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Operating Principle of the Congestion Management Policies 683
Figure 218 Schematic diagram of the first in first out queue
First-In, First-Out (FIFO)
Queuing
As shown in Figure 218, the data packets are input to the first-in, first-out (FIFO)
queue according to the priority order of their arrivals. Data packets that first arrive
are first transmitted, and the data packets that later arrive are transmitted later. All
the packets that will be transmitted from the interface are input to the end of the
FIFO queue of the interface in the priority order of their arrivals. At the time when
the interface transmits the packets, the packets are transmitted in order, starting
from the head of the FIFO queue. During the transmission process of all packets,
there is no difference and no guarantee is provided for the quality of the packet
transmission. Therefore, a single application can occupy all the network resources,
seriously affecting the transmission of key service data.
Priority Queuing (PQ) As shown in Figure 219, the PQ queue is used to provide strict priority levels for
important network data. It can flexibly specify the priority order according to the
network protocol (such as IP or IPX), the interface into which the data are input,
the length of the packet, and the source address, destination address, and other
features.
Figure 219 Schematic diagram of the priority queuing
When the packets arrive at the interface, all of them are first classified (up to 4
classifications), and then they are input to the ends of respective queues according
to the classifications of the packets. Upon the transmission of the packets,
according to different priority levels, the packets in the low priority queue are not
transmitted until all the packets in the high priority queues are transmitted. Thus,
it is guaranteed that, at the network unit where the PQ is utilized, the most
important data can be processed the soonest and the packets of the higher
priority queues have very low delay. Both packet performance exponents of loss
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