Planet Technology Planet Intelligent Gigabit Ethernet Stackable/Routing Switch WGSW-2402A - Manual (Page 163)

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below full wire speed. Note that the rate control function does not shape or manipulate any particular
traffic class. Furthermore, though the average rate of the port can be controlled with this function, the
peak rate will still be full line rate. Two traffic types are allowed: Streaming and Bursting. Bursting
restrict its average rate in longer time period so that it have higher peak rate in short time. However,
the average rate of two traffic type is the same.
Rate control can not use with delay bound. Thus, all the ports should not use profile with delay
sensitive application enabled.
5.15.1.8 WRED Drop Threshold Management Support
To avoid congestion, the Weighted Random Early Detection (WRED) logic drops packets according to
specified parameters. The following table summarizes the behavior of the WRED logic.
T ABLE 5-5 WRED D ROPPING S CHEME
Gigabit Port Megabit Port High Drop Low Drop
Level 1 N ≥240 N ≥120 X% 0%
Level 2 N ≥280 N ≥140 Y% Z%
Level 3 N ≥320 N ≥160 100% 100%
In the table, N is a function of byte-count in queue Px. The WRED logic has three drop levels,
depending on the value of N, which is based on the number of kilobytes in the priority queues. If delay
bound scheduling is used, N equals P7*16+P6*16+P5*8+P4*4+P3*2+P2 on gigabit port and
P3*16+P2*4+P1 on megabit port. If using WFQ scheduling, N equals P7+P6+P5+P4+P3+P2 on gigabit
port and P3+P2+P1 on megabit port. Each drop level from one to three has defined high-drop and
low-drop percentages, which indicate the minimum and maximum percentages of the data that can be
discarded. The X, Y Z percent can be configured on WRED Drop Priority Setting of QoS Global Setting.
In Level 3, all packets are dropped if the bytes in each priority queue exceed the threshold.
Thus, if a queue is a delay-bounded queue, we have a multi-level WRED drop scheme, designed to
control delay and partition bandwidth in case of congestion. If a queue is a WFQ-scheduled queue, we
have a multi-level WRED drop scheme, designed to prevent congestion. In addition to these reasons
for dropping, the Switch also drops frames when global buffer space becomes scarce.
5.15.1.9 QoS Flow Control
Because frame loss is unacceptable for some applications, the Switch provides a flow control option.
When flow control is enabled, scarcity of buffer space in the Switch may trigger a flow control signal;
this signal tells a source port that is sending a packet to this switch, to temporarily hold off.