Information Technology Reference
In-Depth Information
Figure22-5 Policing—Checking Whether the Cell Flow Respects a Specific Traffic Contract
Router
Policing
takes place
These figures are not implying that traffic shaping and policing refer to a common contract and use a
similar algorithm. Misconfigured policing or shaping often leads to cells being discarded by the policer.
It also happens that even though shaping and policing are both set to the same values, policing starts
discarding cells. This is usually the result of a poor shaper or a misbehaving policer.
Understanding Variable Bit Rate Nonreal Time (VBR-NRT)
This chapter provides only an introduction to traffic shaping. If needed, more details can be found in the
Traffic Management specification available on the ATM Forum web site.
In ATM, traffic shaping works by inserting equal times between the cells. For example, if an
OC-3/STM-1 connection is 155 Mbps, only about 149 Mbps can be used for forwarding ATM cells.
(SONET/SDH has approximately 3 percent of overhead.) As a result, the max rate is 353.208 cells
(353.208
8 bits can fit in the OC-3c/STM-1 frames payload in a second). If a user requests a
connection of 74.5 Mbps (half the line rate), equal spaces of 2.83 usec will be inserted between each
cell. 2.83 usec is the time needed to send one cell at OC3c/STM-1 (1/353.208 sec). Because we requested
half the line rate, we can send one cell, wait an equal amount of time, and then start over again.
Now that we've described the generic approach, let's focus briefly on the most classic traffic requested,
variable bit-rate traffic (VBR) shaping as shown in Figure 22-6.
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