Allied Telesis AT-S39 - Manual (Page 175)

AT-S39 User’s Guide
175
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. This can result in the delay of packets reaching their
destinations.
Minor delays are often of no consequence to a network or its
performance. But there are some 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 can be of value. It allows you to manage the flow of
traffic through your 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. If you read Tagged VLAN
Overview on page 128, then you know that a tagged packet contains
information within it that specifies the VLAN to which the packet
belongs.
A tagged packet also contains 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-S62 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 two egress queues, high and low. A packet in a high
priority egress queue is typically transmitted out a port sooner than a
packet in a low priority queue.