D-Link DI-1750 Reference Manual page 386

The QoS provides Weighted Random Early Detection (WRED), Custom Queueing (CQ), and Priority
Queueing (PQ) to deliver differentiated services.
3. QoS Queueing Algorithms
QoS Queueing Algorithms are the important guarantee to achieve QoS configuration. D-LINK router
supports Weighted Fair Queueing (WFQ), Custom Queueing (CQ), Priority Queueing (PQ), Weighted
Random Early Detection (WRED), and the simplest first-in and first-out (FIFO) algorithm.
♦ Weighted Fair Queueing
WFQ is a dynamic scheduling method that provides fair bandwidth allocation to all network traffic.
WFQ applies priority, or weights, to identified traffic to classify traffic into conversations and
determine how much bandwidth each conversation is allowed relative to other conversations. WFQ
is a flow-based algorithm that simultaneously schedules interactive traffic to the front of a queue to
reduce response time and fairly shares the remaining bandwidth among high-bandwidth flows. In
other words, WFQ allows you to give low-volume traffic, such as Telnet sessions, priority over
high-volume traffic, such as FTP sessions. WFQ gives concurrent file transfers balanced use of link
capacity; that is, when multiple file transfers occur, the transfers are given comparable bandwidth.
WFQ overcomes a serious limitation of FIFO queueing. When FIFO is in effect, traffic is sent in the
order received without regard for bandwidth consumption or the associated delays. As a result, file
transfers and other high-volume network applications often generate series of packets of
associated data and depriving other traffic of bandwidth. WFQ provides traffic priority management
that dynamically sorts traffic into messages that make up a conversation to ensure that bandwidth
is shared fairly between individual conversations and that low-volume traffic is transferred in a
timely fashion.
WFQ classifies traffic into different flows based on packet header addressing. For most of traffic are
IP data, thus the WFQ classifies the data packets based on characteristics of IP header, including
such characteristics as source and destination address, source and destination port, protocol types,
and ToS value.
WFQ places packets of the various conversations in the fair queues before transmission. The order
of removal from the fair queues is determined by the virtual time of the delivery of the last bit of
each arriving packet (finish number).
Flow-based WFQ is used as the default queueing mode on most serial interfaces configured to run
at E1 speeds (2.048 Mbps) or below.
See the "WFQ Configuration" for particular configuration of WFQ.
WFQ is only automatically identify flow but does not offer the special service for some peculiar flow.
D-Link router provides Class-Based Weighted Fair Queueing (CBWFQ), which enhanced the standard
WFQ. It can identifies the type of flow by user customization and distribute the authority to the flow.
See the "CBWFQ Configuration" for particular configuration of CBWFQ.
4. Weighted Random Early Detection
Random Early Detection (RED) is a common congestion avoidance mechanism. Random Early
Detection (RED) is a congestion avoidance mechanism that takes advantage of TCP's congestion
control mechanism. By randomly dropping packets prior to periods of high congestion, RED tells the
packet source to decrease its transmission rate. Assuming the packet source is using TCP, it will
decrease its transmission rate until all the packets reach their destination, indicating that the congestion
is cleared.
Weighted RED (WRED) generally drops packets selectively based on IP precedence. Packets with a
higher IP precedence are less likely to be dropped than packets with a lower precedence. Thus, higher
priority traffic is delivered with a higher probability than lower priority traffic. However, you can also
configure WRED to ignore IP precedence when making drop decisions so that non-weighted RED
behavior is achieved.
WRED is useful on any output interface where you expect to have congestion. However, WRED is
usually used in the core routers of a network, rather than the edge. Edge routers assign IP
precedences to packets as they enter the network. WRED uses these precedences to determine how it
treats different types of traffic.
When RED is not configured, output buffers fill during periods of congestion. When the buffers are full,
tail drop occurs; all additional packets are dropped. Since the packets are dropped all at once, global
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