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MSAP2000 AAM User’s Guide
7.11 ATM QoS
ATM Quality of Service (QoS) mechanisms provide the best service on a per-flow
guarantee. ATM network infrastructure was designed to provide QoS. It uses fixed
cell sizes and built-in traffic management (see the following section on traffic
shaping). This allows you to fine-tune the levels of services on the priority of the
traffic flow.
7.12 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.
7.12.1 ATM Traffic Classes
These are the basic ATM traffic classes defined by the ATM Forum Traffic
Management 4.0
Specification.
7.12.1.1 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.
7.12.1.2 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.