Allied Telesis AT-S63 - Manual (Page 133)

AT-S63 Management Software Features Guide
Section II: Advanced Operations 133
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. Some packets destined to be forwarded to an oversubscribed port
from other switch ports may be discarded.
Although minor delays are often of no consequence to a network or its
performance, 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. A delay in the transmission of
packets carrying their data could impact the quality of the audio or video.
This is where CoS can be of value. What it does is it permits a switch to
give higher priority to some packets over other packets.
There are two principal types of traffic found on the ports of a Gigabit
Ethernet switch, one being untagged packets and the other tagged
packets. As explained in “Tagged VLAN Overview” on page 263, one of
the principal differences between them is that tagged packets contain
VLAN information.
CoS applies mainly to tagged packets because, in addition to carrying
VLAN information, these packets can also contain a priority level
specifying how important (delay sensitive) a packet is in comparison to
other packets. It is this number that the switch refers to when determining
a packet’s priority level.
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.
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.
When a tagged packet arrives on a port, the switch examines its priority
value to determine which egress priority queue the packet should be
directed to on the egress port. Table 11 lists the default mappings between
the eight CoS priority levels and the eight egress queues of a switch port.