28D-Link DSL-2680 User Manual
Chapter 3 - Setup
ATM Trafc Shaping
Settings used for ATM trafc shaping are located beneath the VPI/VCI conguration menu. These settings allow the user to adjust ATM Quality of Service (QoS) or
trafc parameters to suit specic trafc requirements. For applications or circumstances where packet loss or packet delay is a concern, ATM QoS can be adjusted to
minimize problems. For most accounts, it will not be necessary to change these settings. Altering QoS settings can adversely affect performance of some commonly
used Internet applications.
If you plan to change QoS or trafc parameters, contact your ISP or network services provider for information on what types of adjustment are available or possible
for your account. Your ISP may not support the class of service you want to use.
To adjust ATM QoS parameters, select one of the Service Categories listed here and type in the PCR value in the entry eld below. For the VBR service category,
an additional parameter (SCR) must also be dened.
UBR – Unspecied Bit Rate, this is the default category used for general-purpose Internet trafc where normal levels of packet loss and delay are acceptable. For
some applications or for multiple connection accounts, it may be desirable to specify the PCR.
CBR – Constant Bit Rate, usually used in circumstances where very low packet loss and very low Cell Delay Variable (CDV) are desirable.
VBR-rt – Real-time Variable Bit Rate. This models bursty trafc with specied peak and sustainable rates. Please note that when VBR-rt is specied, both PCR and
SCR are required (by ATM standards).
VBR-nrt – Non-real-time Variable Bit Rate, usually used when network trafc is characterized by bursts of packets at variable intervals, and some moderate packet
loss and delay is acceptable. This category is typically used for audio and video applications such as teleconferencing. The network must support QoS Class 2 to
use VBR-nrt.
Depending on which Service Category is selected, it will be necessary to change one or more of the following parameters:
PCR - Peak Cell Rate – The PCR is inversely related to the time interval between ATM cells. It is specied for all three service categories (UBR, CBR and VBR) in Kbps.
SCR - Sustainable Cell Rate – The SCR is dened for the VBR service category. This is the rate that can be sustained for “bursty”, on-off trafc sources. It is a function
of Maximum Burst Size (MBS) and the time interval (between cells).
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