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Chapter 14 xDSL Profiles Setup
IES-612-51A User’s Guide
128
14.3 Traffic Shaping
Traffic shaping is an agreement between the carrier and the subscriber to regulate the average
rate and fluctuations of data transmission over an ATM network. This agreement helps
eliminate congestion, which is important for transmission of real time data such as audio and
video connections.
" Traffic shaping controls outgoing (downstream) traffic, not incoming
(upstream).
14.3.1 ATM Traffic Classes
These are the basic ATM traffic classes defined by the ATM Forum Traffic Management 4.0
Specification.
14.3.1.1 Constant Bit Rate (CBR)
Constant Bit Rate (CBR) is an ATM traffic class that provides fixed bandwidth. CBR traffic is
generally time-sensitive (doesn’t tolerate delay). CBR is used for connections that
continuously require a specific amount of bandwidth. Examples of connections that need CBR
would be high-resolution video and voice.
14.3.1.2 Variable Bit Rate (VBR)
The Variable Bit Rate (VBR) ATM traffic class is used with bursty connections. Connections
that use the Variable Bit Rate (VBR) traffic class can be grouped into real time (rt-VBR) or
non-real time (nrt-VBR) connections.
The rt-VBR (real-time Variable Bit Rate) type is used with bursty connections that require
closely controlled delay and delay variation. An example of an rt-VBR connection would be
video conferencing. Video conferencing requires real-time data transfers and the bandwidth
requirement varies in proportion to the video image’s changing dynamics.
The nrt-VBR (non real-time Variable Bit Rate) type is used with bursty connections that do not
require closely controlled delay and delay variation. An example of an nrt-VBR connection
would be non-time sensitive data file transfers.
14.3.1.3 Unspecified Bit Rate (UBR)
The Unspecified Bit Rate (UBR) ATM traffic class is similar to the ABR traffic class for
bursty data transfers. However, while ABR gives subscribers a set amount of bandwidth, UBR
doesn’t guarantee any bandwidth and only delivers traffic when the network has spare
bandwidth.
14.3.2 Traffic Parameters
These are the parameters that control the flow of ATM traffic.