Metrics for monitoring a VOIP call can be obtained from any node in media path of the call flow . Essentially used for analysis via calculation and aggregation , and sometimes used for realtime performance tracking and rectification too .
Rating Factor (R-Factor) and Mean Opinion Score (MOS) are two commonly-used measurements of overall VoIP call quality.
R-Factor: A value derived from metrics such as latency, jitter, and packet loss per ITU‑T Recommendation G.107. It assess the quality-of-experience for VoIP calls on your network. Typical scores range from 50 (bad) to 90 (excellent).
- R factor of 90 , Mos is 4.3 ( Excellent )
- R factor 50 , Mos is 2.6 ( Bad)
MOS: It is derived from the R-Factor per ITU‑T Recommendation G.10 which measures VoIP call quality. PacketShaper measures MOS using a scale of 10-50. To convert to a standard MOS score (which uses a scale of 1-5), divide the PacketShaper MOS value by 10.
The International Telecommunication Union is the United Nations specialised agency in the field of telecommunications, information and communication technologies (ICTs).
TU Telecommunication Standardisation Sector is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardising telecommunications on a worldwide basis.
MOS ( Mean Opinion Score )
MOS is terminology for audio, video and audiovisual quality expressions as per ITU-T P.800.1. It refers to listening, talking or conversational quality, whether they originate from subjective or objective models.
It provides provisions for identifiers regarding the audio bandwidth, the type of interface (electrical or acoustical) and the video resolution too , such as
MOS-AVQE for audiovisual quality;
MOS-CQE is for estimated conversational quality;
MOS-LQE for listening quality;
MOS-TQE is used for talking quality;
MOS-VQE depicts video quality;
For Audio Signal Speech Quality/ AV
– N denotes audio signals upto narrow-band (300-3400 Hz)
– W is for audio signals upto wideband (50-7000 Hz)
– S for upto super-wideband (20-14000 Hz)
– F is obtained for fullband (10-20000 Hz)
For Listening quality LQO
- electrical measurement
performed at electrical interfaces only. In order to predict the listening quality as perceived by the user, assumptions for the terminals are made in terms of intermediate reference system (IRS) or corrected IRS frequency response. A sealed condition between the handset receiver and the user’s ear is assumed.
- acoustical measurement
performed at acoustical interfaces. In order to predict the listening quality as perceived by the user, this measurement includes the actual telephone set products provided by the manufacturer or vendor. In combination with the choice of the acoustical receiver in the laboratory test , there will be a more or less leaky condition between the handset’s receiver and the artificial ear.
Conversational Quality / CQ
Arithmetic mean value of subjective judgments on a 5-point ACR quality scale, is calculated.
Talking Quality / TQ
This describes the quality of a telephone call as it is perceived by the talking party only. Factors affecting TQ include echo signal , background noise , double talk etc. It is calculated based on the arithmetic mean value of judgments on a 5-point ACR quality scale.
Video Quality / VQ
To account for differentiation in perceived quality for mobile and fixed devices and to allow for proper handling of different use-cases as
– M for mobile screen such as a smartphone or tablet (approximately 25 cm or less)
– T for PC/TV monitors
It is calculated based on the arithmetic mean value of subjective judgments, typically on a 5-point quality scale
Audio Visual Quality / AVQ
Refers to quality of audio visual stream under corresponding networking conditions. It is also calculated based on the arithmetic mean value of judgments on a 5-point ACR quality scale.
Other parameters also contributing to VoIP metric Analysis
It is the time required for packets to travel from one end to another, in milliseconds.
If the sum of measured latency is 800 ms and the number of latency samples is 20, then the average latency is 40 ms.
Header of the RTP packets carry timestamps which later can also be used to calculate round-trip time.
packet loss percentage performed per RFC 3550 using RTP header sequence numbers.
The variation in the delay of received packets in a flow, measured by comparing the interval when RTP packets were sent to the interval at which they were received.
For instance, if packet #1 and packet #2 leave 30 milliseconds apart and arrive 50 milliseconds apart, then the jitter is 20 milliseconds.
Methods for objective and subjective assessment of speech and video quality.
Mapping R-value to calculate MOS
To map MOS from R value using above defined metrics , a standard formula is used. First the latency and jitter are added and defined value for computation time is also added , resulting in effective latency
effectiveLatency = latency + jitter * latencyImpact + compTime
Subtracting effective latency from defined R
R = 93 – (effectiveLatency / factorLatencyBased)
Calculate percentage of packet loss
R = R – (lostPackets * impact) MOS = ( (R - 60) * (100 – R) * 0.000007R) + 0.035R + 1)