Allied Telesis AT-S101 - Manual (Page 62)

Chapter 4: Quality of Service (QoS)
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Overview
When a port on an Ethernet switch becomes oversubscribed—its egress
queues contain more packets than the port can handle in a timely
manner—the port may be forced to delay the transmission of some
packets, which delays packets from reaching their destinations. A port
may be forced to delay transmission of packets while it handles other
traffic, and, in some situations, some packets destined to be forwarded to
an oversubscribed port from other switch ports may be discarded.
Minor delays are often inconsequential to a switch or its performance. But
there are applications, referred to as delay or time sensitive applications,
that can be impacted by packet delays. Voice transmission and video
conferencing are two examples. If packets carrying data for either of these
are delayed from reaching their destination, the audio or video quality may
suffer.
This is where the QoS feature can be of value. It allows you to manage the
flow of traffic through a switch by having the switch ports give higher
priority to some packets, such as delay sensitive traffic, over other
packets. This is referred to as prioritizing traffic.
The QoS feature actually consists of several different elements. The
element supported by the AT-GS950/8POE switch is called Class of
Service (CoS) and which applies primarily to tagged packets and DSCP
which priorities IP packets. As explained in “Tagged VLAN Overview” on
page 105, a tagged packet contains information that specifies the VLAN to
which the packet belongs.
A tagged packet can also contain a priority level. This priority level is used
by switches and other networking devices to determine how important
(delay sensitive) a packet is in comparison to other packets. Packets of a
high priority are typically handled before packets of a low priority.
CoS, as defined in the IEEE 802.1p standard, has eight levels of traffic
classes. In the AT-S101 software, the priorities are 0 to 7, with 0 the
lowest priority and 7 the highest priority.
When a tagged packet is received by a port, it is examined by the
AT-S101 software for its priority. The switch software uses the priority to
determine which egress priority queue the packet should be stored in on
the egress port.
Each port on the AT-GS950/8POE switch has four priority queues, 0 (low)
to 3 (high). When a tagged packet enters a switch port, the switch
responds by placing the packet into one of the queues according to the
assignments shown in Table 1 on page 63. A packet in a high priority
egress queue is typically transmitted from a port sooner than a packet in a
low priority queue.