traffic. This example is illustrated in
Figure 7 .
Quality of Service (QoS) is a licensed traffic shaping feature available in Fabric OS. QoS allows the
prioritization of data traffic based on the SID and DID of each frame. Through the use of QoS zones,
traffic can be divided into three priorities: high, medium, and low, as shown in
Figure 7 . The seven data
virtual channels (VC8 through VC14) are used to multiplex data frames based upon QoS zones when
congestion occurs. For more information on QoS zones, refer to
Optimizing Fabric Behavior on page
375.
Congestion versus over-subscription
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