Hardware Reference Manual
CP945FP-12 CompactPCI, RM945FP-12 VME, & RM948 With Embedded Layer 2
Switch, Front Input/Output
DDC No. Rx-URMH 122 Rev A
Issued 5 December 2003
9 of 18
The following are the transmission scheduling exceptions:
1. Packets with high priority and low priority drop will be transmitted before any in the low priority queue.
2. If the high priority delay is => 2ms, then transmit from the high priority queue.
3. If the high priority delay is < 1ms and the low priority queue is congested, then transmit from the low priority
queue.
4. If the high priority delay is < 2ms and the low priority queue is full, then transmit from the low priority queue.
The QoS capabilities of the fabric are enabled by loading the appropriate values into the configuration registers
and by either enabling or disabling the flow control. The values are loaded from SROM. To alter the QoS
characteristics, modify the SROM configuration.
When Flow Control and QoS are enabled, the fabric will utilize WRR to schedule packet transmission, and will
use either backpressure or 802.3x flow control to handle situations of buffer memory congestion. When Flow
Control is disabled and Qos is enabled, the fabric will use RED/RIO to drop random packets in order to handle
situations of buffer memory congestion. In this method, only certain packet flows are slowed down, while the
remaining see no impact from the network traffic congestion. RED/RIO is a method of handling traffic congestion
in the absence of Flow Control mechanisms. When Flow Control is enabled, all devices that are connected to a
switch node that is exercising flow control are effectively unable to transmit, even nodes that are not directly
responsible for the congestion problem. RED/RIO allows traffic to continue flowing into ports on a switch and
randomly drops packets with different probabilities based upon each packet’s priority markings. As the switch
congestion increases, the probability of dropping an input packet increases. As congestion decreases, the
probability of dropping an input packet decreases. In this manner, only traffic flows that have had packets
dropped will be affected by the congestion. Other traffic flows will see no effect.
2.1.3 Port Configuration
By setting parameters in the SROM, each port can be individually configured in speed (i.e., 10 or 100Mbs) and
duplex (i.e., full or half).
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