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| 1.2 |  | Key Characteristics of Various Switching Technologies |
| 1.2.6 |  | Quality of Service (QoS) |

QoS maps well to the multilayer campus design. Packet classification is a multilayer service that is applied at the wiring closet switch, the ingress point to the network.

QoS for voice or video over IP consists of providing minimal packet loss and minimal delay so that voice/video quality is not affected by conditions in the network. A brute-force solution is to simply provide sufficient bandwidth at all points in the network to minimize packet loss and queuing delay. A better alternative is to apply congestion management and congestion avoidance at oversubscribed points in the network. Most modern Layer 3+ switches support configuration options for congestion management and congestion avoidance.

A reasonable design goal for end-to-end network delay for Voice over IP (VoIP) is 150 milliseconds. Providing a separate outbound queue for real-time traffic is sufficient to guarantee low delay for voice at campus speeds. Bursty traffic, such as file transfers, is placed in a different queue from real-time traffic. Low-latency queuing (LLQ) is the preferred method for queuing in this context, giving priority to voice traffic while data traffic is typically governed by weighted fair queuing. Auxiliary VLANs also provide an effective means for optimizing voice traffic in a campus LAN. Another option for achieving low packet loss and high throughput in any queue that experiences bursty data traffic flow is Weighted Random Early Detection (WRED).

QoS plays a significant role in designing a modern switched network. With VoIP and streaming video now commonplace and videoconferencing on the rise, it is important to understand the particular demands of these types of traffic on the network and how to design the switched network to optimally support it.