ZyXEL Communications MSC1000G Series, User Manual

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Chapter 11 Profile Screens
Management Switch Card User’s Guide
361
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.
11.12.2.2 Sustained Cell Rate (SCR)
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.
11.12.2.3 Maximum Burst Size (MBS)
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.
Note: 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 230
PCR, SCR and MBS in Traffic Shaping
11.12.2.4 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.
11.12.2.5 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.
11.12.2.6 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.