EnGenius EWS5912FP, Руководство пользователя

Страница: 155 / 207

155
QoS
Quality of Service (QoS) provides the ability to implement
priority queuing within a network. QoS is a means of
providing consistent and predictable data delivery to
the Switch by distinguishing between packets that have
stricter timing requirements from those that are more
tolerant of delays. QoS enables traffic to be prioritized while
avoiding excessive broadcast and multicast traffic. Traffic
such as Voice and Video streaming which require minimal
delays can be assigned to a high priority queue, while other
traffic can be assigned to a lower priority queue, resulting
in uninterrupted actions. Without QoS, all traffic data is as
likely to be dropped when the network is congested. This
can result in reductions in network performance and hinder
the network in time-critical situations.
In a Switch, multiple queues per port are often provided
to give preference to certain packets over others based
on user-defined criteria. When a packet is queued for
transmission within a port, the rate at which it is processed
depends on how the queue is configured and the amount
of traffic present within other queues on the port. If a delay
is necessary, packets are held in the queue until they are
authorized for transmission.
Global Settings
There are two options for applying QoS information onto
packets: the 802.1p Class of Service (CoS) priority field
within the VLAN tag of tagged Ethernet frames, and Dif-
ferentiated Services (DiffServ) Code Point (DSCP). Each
port on the Switch can be configured to trust one of the
packet fields (802.1p , DSCP or DSCP+802.1p). Packets
that enter the Switch’s port may carry no QoS informa-
tion as well. If so, the Switch places such information
into the packets before transmitting them to the next
node. Thus, QoS information is preserved between nodes
within the network and the nodes know which label to
give each packet. A trusted field must exist in the packet
for the mapping table to be of any use. When a port is
configured as untrusted, it does not trust any incoming
packet priority designations and uses the port default
priority value instead to process the packet.