background image
1-23
Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
956467
Chapter 1 Overview
What is the Quality of Service Toolset?
Policing and Shaping Tools
Policers and shapers are the oldest forms of QoS mechanisms. These tools have the same objectives,
namely to identify and respond to traffic violations.
Policers and shapers usually identify traffic violations in an identical manner; however, their main
difference is the manner in which they respond to violations:
·
A policer typically drops traffic.
·
A shaper typically delays excess traffic using a buffer, or queueing mechanism, to hold packets and
shape the flow when the data rate of the source is higher than expected.
Shaping tools meter the transmit rate of frames from a source router to a destination router. This
metering is typically done at a value that is lower than the line or circuit rate of the transmitting interface,
as shown in
Figure 1-14
. The metering is done at this rate to account for the circuit speed mismatches
that are common in current non-broadcast multiple-access (NBMA) networks, such as Frame-Relay and
ATM.
Figure 1-14 Shaping Traffic at Less than Line Rate for NBMA Networks
Tip
For more information, see Policing and Shaping in the Cisco IOS Quality of Service Solutions
Configuration Guide
, Release 12.2.
Link-Efficiency Mechanisms
Newer mechanisms have been developed to address link-specific requirements, such as minimizing
serialization delay and traffic compression.
Tip
For more information, see Link Efficiency Mechanisms in the Cisco IOS Quality of Service Solutions
Configuration Guide
, Release 12.2.
Link Fragmentation and Interleave
To offset link-specific delays, larger data packets can be fragmented and interleaved with higher priority
voice or video packets. This helps reduce the variation in delay caused by periodic congestion on
slow-link edges.
Figure 1-15
illustrates the process of link fragmentation and interleaving (LFI).
Traffic shaping limits the transmit rate
to a value (R) lower than line rate
Without Traffic Shaping
With Traffic Shaping
Line Rate
R
74643