ZyXEL Communications AAM1212, User Manual

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AAM1212 User’s Guide
98 Chapter 8 ADSL Port Setup
Figure 47 PCR, SCR, MCR and MBS in Traffic Shaping
8.12.2.5 Cell Delay Variation Tolerance (CDVT)
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.
8.12.2.6 Burst Tolerance (BT)
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.
8.12.2.7 Theoretical Arrival Time (TAT)
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 48 TAT, CDVT and BT in Traffic Shaping