ZyXEL Communications P-660HN-51 User Manual page 58

Chapter 5 Broadband
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.
P-660HN-51 User's Guide
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