User guide

Introduction QOS
7-1
iMG/RG Software Reference Manual (Quality of Service)
7. Quality of Service
7.1 QOS
7.1.1 Introduction
The Quality of Service (QoS) support within gateway units allows different classes of traffic, such as specific
applications or users of a network, to be offered different levels of service. The key features offered are:
• Quality of service for traffic managed by the ADSL module.
• Quality of service for local applications such as Voice over IP (VoIP) traffic.
• Architecture compatible with RFC 2475 An Architecture for Differentiated Services and RFC 2474 Definition of
the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers.
7.1.2 QoS architecture overview
The basic building blocks of DIFFSERV functionality are various traffic conditioning functions, such as:
• Classification
• Policing
• Marking
• Shaping
The gateway provide a set of software tools that implement these traffic-conditioning functions. Hence, these
tools can configure DIFFSERV or any other QoS behaviour. These software tools are:
• The Classifier; it classifies incoming packets to ensure that important packets are marked as high priority. It
will also communicate the scheduling priority and drop priority to the Scheduler and communicate the
meter-id to the Meter
• The Meter; it measures the temporal properties of the incoming stream against configured parameters and
communicates the drop priority to the Scheduler.
• The Scheduler; it is used for scheduling packets for transmission on an outgoing interface based on informa-
tion received from the Meter and the Classifier. It can be configured to provide Priority Scheduling or
WF2Q+ Scheduling behavior.
Priority Scheduling is used to send out one class of packets with absolute priority over other classes. WF2Q+
Scheduling is used to provide a fair sharing of a single outgoing link between multiple classes (Assured forwarding)
or to send out one class of packets with priority over other classes while ensuring that lower priority traffic is
not completely starved (Expedited Forwarding).