ZyXEL Communications VMG1312-B Series User Manual page 103

Chapter 6 Broadband
The following figure illust rat es t he relat ionship bet ween PCR, SCR and MBS.
Figure 28 Exam ple of Traffic Shaping
ATM Traffic Classes
These are t he basic ATM t raffic classes defined by t he ATM Forum Traffic Managem ent 4.0
Specificat ion.
Const ant Bit Rat e ( CBR)
Const ant Bit Rat e ( 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- sensit ive ( doesn't t olerat e delay) . CBR is used for connect ions
t hat cont inuously require a specific am ount of bandwidt h. A PCR is specified and if t raffic exceeds
t his rat e, cells m ay be dropped. 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 burst y connect ions. Connect ions t hat use
t he Variable Bit Rat e ( VBR) t raffic class can be grouped int o real t im e ( VBR- RT) or non- real t im e
( VBR- nRT) connect ions.
The VBR- RT ( real- t im e Variable Bit Rat e) t ype is used wit h burst y connect ions t hat require closely
cont rolled delay and delay variat ion. I t also provides a fixed am ount of bandwidt h ( a PCR is
specified) but is only available when dat a is being sent . An exam ple of an VBR- RT connect ion would
be video conferencing. Video conferencing requires real- t im e dat a t ransfers and t he bandwidt h
requirem ent varies in proport ion t o t he video im age's changing dynam ics.
The VBR- nRT ( non real- t im e Variable Bit Rat e) t ype is used wit h burst y connect ions t hat do not
require closely cont rolled delay and delay variat ion. I t is com m only used for " burst y" t raffic t ypical
on LANs. PCR and MBS define t he 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 sensit ive dat a file t ransfers.
Unspecified Bit Rat e ( UBR)
The Unspecified Bit Rat e ( UBR) ATM t raffic class is for burst y dat a t ransfers. However, UBR doesn't
guarant ee any bandwidt h and only delivers t raffic when t he net work has spare bandwidt h. An
exam ple applicat ion is background file t ransfer.
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VMG1312-B Series User's Guide