18. Quality Of Service; Introduction; Feature Overview - Nokia ESB26 User Manual

18. Quality of Service

Introduction

Today's networks transmit data streams for various applications using many different
protocols. Different types of traffic sharing a data path through the network can interact in
ways that affect their application performance. Traffic prioritization becomes especially
important when delay-sensitive, interactive applications are supported across the network. In
many cases a guaranteed level of throughput is part of contractual obligations between the
network operator and customers or third-party service providers.
Policy-based Quality of Service (QoS) allows you to specify different service levels for traffic
traversing the switch. Policy-based QoS is an effective control mechanism for networks that
have heterogeneous traffic patterns. Using Policy-based QoS, you can specify the service
level for a traffic type or host.
QoS controls congestion by determining the order in which packets are transmitted based on
priorities assigned to those packets. QoS queuing policies can protect bandwidth for important
categories of applications, or specifically limit the bandwidth associated with less critical
traffic. For example, if Voice Over IP (VOIP) traffic requires a reserved amount of bandwidth
to function properly, QoS policies can reserve sufficient bandwidth for this type of
application. Other applications deemed less critical can be limited in their bandwidth usage.
During periods of light traffic, QoS policies have little effect, and packets are transmitted as
soon as they arrive. During periods of congestion, outbound packets accumulating at an
interface are sorted into eight queues. They are transmitted from the queues according to the
queuing mechanism configured for the interface.

Feature Overview

When using QoS feature, each physical port sorts inbound and outbound traffic into eight
queues for the QoS processing.
You control Quality of Service behavior in two ways:
By configuring the criteria used to sort inbound and outbound packets into the eight
queues. By default, values of the 802.1p priority field of the packet header are mapped to the
eight QoS queues. You can also map destination MAC address priority values to the QoS
queues.
By selecting the queuing mechanism to apply to the outbound queues. Two basic queuing
mechanisms are provided:
•
Weighted Round-Robin queuing lets you assign a relative weight to each queue,
which determines the bandwidth assigned to each queue relative to the others.
•
Strict Priority queuing sets the eight queues in a rigid order, and always transmits
packets from the highest-priority queue that has packets waiting.
In addition, two hybrid queuing schemes are available, which combine the Weighted Round
Robin and Strict Priority mechanisms.
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