Chapter 6 Broadband
VMG1312-B Series User’s Guide
103
The following figure illustrates the relat ionship bet ween PCR, SCR and MBS.
Figure 28 Exam ple of Traffic Shaping
ATM Traffic Classes
These are t he basic ATM traffic classes defined by t he ATM Forum Traffic Managem ent 4.0
Specificat ion.
Const ant Bit Rat e ( CBR)
Constant Bit Rate (CBR) provides fixed bandwidt h t hat is always available even if no dat a is being
sent. CBR t raffic is generally t im e- sensitive ( doesn't t olerate delay) . CBR is used for connect ions
that continuously require a specific amount of bandwidt h. A PCR is specified and if traffic exceeds
this rate, cells m ay be dr opped. Exam ples of connect ions t hat need CBR would be high- resolut ion
video and voice.
Variable Bit Rat e ( VBR)
The Variable Bit Rat e ( VBR) ATM t raffic class is used wit h bursty connect ions. Connections t hat use
the Variable Bit Rat e (VBR) traffic class can be grouped into real t im e (VBR- RT) or non-real t im e
( VBR- nRT) connections.
The VBR- RT ( r eal- t im e Variable Bit Rat e) type is used w ith bursty connect ions that require closely
cont r olled delay and delay variat ion. I t also provides a fixed am ount of bandwidth ( a PCR is
specified) but is only available when dat a is being sent. An exam ple of an VBR- RT connection would
be video conferencing. Video conferencing requires real- t im e dat a t ransfers and t he bandwidth
requirem ent varies in proportion t o the video im age's changing dynam ics.
The VBR- nRT (non real-t im e Variable Bit Rate) type is used w ith bursty connect ions that do not
require closely controlled delay and delay variat ion. I t is com m only used for "burst y" traffic t ypical
on LANs. PCR and MBS define the burst levels, SCR defines t he m inim um level. An exam ple of an
VBR- nRT connect ion would be non-t im e sensitive data file transfers.
Unspecified Bit Rat e ( UBR)
The Unspecified Bit Rate (UBR) ATM t raffic class is for burst y data transfers. However, UBR doesn't
guarantee any bandwidt h and only delivers t raffic when the network has spare bandwidth. An
exam ple application is background file transfer.
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