Allied Telesis AT-S63 - Manual (Page 314)

Chapter 15: Class of Service
314 Section II: Advanced Operations
Class of Service 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, resulting in the delay of packets 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 of no consequence to a network 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 CoS is 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.
CoS applies primarily to tagged packets. A tagged packet, as explained in
“Tagged VLAN Overview” on page 598, contains information within it that
specifies the VLAN to which the packet belongs.
A tagged packet can also contain a priority level. This priority level is used
by network switches and other networking devices to know how important
(delay sensitive) that packet is compared 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 priority.
The priorities are 0 to 7, with 0 the lowest priority and 7 the highest.
When a tagged packet is received on a port on the switch, it is examined
by the AT-S63 software for its priority. The switch software uses the
priority to determine which egress priority queue the packet should be
directed to on the egress port.
Each switch port has eight egress queues, labeled Q0, Q1, Q2, Q3, Q4,
Q5, Q6, Q7. Q0 is the lowest priority queue and Q7 is the highest. A
packet in a high priority egress queue is typically transmitted out a port
sooner than a packet in a low priority queue.