Copyright Telchemy Inc 2001 Embedded Passive Monitoring of Voice over IP Service Quality Alan Clark Telchemy, Inc Atlanta, GA
Copyright Telchemy Inc 2001 Embedded Passive Monitoring IP Network Gateway Monitoring Agent embedded into VoIP Gateway QoS metrics
Copyright Telchemy Inc 2001 Active vs Passive Testing Active Measurement –Establish looped back test call –Send recorded speech file –Compare sent and received files (PSQM/PESQ) Passive Monitoring –Measure characteristics of live calls –Determine Transmission Quality metric –Estimate Speech Quality metrics
Copyright Telchemy Inc 2001 The E Model “Mouth to ear” transmission quality measurement Produces an “R” factor typically in the range 50 (bad) -95 (good) R factor can be related to MOS score, Terminate Early (TME) etc. ITU G.107/ G.108 and ETSI ETR250
Copyright Telchemy Inc 2001 E Model R = Ro - Is - Id - Ie + A Base R value - Noise level Impairments that occur simultaneously with speech - received speech level - sidetone level - quantization noise Impairments that are delayed with respect to speech - talker echo - listener echo - round trip delay Equipment Impairment Factor - CODEC - multiplexing effects Advantage factor
Copyright Telchemy Inc 2001 E Model Delay, measured using RTCP R Factor E Model IeIe Packet Loss Jitter Codec type Loss Model Jitter Model Codec Model
Copyright Telchemy Inc 2001 R Factor vs MOS R FactorMOS
Copyright Telchemy Inc 2001 Impairments can vary with time Interaction of streams in IP network causes many impairments to vary with time Adaptive jitter buffer Call re-routing CODEC type changed during call
Copyright Telchemy Inc 2001 Measuring Packet Loss Arriving RTP packets Jitter buffer Late packets discarded Monitor jitter and packet loss after jitter buffer CODEC RTCP statistics report every 5 seconds
Copyright Telchemy Inc 2001 Burst vs average loss Burst of packet loss Zero packet loss Non-bursty packet loss Is not the same as
Copyright Telchemy Inc 2001 Effects of packet loss burst Transmission Quality High Low 4-5 seconds seconds User perceived quality profile
Copyright Telchemy Inc 2001 Loss Model - Markov model 3 Lost 1 Rcvd P 13 P 22 P 11 P 31 2 Rcvd P 23 P 32 4 Lost P 14 P 41 Burst state Gap state
Copyright Telchemy Inc 2001 Frame loss event driven Burst of packet loss Zero packet loss Lost( )
Copyright Telchemy Inc 2001 Loss Model - Markov model 3 Lost 1 Rcvd P 13 P 22 P 11 P 31 2 Rcvd P 23 P 32 4 Lost P 14 P 41 Burst state Gap state Model parameters reconstructed at end of call
Copyright Telchemy Inc 2001 Loss Model - mapping loss to I e Curve is CODEC dependant
Copyright Telchemy Inc 2001 Determining QoS metrics 1. Determine “good” and “bad” state Ie Factor
Copyright Telchemy Inc 2001 Determining QoS metrics t = 5 t = Determine “good” and “bad” state Ie Factor 2. Estimate Instantaneous R Factor for each state
Copyright Telchemy Inc 2001 Determining QoS metrics t = 5 t = Determine “good” and “bad” state Ie Factor 2. Estimate Instantaneous R Factor for each state 3. Determine average Ie
Copyright Telchemy Inc 2001 Measuring Delay CODEC Accumulate frame Encode Transmission Jitter buffer Decode RTCP exchange
Copyright Telchemy Inc 2001 Delay Model 175 mS “knee” End to end delay (mS) R Factor Reduction
Copyright Telchemy Inc 2001 “Recency” effect “Good” 4.3MOS “Bad” 1.8 MOS (3dB SNR) MOS 3.82 MOS 3.28 MOS 3.18 Source AT&T T1A1.7/ second call
Copyright Telchemy Inc 2001 Estimation of recency effect Delay since last significant burst Effects decay over seconds Average for call
Copyright Telchemy Inc 2001 Integration with VoIP SMS VQmon SNMP Set, Get, Trap End of call msg (DRQ) CDR Service Mgt System Network Mgt System
Copyright Telchemy Inc 2001 H.323/H.225 based reporting H.225 Disengage Request (DRQ) nonStandardData field R end INTEGER (0..100) R av INTEGER (0..100) R min INTEGER (0..100) R max INTEGER (0..100)
Copyright Telchemy Inc 2001 Embedded Passive Monitoring RTP stream CODECCODEC IP Network CODECCODEC 1 Measure Packet Loss Distribution, Jitter, Delay, CODEC type 2 Produce a transmission quality rating (R Factor) using extension to the E-Model 3 Map R Factor to an estimated MOS Score