ZyXEL Communications P-2612HW-F1 User Manual page 120

Chapter 6 WAN Setup
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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P-2612HW-F1 User's Guide