ZyXEL Communications ALC1224, Руководство пользователя

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ALC1224 User’s Guide
Virtual Channel Management 4-5
Figure 4-2 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 4-3 TAT, CDVT and BT in Traffic Shaping