Allied Telesis AT-S79 manual QoS Overview

Chapter 8: Quality of Service (QoS)

QoS Overview

When a port on an Ethernet switch becomes oversubscribed—its egress queues contain more packets than the port can handle in a timely manner—the port may be forced to delay the transmission of some packets, resulting in the delay of packets from reaching their destinations. A port may be forced to delay transmission of packets while it handles other traffic, and, in some situations, some packets destined to be forwarded to an oversubscribed port from other switch ports may be discarded.

Minor delays are often of no consequence to a network or its performance. But there are applications, referred to as delay or time sensitive applications, that can be impacted by packet delays. Voice transmission and video conferencing are two examples. If packets carrying data for either of these are delayed from reaching their destination, the audio or video quality may suffer.

This is where QoS can be of value. It allows you to manage the flow of traffic through a switch by having the switch ports give higher priority to some packets, such as delay sensitive traffic, over other packets. This is referred to as prioritizing traffic.

QoS actually consists of several different elements. The element supported by the AT-GS950/16 and AT-GS950/24 switches is called Class of Service (CoS). CoS applies primarily to tagged packets. As explained in “Tagged VLAN Overview” on page 80, a tagged packet contains information within it that specifies the VLAN to which the packet belongs.

A tagged packet can also contain a priority level. This priority level is used by network switches and other networking devices to know how important (delay sensitive) that packet is in comparison to other packets. Packets of a high priority are typically handled before packets of a low priority.

CoS, as defined in the IEEE 802.1p standard, has eight levels of priority.

The priorities are 0 to 7, with 0 the lowest priority and 7 the highest.

When a tagged packet is received on a port on the switch, it is examined by the AT-S79 software for its priority. The switch software uses the priority to determine which egress priority queue the packet should be stored in on the egress port.

Each port on the AT-GS950/16 and AT-GS950/24 switches has four priority queues, 0 (low) to 3 (high). When a tagged packet enters a switch port, the switch responds by placing the packet into one of the queues according to the assignments shown in Table 2. A packet in a high priority egress queue is typically transmitted out a port sooner than a packet in a low priority queue.

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