ATM Connections 7-23
Traffic Policing Examples
Traffic Policing Examples
Traffic Policing, also known as Usage Parameter Control (UPC), is implemented using either an
ATM Forum single or dual-leaky bucket algorithm. The buckets represent a GCRA (Generic Cell
Rate Algorithm) defined by two parameters:
• Rate (where I, expected arrival interval is defined as 1/Rate)
• Deviation (L)
If the cells are clumped too closely together, they are non-compliant and are tagged or discarded as
applicable. If other cells arrive on time or after their expected arrival time, they are compliant, but
three is no accrued credit.
Dual-Leaky Bucket (An Analogy)
A GCRA viewpoint is as follows:
• For a stream of cells in an ATM connection, the cell compliance is based on the theoretical arrival
time (TAT).
• The next TAT should be the time of arrival of the last compliant cell plus the expected arrival
interval (I) where I = 1/rate.
• If the next cell arrives before the new TAT, it must arrive no earlier than new TAT - CDVT to be
compliant.
• If the next cell arrives after the new TAT, it is compliant, but there is no accrued credit.
CBR Traffic Policing Examples
CBR traffic is expected to be at a constant bit rate, have low jitter, and is configured for a constant
rate equal to Peak Cell Rate (PCR). The connection is expected to be always at peak rate.
When a connection is added, a VPI.VCI address is assigned, and the UPC parameters are configured
for the connection. For each cell in an ATM stream seeking admission to the network, the VPI.VCI
addresses are verified and each cell is checked for compliance with the UPC parameters. The CBR
cells are not enqueued, but are processed by the policing function and then sent to the network unless
discarded.
For CBR, traffic policing is based on:
• Bucket 1
— PCR(0+1), Peak Cell Rate
— CDVT(0+1), Cell Delay Variation
The CBR connection may be configured with policing selected as either 4 or 5. With policing set to
5, there is no policing. With policing set to 4, there is single leaky bucket PCR policing as shown in
Figure 7-11. The single leaky bucket polices the PCR compliance of all cells seeking admission to
the network, both those with CLP = 0 and those with CLP =1. Cells seeking admission to the network
with CLP set equal to 1 may have either encountered congestion along the user’s network or may
have lower importance to the user and have been designated as eligible for discard in the case
congestion is encountered. If the bucket depth CDVT (0+1) limit is exceeded, it discards all cells
seeking admission. It does not tag cells. If leaky bucket 1 is not full, all cells (CLP =0 and CLP=1)
are admitted to the network.
