ZyXEL Communications IES-612-51A User Manual

Chapter 14 xDSL Profiles Setup

IES-612-51A User’s Guide

Burst Tolerance (BT) is the maximum number of cells that the port is guaranteed to handle 
without any discards. BT controls the time scale over which the SCR is enforced. BT is used to 
determine if a cell arrived too early in relation to SCR. Use this formula to calculate BT: (MBS 
– 1) x (1 / SCR – 1 / PCR) = BT.

The Theoretical Arrival Time (TAT) is when the next cell (in an ATM connection’s stream of 
cells) is expected to arrive. TAT is calculated based on the PCR or SCR. 

The following figure illustrates the relationship between TAT, CDVT and BT. If a cell arrives 
at time A, then according to PCR or SCR, the next cell is expected to arrive at time B. If the 
next cell arrives earlier than time C, it is discarded or tagged for not complying with the TAT. 
Time C is calculated based on the CDVT or BT.

Figure 57   TAT, CDVT and BT in Traffic Shaping

Upstream policing is an agreement between the carrier and the subscriber to regulate the 
average rate and fluctuations of data transmission coming from the subscriber's device to the 
IES-612-51A.

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Upstream policing controls incoming (upstream) traffic, not outgoing 
(downstream).

The ATM traffic classes and parameters are identical with downstream shaping.

Upstream policing can control the upstream incoming traffic rate on specific PVCs. Upstream 
ATM cell traffic that violates the policing profile will be discarded. Traffic shaping must also 
be enabled on the subscriber's device in order to use upstream policing. If a subscriber 
attempts to enlarge his device's PVC shaping parameters in order to get more upstream traffic 
bandwidth, it will violate the IES-612-51A's upstream policing profile and the traffic will be 
discarded. Operators can use this feature to prevent subscribers from changing their device 
settings.