Cisco Systems 520 series QoS

B-7

Cisco Secure Router 520 Series Software Configuration Guide

OL-14210-01

Appendix B Concepts

QoS

QoS

This section describes Quality of Service (QoS) parameters, including the following:

• IP Precedence

• PPP Fragmentation and Interleaving

• CBWFQ

• RSVP

• Low Latency Queuing

QoS refers to the capability of a network to provide better service to selected network traffic over various

technologies, including ATM, Ethernet and IEEE 802.1 networks, and IP-routed networks that may use

any or all of these underlying technologies. Primary goals of QoS include dedicated bandwidth,

controlled jitter and latency (required by some real-time and interactive traffic), and improved loss

characteristics. QoS technologies provide the elemental building blocks for future business applications

in campus, WAN, and service provider networks.

QoS must be configured throughout your network, not just on your router running VoIP, to improve voice

network performance. Not all QoS techniques are appropriate for all network routers. Edge routers and

backbone routers in your network do not necessarily perform the same operations; the QoS tasks they

perform might differ as well. To configure your IP network for real-time voice traffic, you need to

consider the functions of both edge and backbone routers in your network.

QoS software enables complex networks to control and predictably service a variety of networked

applications and traffic types. Almost any network can take advantage of QoS for optimum efficiency,

whether it is a small corporate network, an Internet service provider, or an enterprise network.

You can partition traffic in up to six classes of service using IP Precedence (two others are reserved for

internal network use). The queuing technologies throughout the network can then use this signal to

expedite handling.

Features such as policy-based routing and committed access rate (CAR) can be used to set precedence

based on extended access-list classification. This allows considerable flexibility for precedence

assignment, including assignment by application or user, by destination and source subnet, and so on.

Typically this functionality is deployed as close to the edge of the network (or administrative domain)

as possible, so that each subsequent network element can provide service based on the determined policy.

IP Precedence can also be set in the host or network client with the signaling used optionally. IP

Precedence enables service classes to be established using existing network queuing mechanisms (such

as class-based weighted fair queuing [CBWFQ]) with no changes to existing applications or complicated

network requirements.

With multiclass multilink PPP interleaving, large packets can be multilink-encapsulated and fragmented

into smaller packets to satisfy the delay requirements of real-time voice traffic; small real-time packets,

which are not multilink encapsulated, are transmitted between fragments of the large packets. The

interleaving feature also provides a special transmit queue for the smaller, delay-sensitive packets,

enabling them to be transmitted earlier than other flows. Interleaving provides the delay bounds for

delay-sensitive voice packets on a slow link that is used for other best-effort traffic.