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12.1 Overview
This chapter introduces the configuration and functions of the QoS (Quality of Service) screen.
The QoS (Quality of Service) feature allows you to prioritize the flow of data passing through the Switch.
Occasionally, data might be delayed, depending on the volume of traffic and the capacity of the
equipment. Numeric and text data are usually not affected by delays, because they are reassembled
at the destination. However, when VoIP and streaming videos are reassembled, they might have some
troublesome gaps. Without QoS, all traffic data is equally likely to be dropped when the network is
congested. This can cause a reduction in network performance and make the network inadequate for
time-critical applications such as VOD (Video on Demand).
You can enable QoS to have the Switch assign each packet a priority and then queues the packet
accordingly. Packets assigned a high priority are processed more quickly than those with low priority if
there is congestion, allowing time-sensitive applications to flow more smoothly. Time-sensitive
applications include both those that require a low level of latency (delay) and a low level of jitter
(variations in delay) such as Voice over IP (VoIP) or Internet gaming, and those for which jitter alone is a
problem such as Internet radio or streaming video.
12.2 What You Need to Know
The Switch can put packets into the queues according to the port on which the packet is received or
the priority tag in the packet.
12.2.1 Port-Based QoS
The Port-Based QoS feature assigns priority to data transmitted through a particular port. When the data
arrives to a port it begins a queue. Therefore, the Switch has a queue for each port. If data arrives at the
same time to all ports, ports with higher priority will be first to transmit the data received. The higher the
priority of the port, the less delays the data passing through will have.
12.2.2 IEEE 802.1p QoS
IEEE 802.1p defines a 3-bit field called PCP (Priority Code Point) within the IEEE 802.1Q VLAN tag, which is
also referred to as a CoS (Class of Service) value and indicates the frame priority level. IEEE 802.1p QoS
uses the priority value (from 0 to 7) to define up to 8 traffic types. That is, each priority level defines a
class of service. The table below shows the IEEE recommendations for traffic types, these may vary or be
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