2-2 QoS Explained

Key Concepts

• QoS Overview
• Classification and Marking
• Congestion Management
• Congestion Avoidance
• Policing and Shaping

QoS Overview

Quality of Service (QoS) is a set of techniques used to manage network traffic and minimize congestion. QoS ensures that critical applications receive the necessary bandwidth and priority, improving overall network performance and user experience.

Classification and Marking

Classification and marking are the first steps in implementing QoS. Classification involves identifying and categorizing different types of traffic based on criteria such as source IP, destination IP, protocol, and port number. Marking, often done using Differentiated Services Code Point (DSCP) values, assigns a priority to each classified traffic type.

Example

A network administrator classifies VoIP traffic as high priority and marks it with a DSCP value of 46 (EF). This ensures that VoIP packets are given priority over other types of traffic, such as web browsing or file transfers.

Congestion Management

Congestion management techniques ensure that traffic is handled efficiently during periods of high demand. Common methods include queuing algorithms like Weighted Fair Queuing (WFQ) and Priority Queuing (PQ). These algorithms allocate bandwidth to different queues based on their priority, ensuring that high-priority traffic is served first.

Example

In a network with three queues (high, medium, and low priority), WFQ ensures that high-priority traffic (e.g., VoIP) receives the most bandwidth, followed by medium-priority traffic (e.g., video streaming), and finally low-priority traffic (e.g., email).

Congestion Avoidance

Congestion avoidance techniques prevent network congestion before it occurs. Common methods include Random Early Detection (RED) and Weighted RED (WRED). These techniques monitor the queue length and drop packets probabilistically when the queue reaches a certain threshold, preventing congestion and ensuring smoother traffic flow.

Example

WRED monitors the queue length for high-priority traffic and starts dropping packets only when the queue is almost full. This ensures that high-priority traffic is minimally affected while preventing congestion for lower-priority traffic.

Policing and Shaping

Policing and shaping are used to control the rate of traffic entering the network. Policing involves monitoring the traffic and discarding packets that exceed a specified rate. Shaping, on the other hand, buffers excess traffic and releases it at a controlled rate, ensuring that the traffic conforms to the specified rate without being dropped.

Example

A network administrator sets a policing rate of 1 Mbps for a specific application. If the application sends traffic at 1.5 Mbps, the excess 0.5 Mbps is dropped. In contrast, shaping would buffer the excess traffic and release it at 1 Mbps, ensuring no packets are dropped but maintaining the specified rate.

Conclusion

Understanding Quality of Service (QoS) is essential for managing network traffic and ensuring optimal performance for critical applications. By mastering classification and marking, congestion management, congestion avoidance, and policing and shaping, network professionals can implement effective QoS strategies that enhance network efficiency and user experience.