Chapter 4 WAN
Basic Home Station VDSL2 P8701T User’s Guide
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The following figure illustrates the relationship between PCR, SCR and MBS.
Figure 32 Example of Traffic Shaping
ATM Traffic Classes
These are the basic ATM traffic classes defined by the ATM Forum Traffic Management 4.0
Specification.
Constant Bit Rate (CBR)
Constant Bit Rate (CBR) provides fixed bandwidth that is always available even if no data is being
sent. CBR traffic is generally time-sensitive (doesn't tolerate delay). CBR is used for connections
that continuously require a specific amount of bandwidth. A PCR is specified and if traffic exceeds
this rate, cells may be dropped. Examples of connections that need CBR would be high-resolution
video and voice.
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 (VBR-RT) or non-real time
(VBR-nRT) connections.
The VBR-RT (real-time Variable Bit Rate) type is used with bursty connections that require closely
controlled delay and delay variation. It also provides a fixed amount of bandwidth (a PCR is
specified) but is only available when data is being sent. An example of an VBR-RT 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 VBR-nRT (non real-time Variable Bit Rate) type is used with bursty connections that do not
require closely controlled delay and delay variation. It is commonly used for "bursty" traffic typical
on LANs. PCR and MBS define the burst levels, SCR defines the minimum level. An example of an
VBR-nRT connection would be non-time sensitive data file transfers.
Unspecified Bit Rate (UBR)
The Unspecified Bit Rate (UBR) ATM traffic class is for bursty data transfers. However, UBR doesn't
guarantee any bandwidth and only delivers traffic when the network has spare bandwidth. An
example application is background file transfer.
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