ZyXEL Communications Prestige 861H-G1, Руководство пользователя

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Prestige 861H User’s Guide
WAN Setup 7-3
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, MCR and MBS.
Figure 7-1 PCR, SCR, MCR and MBS in Traffic Shaping
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.
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.
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 7-2 TAT, CDVT and BT in Traffic Shaping