D-Link DSL-G604T User Manual page 45

The table below provides a description of the ATM settings for traffic shaping.
ATM Parameters
Service Category
PCR
SCR
CDVT
MBS
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DSL-G604T Generation II ADSL2+ Wireless Modem Router
Description
The ATM settings allow the user to adjust ATM Quality of Service (QoS) or traffic parameters to suit
specific traffic 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 traffic 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 field below. For the VBR service category, an additional parametre (SCR) must
also be defined.
UBR
– Unspecified Bit Rate, this is the default category used for general-purpose Internet traffic
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-nt
– Real-time Variable Bit Rate. This models bursty traffic with specified peak and sustainable
rates. Please note that when VBR-rt is specified, both PCR and SCR are required (by ATM
standards).
VBR-nrt
– Non-real-time Variable Bit Rate, usually used when network traffic 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.
Peak Cell Rate – The PCR is inversely related to the time interval between ATM cells. It is
specified for all three service categories (UBR, CBR and VBR) in Kbps.
Sustainable Cell Rate – The SCR is defined for the VBR service category. This is the rate that
can be sustained for "bursty", on-off traffic sources. It is a function of Maximum Burst Size
(MBS) and the time interval (between cells).
Cell Delay Variation Tolerance – CDVT is a measure of the cell clumping phenomenon by which
cells are delayed in the network and are
clumped together and arrive at a system at a faster rate than negotiated.
Increasing the CDVT creates greater bucket depth.
Maximum Burst Size – The MBS is the maximum number of bytes that can be sent
continuously from the source to the destination dropping any packets. Some ATM providers set
the MBS and CDVT very low and adjust up if problems occur.
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