Hi all,
I tried to understand how IP SLA Manager calculate MOS but I guet incoherent results:
1) The MOS calculation is based on the The Calculated Planning Impairment Factor (ICPIF):
Icpif = Io + Iq + Idte + Idd + Ie - A
2) For an IP network (Io=0, Iq=0, Idte=0 and A=0) this formula is reduced to:
Icpif = Idd + Ie where:
- Idd represents impairments caused by one-way transmission times (one-way delay)
- Ie represents impairments caused by equipment effects, such as the type of codec used for the call and packet loss
3) Idd: This parameter depends on one way delay
Table 4 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 |
4) Ie: This parameter depends on Packets loss:
Table 5 Sample Correspondence of Measured Packet Loss to ICPIF Equipment Impairment Packet Loss
(as a percentage of total number of packets sent) | Equipment Impairment Factor for PCM (G.711) Codecs | Equipment Impairment Factor for the CS-ACELP (G.729A) Codec |
|---|
2% | 12 | 20 |
4% | 22 | 30 |
6% | 28 | 38 |
8% | 32 | 42 |
5) MOS and ICPIF correspondance:
Table 7 Correspondence of ICPIF Values to MOS Values ICPIF Range | MOS | Quality Category |
|---|
0 - 3 | 5 | Best |
4 - 13 | 4 | High |
14 - 23 | 3 | Medium |
24 - 33 | 2 | Low |
34 - 43 | 1 | Poor |
6) The incoherence:
- In my network all one way delay is less than 50 ms (about 10 ms), so Idd= 0 (see table in point 3)
Scenario1:
When there is no paquets loss I suppose that Ie=0 so Icpif = Idd + Ie = 0+0= 0
then MOS= 5
But in IPSLA manager I guet MOS = 4,34 !!!!!
Scenario2:
When there is paquets loss about 50%, the Ie parameter is greater than 32 (see table in point 4)
so Icpif = Idd + Ie > 32
then the must be between 1 and 3
But in IPSLA manager I guet MOS = 3,5 !!!!!
For more details see this URL
http://www.cisco.com/en/US/docs/ios/12_4/ip_sla/configuration/guide/hsvoipj.html
Do you have any explanation to this incoherence ?
Thanks
