ZyXEL Communications Corporation P873HNUP51B Manuel D’Utilisation

Chapter 6 Broadband

P-873HNUP-51B User’s Guide

NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a host in 
a packet, for example, the source address of an outgoing packet, used within one network to a 
different IP address known within another network.

Traffic Shaping is an agreement between the carrier and the subscriber to regulate the average rate 
and fluctuations of data transmission over an ATM network. This agreement helps eliminate 
congestion, which is important for transmission of real time data such as audio and video 
connections.

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.