Motorola Netopia 7000, Administrator'S Handbook

Page: 76 / 368

Administrator’s Handbook
76
ATM Traffic Shaping:
You can prioritize delay-sensitive data by configuring the Quality of Ser vice (QoS)
characteristics of the vir tual circuit. Click the
ATM Traffic Shaping
link.
You can choose UBR (Unspecified Bit Rate), CBR (Constant Bit Rate), or VBR (Variable Bit Rate) from the
pull-down menu and set the Peak Cell Rate (PCR) in the editable field.
UBR (Unspecified Bit Rate)
guarantees no minimum transmission rate. Cells are transmitted on a
“best effor t” basis. However, there is a cap on the maximum transmission rate for UBR VCs. In a practi-
cal situation:
• UBR VCs should be transmitted at a priority lower than CBR.
• Bandwidth should be shared equally among UBR VCs.
UBR applications are non-real-time traffic such as IP data traffic.
CBR (Constant Bit Rate)
guarantees a cer tain transmission rate (although the application may
underutilize this bandwidth). A Peak Cell Rate (PCR) characterizes CBR. CBR is most suited for real time
applications such as real time voice / video, although it can be used for other applications.
VBR (Variable Bit Rate)
This class is characterized by:
• a
Peak Cell Rate
(PCR), which is a temporar y burst, not a sustained rate, and
• a
Sustained Cell Rate
(SCR),
• a Burst Tolerance (BT), specified in terms of
Maximum Burst Size
(MBS). The MBS is the maximum
number of cells that can be transmitted at the peak cell rate and should be less than, or equal to the
Peak Cell Rate, which should be less than, or equal to the line rate.
VBR has two sub-classes:
a.
VBR non-real-time (VBR-nr t): Typical applications are non-real-time traffic, such as IP data traffic. This
class yields a fair amount of Cell Delay Variation (CDV).
b.
VBR real time (VBR-r t): Typical applications are real-time traffic, such as compressed voice over IP
and video conferencing. This class transmits cells with a more tightly bounded Cell Delay Variation. The
applications follow CBR.
☛
Note:
The difference between VBR-r t and VBR-nr t is the tolerated Cell Delay Variation range and the
provisioned Maximum Burst Size.