ICPIF value
computation with the Cisco NX-OS software is based primarily on the two main
factors that can impair voice quality: delayed packets and lost packets.
Because packet delay and packet loss can be measured by IP SLAs, the ICPIF
formula,
Icpif=Io+Iq+Idte+Idd+Ie-A , is
simplified by assuming that the values of
Io,
Iq, and
Idte are zero,
as follows:
Total
Impairment
Factor
(Icpif)
=
Delay
Impairment
Factor
(Idd)
+
Equipment
Impairment
Factor
(Ie)
—
Expectation/Advantage
Factor
(A)
The ICPIF value is
computed by adding a Delay Impairment Factor, which is based on a measurement
of delayed packets, and an Equipment Impairment Factor, which is based on a
measurement of lost packets. From this sum of the total impairments measured in
the network, an impairment variable (the Expectation Factor) is subtracted to
yield the ICPIF.
Cisco gateways use
this formula to calculate the ICPIF for received VoIP data streams.
Delay
Impairment Factor
The Delay
Impairment Factor (Idd
) is a number based on two values. One value is fixed and is derived using the
static values (as defined in the ITU standards) for Codec Delay, Look Ahead
Delay, and Digital Signal Processing (DSP) Delay. The second value is a
variable and is based on the measured one-way delay (round-trip time
measurement divided by 2). The one-way delay value is mapped to a number using
a mapping table that is based on a G.107 (2002 version) analytic expression.
The following table
shows sample correspondences between the one-way delay measured by IP SLAs and
Delay Impairment Factor values.
Table 3. Sample
Correspondence of One-Way Delay to ICPIF Delay Impairment
|
One-Way
Delay (ms)
|
Delay
Impairment Factor
|
|
50
|
1
|
|
100
|
2
|
|
150
|
4
|
|
200
|
7
|
Equipment
Impairment Factor
The Equipment
Impairment Factor (Ie ) is a number
based on the amount of measured packet loss. The amount of measured packet
loss, expressed as a percentage of total number of packets sent, corresponds
with an Equipment Impairment Factor that is defined by the codec.
The following table
shows sample correspondences between the packet loss measured by IP SLAs and
Equipment Impairment Factor values corresponding with each other.
Table 4. Sample
Correspondence of Measured Packet Loss to ICPIF Equipment Impairment
|
Packet Loss
(as a percentage of total number of packets sent)
|
Equipment
Impairment Value for PCM (G.711) Codecs
|
Equipment
Impairment Value for the CS-ACELP (G.729A) Codec
|
|
2%
|
12
|
20
|
|
4%
|
22
|
30
|
|
6%
|
28
|
38
|
|
8%
|
32
|
42
|
Expectation
Factor
The Expectation
Factor, also called the Advantage Factor (A ), represents the expectation that users might
accept some degradation in quality in return for ease of access. For example, a
mobile phone user in a hard-to-reach location might expect that the connection
quality will not be as good as a traditional land-line connection. This
variable is also called the Advantage Factor (short for Access Advantage
Factor) because it attempts to balance an increased access advantage against a
decline in voice quality.
The table below,
adapted from ITU-T Rec. G.113, defines a set of provisional maximum values for
A in terms
of the service provided.
Table 5. Advantage
Factor Recommended Maximum Values
|
Communication Service
|
Advantage/Expectation Factor:
Maximum
value of A
|
|
Conventional wire-line (land-line)
|
0
|
|
Mobility
(cellular connections) within a building
|
5
|
|
Mobility
within a geographical area or moving in a vehicle
|
10
|
|
Access to
hard-to-reach location; (for example, via multi-hop satellite connections)
|
20
|
These values are
only suggestions. To be meaningful, you should use the factor
A and its
selected value in a specific application consistently in any planning model
that you adopt. However, the values in the table should be considered as the
absolute upper limits for
A.
The default
Advantage Factor for IP SLAs VoIP UDP jitter operations is always zero.
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