Table 12: Taf Impact On Shared Buffer Average Utilization Calculation - Alcatel-Lucent 7950 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:
SBAU
SBAU
SBU = The instantaneous shared buffer utilization
TAF = The time average factor
Table 12
instantaneous SBU and the previous SBAU (SBAU
(SBAU

Table 12: TAF Impact on Shared Buffer Average Utilization Calculation

TAF
0
1
2
3
4
Page 50

SBAU = SBU

n
= Shared buffer average utilization for event n
n
= Shared buffer average utilization for event (n-1)
n-1
shows the effect the allowed values of TAF have on the relative weighting of the
).
n
Equates
TAF
2
To
0 1
2
1 2
2
2 4
2
3 8
2
4 16
2


1 2
×  ×
----------- - -------------------- -
+ SBAU

n 1 –

TAF
2
n-1)
Shared Buffer
Instantaneous
Utilization Portion
1/1 (1)
1/2 (0.5)
1/4 (0.25)
1/8 (0.125)
1/16 (0.0625)
7950 XRS Quality of Service Guide

TAF
– 1


TAF
2
has on the calculating the current SBAU
Shared Buffer Average
Utilization Portion
0 (0)
1/2 (0.5)
3/4 (0.75)
7/8 (0.875)
15/16 (0.9375)