ZyXEL P-663H-51 Guia Do Utilizador

Chapter 5 WAN Setup

P-663H-51 User’s Guide

Note: Traffic shaping controls outgoing (upstream) traffic, not incoming 

(downstream).

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.

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 21   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 realtime or non realtime  
connections.