Frame Header Based Speech Quality Analysis Method in a Circuit-Switched Media Gateway

1. Frame Header Based Speech Quality Analysis Methodin a Circuit-Switched Media Gateway Master’s Thesis Presentation 18.10.2005 Author: Mika VäisänenSupervisor: Prof. Raimo KantolaInstructor: Ph.D. Peter Jungner

2. Contents • Introduction • Circuit-Switched Media Gateway • Speech Coding • Iu and Nb User Plane Protocols • Speech Quality Measurement • Estimation Method development • Analysis of the Method • Conclusions

3. Introduction • Background • On UMTS networks coded speech is transported in frames • On ideal situation only the used speech coding method degrades the speech quality of a call • In practise, frames are damaged on air-interface and lost on core network congestion • Problem • Operator may not know, how customers are perceiving the quality of the network • Operator will lose customers, if speech quality in the network drops • Operator must be able to monitor the speech quality in the network in real time • Objectives • To develop a method that can estimate speech quality of calls in UMTS Core Network by analysing only the speech frame headers

4. Circuit-Switched Media Gateway (CS-MGW) • Adapts different Access Networks to the Core Network • Main functions: • Media conversion (ATM, IP, TDM) • Bearer control (Resource reservation) • Payload processing (Transcoding, echo cancelling, …)

5. Speech Coding • Adaptive Multi-Rate (AMR) coding used in UTRAN • Variable bit-rate modes from 4.75 to 12.2 kbps • Source Controlled Rate of operation • During silence only Silence Descriptor (SID) frames are sent with low bit-rate • Uses efficient error concealment • Lost or damaged frames are “faded away” • Frame substitution and muting • AMR end-to-end = Transcoder Free Operation (TrFO) • Pulse Code Modulation (PCM) possibly used in CN • Compressed, 64 kbps • No error concealment • AMR-PCM-AMR = Coder tandeming, transcoding

6. Iu and Nb User Plane Protocols • Speech is carried in User Plane frames • 1 AMR frame in each Iu/Nb frame • 40 PCM samples in each Nb frame • Besides speech the Iu/Nb frames contain information • Frame numbering to detect lost frames • Frame Quality Classification (FQC) • Information of the frame type (AMR bit-rate, SPEECH/SID) • Transcoding in Tandem call cases re-creates the frame stream • All information regarding quality in the frame headers is lost

7. Speech Quality Measurement • Listening tests • Absolute Category Rating (ACR), scale 1-5 • Mean Opinion Score (MOS) • Objective methods • Emulate listening tests • Speech signal based • Resource consuming • Perceptual Evaluation of Speech Quality (PESQ) • PESQ score, ranging from -0.5 to 4.5. • Correlation against listening tests 0.935. • Parameter based • Light, but not as accurate • ITU E-Model • PsyVoIP, VQMon

8. Estimation Method Development • Establish a model between frame loss/damage and speech quality • Frame losses and damages in simulated environment • Lost SID frames ignored, because they are 100 times less important than speech frames • Speech quality analysis with PESQ • Find out a way to determine types of lost frames • In PCM case simple, as all frames can be considered equal. • In AMR case SID frames complicate the determination • Create a method implementation to be run in CS-MGW

9. Analysis of the Method • AMR TrFO case (AMR 12.2 kbps all the way) • Correlation of 0.90 was established between the method and real PESQ scores • Mean estimation error 0.14 PESQ-MOS units

10. Analysis of the Method • Tandem case (AMR 12.2 - PCM – AMR 12.2) • Correlation of 0.83 was established between the method and real PESQ scores • Mean estimation error 0.19 PESQ-MOS units

11. Conclusions • The method proven to be surprisingly accurate, despite its simple implementation • PESQ-MOS differences < 0.5 are barely audible • Being able to determine the frame content (silence/speech) helps to improve the estimation • Ideal solution for operators using a leased RAN • In addition to price, also speech quality can be used to compare alternative networks