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APPENDIX A
Traffic Shaping
Traffic Shaping is an agreement between the carrier and the subscriber to regulate the
average rate and "burstiness" or fluctuation of data transmission over an ATM network. This
agreement helps eliminate congestion, which is important for transmission of real time data
such as audio and video connections.
Peak Cell Rate (PCR) is the maximum rate at which the sender can send cells. This
parameter may be lower (but not higher) than the maximum line speed. 1 ATM cell is 53
bytes (424 bits), so a maximum speed of 832 Kbps gives a maximum PCR of 1962 cells/sec.
This rate is not guaranteed because it is dependent on the line speed.
Sustained Cell Rate (SCR) is the mean cell rate of a bursty, on-off traffic source that can be
sent at the peak rate, and a parameter for burst-type traffic. SCR may not be greater than the
PCR; the system default is 0 cells/sec.
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. The following figure
illustrates the relationship between PCR, SCR and MBS.
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