S350 Series 24-Port (PoE+) and 48-Port Gigabit Ethernet Smart Managed Pro Switches
Configure Quality of Service User Manual184
Quality of Service Concepts
In a switch, each physical port consists of one or more queues for transmitting packets on the
attached network. 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 in a port, the rate at which it is serviced depends on how the queue is
configured and possibly the amount of traffic present in the other queues of the port. If a
delay is necessary, packets are held in the queue until the scheduler authorizes the queue for
transmission. As queues become full, packets can no longer be held for transmission and are
dropped by the switch.
Quality of Service (QoS) is a means of providing consistent, predictable data delivery by
distinguishing packets with strict timing requirements from those that are more tolerant of
delay
. Packets with strict timing requirements are given special treatment in a QoS-capable
network. With this in mind, all elements of the network must be QoS capable.
The presence
of at least one node that is not QoS capable creates a deficiency in the network path, and the
performance of the entire packet flow is compromised.
Manage Class of Service
The Class of Service (CoS) queueing feature lets you directly configure certain aspects of
switch queueing. This provides the desired QoS behavior for different types of network traffic
when the complexities of DiffServ are not required. The priority of a packet arriving at an
interface can be used to steer the packet to the appropriate outbound CoS queue through a
mapping table. CoS queue characteristics that affect queue mapping, such as minimum
guaranteed bandwidth or transmission rate shaping, are user configurable at the queue (or
port) level.
Note: Models GS324T and GS324TP support four hardware queues per
port. Model GS348T supports eight hardware queues per port.
CoS Configuration Concepts
You can set the Class of Service trust mode for an interface. Each port in the switch can be
configured to trust one of the packet fields (802.1p or IP DSCP), or to not trust any packet’s
priority designation (untrusted mode). If the port is set to a trusted mode, it uses a mapping
table appropriate for the trusted field being used. This mapping table indicates the CoS
queue to which the packet must be forwarded on the appropriate egress port. Of course, the
trusted field must exist in the packet for the mapping table to be of any use. If this is not the
case, default actions are performed. These actions involve directing the packet to a specific
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