ZyXEL Communications P-660RU-T1 v3, User Manual, Page: 72 / 230

Chapter 7 Internet Setup
P-660RU-Tx v3 User’s Guide
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7.3.7 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.