Tuning The Shared Buffer Utilization Calculation - Alcatel-Lucent 7450 Quality Of Service Manual

QoS Policies

Tuning the Shared Buffer Utilization Calculation

The router allows tuning the calculation of the Shared Buffer Average Utilization (SBAU)
after assigning buffers for a packet entering a queue as used by the RED slopes to calculate a
packet's drop probability. The router implements a time average factor (TAF) parameter in
the buffer policy which determines the contribution of the historical shared buffer utilization
and the instantaneous Shared Buffer Utilization (SBU) in calculating the SBAU. The TAF
defines a weighting exponent used to determine the portion of the shared buffer instantaneous
utilization and the previous shared buffer average utilization used to calculate the new shared
buffer average utilization. To derive the new shared buffer average utilization, the buffer pool
takes a portion of the previous shared buffer average and adds it to the inverse portion of the
instantaneous shared buffer utilization (SBU). The formula used to calculated the average
shared buffer utilization is:
where:
SBAUn = Shared buffer average utilization for event n
SBAUn-1 = Shared buffer average utilization for event (n-1)
SBU = The instantaneous shared buffer utilization
TAF = The time average factor
Table 14
instantaneous SBU and the previous SBAU (SBAUn-1) has on the calculating the current
SBAU (SBAUn).
62
shows the effect the allowed values of TAF have on the relative weighting of the
Table 14: TAF Impact on Shared Buffer Average Utilization Calculation
TAF Equates
TAF
2
To
0 1
0
2
1 1 2
2
Figure 8: Equation
Shared Buffer
Instantaneous
Utilization Portion
1/1 (1)
1/2 (0.5)
Shared Buffer Average
Utilization Portion
0 (0)
1/2 (0.5)
Quality of Service Guide