ATL Telecom R1-SW Ethernet Switch - Manual (Page 179)

Configuring QoS
9-11
WFQ (Weight Fair Queuing)
WFQ method divides whole packet in queue into bit unit to solve the problem of WRR and
transmits the bits at weight ratio of queues, then reassembles the bits.
This method can transmit packets without the packet size at the ratio that is specified in the
queue, but it is complicated to implement.
DWRR (Deficit Weight Round Robin)
DWRR method enhances disadvantage of WRR and WFQ. DWRR defines weight, quantum and
deficit counter to each queue. Quantum is the maximum packet size that is processed by weight
ratio. Deficit counter is set to ‘0’ by default. Deficit counter is merged with quantum when data
of a queue is serviced. The packet of queue can be serviced up to deficit counter. After the
packet is serviced, deficit counter is decreased to the packet size.
For example, there is a queue that quantum value is 1000bytes. If 500byte packet, 300byte
packet, and 300byte packet are in a queue, only 500byte packet and 300byte packet can be
processed because the queue can process up to 1000bytes. Then, deficit counter becomes 200.
After other queues process their packet, the queue become in the order. The deficit counter
value becomes 1200, and the queue can process up to 1200byte.
Deficit counter memorizes the size of packet that was not transmitted as the ratio of weight, and
transmits the packet next time.
Let’s look at the operation principal of DWRR. There are three queues in an output port as
below. In each queue, 2:1:1 of weight is assigned. The quantum values of each queue are set as
1000byte, 500byte and 500byte. The deficit counter values are set as ‘0’ (Picture 1).
Bit-by-Bit Service Ratio
= Q1:Q2:Q3 = 2:1:1
[Q1] Weight: 2
[Q2] Weight: 1
[Q3] Weight: 1
300B 200B 400B 300B 100B
600B 300B 400B
400B 500B 500B
Pa
cket
Segmentation
Last bit of
300B Pkt
Last bit of
100B Pkt
Last bit of
400B Pkt
Last bit of
500B Pkt
Last bit of
300B Pkt
Last bit of
600B Pkt
Last bit of
200B Pkt
Last bit of
300B Pkt
Last bit of
500B Pkt
Last bit of
400B Pkt
Last bit of
400B Pkt
1 bit
Output Port
600B 300B 400B 500B 300B 200B 400B 300B 100B 500B 400B
Bit-by-Bit WRR
Scheduler
Packet
Reassembler