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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
tolerantofdelays.QoSenablestrafctobeprioritizedwhile
avoidingexcessivebroadcastandmulticasttrafc.Trafc
such as Voice and Video streaming which require minimal
delays can be assigned to a high priority queue, while other
trafccanbeassignedtoalowerpriorityqueue,resulting
inuninterruptedactions.WithoutQoS,alltrafcdataisas
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-dened criteria. When a packet is queued for
transmission within a port, the rate at which it is processed
dependsonhowthequeueisconguredandtheamount
oftrafcpresentwithinotherqueuesontheport.Ifadelay
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:the802.1pClassofService(CoS)priorityeld
within the VLAN tag of tagged Ethernet frames, and Dif-
ferentiated Services (DiffServ) Code Point (DSCP). Each
portontheSwitchcanbeconguredtotrustoneofthe
packetelds(802.1p,DSCPorDSCP+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
giveeachpacket.Atrustedeldmustexistinthepacket
for the mapping table to be of any use. When a port is
conguredasuntrusted,itdoesnottrustanyincoming
packet priority designations and uses the port default
priority value instead to process the packet.
