ENA Cisco IOS IP SLA can simulate VoIP traffic across the IP network, using 3 standard CODECs, and then measures network performance. ENA reports on consistent voice quality scores (MOS and IPCIF) between Cisco IOS devices.
ENA Cisco IOS IP SLA’s UDP Jitter VoIP solution is useful, for example, as a due diligence tool for administrators determining whether the network is ready for a full VoIP installation. The metrics collected by the UDP Jitter VoIP operation contribute to ICPIF and MOS measurements:
- ICPIF (Impairment/Calculated Planning Impairment Factor) attempts to quantify the key impairments to voice quality that are encountered in the network. ICPIF values are expressed in a typical range of 5 (very low impairment) to 55 (very high impairment). ICPIF values numerically less than 20 are generally considered adequate. While intended to be an objective measure of voice quality, the ICPIF value is also used to predict the subjective effect of combinations of impairments.
- MOS (Mean Opinion Score) is a common benchmark used to determine the quality of sound produced by specific CODECs. A wide range of listeners have judged the quality of voice samples sent using particular CODECs, on a scale of 1 (poor quality) to 5 (excellent quality). The opinion scores are averaged to provide the mean for each sample.
Synchronizing device clocks
Unidirectional latency, ICPIF and VoIP metrics all require clock synchronization between the source and destination devices. When synchronization fails, these metrics are not returned and gaps appear in the data. Use NTP (Network Time Protocol) to synchronize device clocks.
ICPIF identifies and rates 5 types of impairment and a user expectation factor:
Icpif = Io + Iq + Idte + Idd + Ie - A
- Io - impairments caused by non-optimal loudness rating or high noise.
- Iq - impairments caused by PCM type quantizing distortion.
- Idte - impairments caused by talker echo.
- Idd - impairments caused by one-way transmission times (one-way latency).
- Ie - impairments caused by equipment effects, e.g. type of CODEC, packet loss.
- A - advantage factor (user expectation factor).
ICPIF values range from 0 to 55, with 0 considered a perfect score:
|ICPIF Upper Limit||Speech Communication Quality|
|45||exceptionally limiting case.|
|55||customers likely to react strongly.|
Ie - equipment impairment factors
The G.711 CODEC delivers a better quality of service than G.729a, but both operate within a packet environment. The actual impairment depends on the CODEC used and packet loss. The greater the packet loss, the greater the impairment. Cisco has measured voice quality with PSQM (ITU P.861) at discrete packet loss levels. ENA uses this scale to translate packet loss levels for given CODECs into voice distortion values.
|Packet Loss (%)||G.711 Codec||G.729a Codec|
Idd - transmission delay impairment
The Idd value represents one-way latency. Times are translated into values using the table taken from the ITU-T recommendation G.113.
Delivery of service is about matching customer perception with their expectation of voice quality. Expectations vary according to the communication service, e.g. landline users have higher expectations than mobile phone users. The advantage factor allows this expectation to be included in the ICPIF calculation. The default advantage factor for IP SLAs VoIP UDP Jitter operations is always zero.
|Communication Service||Maximum A|
|Wireless (in building)||5|
|Cellular outside/moving vehicle||10|
|Difficult location, multi-hop satellite link||20|
MOS is used by knowing the used CODEC, monitoring the level of transmission impairment, and then deriving the MOS value. This MOS value indicates the user's subjective experience of voice transmission.
Combining ICPIF and MOS
Combining ICPIF and MOS measurements involves combining both objecting and subjectiev VoIP quality of service measurements.
|4-13||3-4||just perceptible, not annoying.|
|14-23||2-3||perceptible, slightly annoying.|
|24-33||1-2||annoying, not objectionable.|
|34-43||0-1||very annoying, objectionable.|
IP SLA ICPIF and MOS events and incidents in ENA
There are 4 events associated with ICPIF and MOS:
- IP SLA Low MOS Incident
- IP SLA Problem Incident
Please see this article for help and information on how to edit a device's IP SLA threshold.