An Overview of QoS Mechanisms


• Classification: Each class-oriented QoS mechanism has to support some type of classification

• Marking: Used to mark packets based on classification and/or metering

• Congestion Avoidance: Used to drop packets early in order to avoid congestion later in the network

• Congestion Management: Each interface must have a queuing mechanism to prioritize transmission of packets

• Policing and Shaping: Used to enforce a rate limit based on the metering (Example: Frame Relay traffic shaping)

• Link Efficiency: Used to improve bandwidth efficiency through compression (or link fragmentation and interleaving)

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From the moment an IP packet enters the network, it may get the required service needed by the provision of various QoS mechanisms. A packet may be classified and then usually marked with its class identification. From that point on, the packet may be treated by other IP QoS mechanisms, depending on its packet classification. The figure above and the text below outline the main categories of IP QoS mechanisms.

Classification and marking mechanisms identify and split traffic into different classes. Traffic classes get a mark according to the traffic behavior and the intended business policies.

With congestion avoidance various mechanisms discard specific packets based on the markings. These mechanisms attempt to prevent or reduce network congestion.

Congestion management mechanisms attempt to prioritize, protect, and isolate traffic based on the markings.

Policing and shaping mechanisms attempt to condition the traffic; policing drops misbehaving traffic to maintain network integrity; shaping controls bursts by queuing network traffic.

Link efficiency mechanisms also provide QoS. One type of link efficiency mechanism is packet header compression to improve the bandwidth efficiency of a link. Another technology is Link Fragmentation and Interleaving (LFI) that can decrease the "jitter" of voice transmission by reducing voice packet delay.

QoS Mechanisms and Remote Access

This topic describes the issues that must be considered when you are applying QoS mechanisms to remote access situations.

Which QoS Mechanisms for Remote Access?


© 2004 Cisco Systems, Inc. All rights reserved. bcran v2.1—10-9

To provide end-to-end QoS, both the enterprise and service provider must implement the proper QoS mechanisms to ensure the proper traffic handling across the whole network.

Until recently, IP QoS was not an issue in an enterprise campus network because bandwidth was plentiful. Recent applications such as IP telephony, videoconferencing, e-learning as well as traditional mission-critical data applications have changed the requirement. Now network administrators must address the issues of buffer management and additional bandwidth.

In addition, IP QoS functions such as classification, scheduling, and provisioning are now required within the enterprise to manage bandwidth and buffers to minimize loss, delay, and jitter.

This figure lists some of the requirements within the different building blocks that make up the end-to-end enterprise network.

Most of the more complex QoS configurations of specific interest for remote access occur at the WAN edges. Some QoS tools used specifically at the WAN edge are the following:

Congestion avoidance using weighted random early detection (WRED)

Congestion management using queuing

Link efficiency using compression.

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