ZyXEL Communications P-660HN-TxA - Manual (Page 87)

Chapter 6 WAN Setup
P-660HN-TxA User’s Guide
87
Sustained Cell Rate (SCR) is the mean cell rate of each bursty traffic source. It
specifies the maximum average rate at which cells can be sent over the virtual
connection. SCR may not be greater than the PCR.
Maximum Burst Size (MBS) is the maximum number of cells that can be sent at
the PCR. After MBS is reached, cell rates fall below SCR until cell rate averages to
the SCR again. At this time, more cells (up to the MBS) can be sent at the PCR
again.
If the PCR, SCR or MBS is set to the default of "0", the system will assign a
maximum value that correlates to your upstream line rate.
The following figure illustrates the relationship between PCR, SCR and MBS.
Figure 35 Example of Traffic Shaping
6.5.1 ATM Traffic Classes
These are the basic ATM traffic classes defined by the ATM Forum Traffic
Management 4.0 Specification.
Constant Bit Rate (CBR)
Constant Bit Rate (CBR) provides fixed bandwidth that is always available even if
no data is being sent. CBR traffic is generally time-sensitive (doesn't tolerate
delay). CBR is used for connections that continuously require a specific amount of
bandwidth. A PCR is specified and if traffic exceeds this rate, cells may be
dropped. Examples of connections that need CBR would be high-resolution video
and voice.
Variable Bit Rate (VBR)
The Variable Bit Rate (VBR) ATM traffic class is used with bursty connections.
Connections that use the Variable Bit Rate (VBR) traffic class can be grouped into
real time (VBR-RT) or non-real time (VBR-nRT) connections.