NTP protocols. In this situation, to guarantee that higher priority data will not be lost,
some quality at the network services should be provided.
Where traffic cannot be dropped, it must use some Quality-of-Service (QoS)
mechanism to ensure that data will not be lost. This mechanism will guard a part of
its bandwidth to be used only by these messages. General traffic will be stored in a
queue to be forwarded, and higher priority traffic will have different queue to be
stored before it is forwarded. When forwarding data, the switch will deliver firstly
higher priority data, and will forward remaining traffic after high priority data queue
is empty. If lower priority data reach its bandwidth, there will be lost of data, but the
higher priority data will not be affected, as it has guarantee of bandwidth. Figure
below shows an example of such situation.
Figure 51: Network with prioritization of traffic
There are several ways to separate prioritized traffic from general purposes traffics.
At data link layer, there are the Priority bits in 802.1Q frame that can be used. At
network layer, DSCP information in IP protocol header can also be used, and so on.
10.2 Class-of-Service (CoS) Bits QoS
First development of a standard to incorporate traffic prioritization started at the IEEE
802.1p standard, which nowadays is inside the IEEE 802.1Q standard. At the last one,
there is a 3-bit length field, the class-of-service bits (CoS), that is used to set which
priority the network should use for this frame. Next figure shows the 802.1Q and CoS
bits, called Priority Code Point (PCP).
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