ZyXEL Communications P-660HN-T1H User Manual

 Chapter 6 WAN Setup

P-660HN-T1H User’s Guide

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 34   Example of Traffic Shaping

These are the basic ATM traffic classes defined by the ATM Forum Traffic 
Management 4.0 Specification. 

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.

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.