ZyXEL Communications IES-612-51A User Manual

 Chapter 14 xDSL Profiles Setup

IES-612-51A User’s Guide

Peak Cell Rate (PCR) is the maximum rate at which the sender can send cells. This parameter 
may be lower (but not higher) than the maximum line speed. 1 ATM cell is 53 bytes (424 bits), 
so a maximum speed of 832Kbps gives a maximum PCR of 1962 cells/sec. This rate is not 
guaranteed because it is dependent on the line speed. 

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. 

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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 56   PCR, SCR and MBS in Traffic Shaping

Cell Delay Variation Tolerance (CDVT) is the accepted tolerance of the difference between a 
cell’s transfer delay and the expected transfer delay. CDVT controls the time scale over which 
the PCR is enforced. CDVT is used to determine if a cell arrived too early in relation to PCR.