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Frame Header Speech Quality Analysis in Circuit-Switched Media Gateway

This Master's thesis presentation explores a method for estimating speech quality in UMTS Core Network by analyzing speech frame headers in a Circuit-Switched Media Gateway (CS-MGW). The method aims to help operators monitor real-time speech quality, addressing issues such as frame damage and loss affecting network performance. The study covers topics like speech coding, Iu/Nb User Plane Protocols, and objective quality measurement methods like PESQ. The analysis of the developed method shows promising accuracy in estimating speech quality, vital for maintaining customer satisfaction and network competitiveness. The research highlights the importance of efficient frame analysis for network operators and provides valuable insights into improving speech quality monitoring.

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Frame Header Speech Quality Analysis in Circuit-Switched Media Gateway

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  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

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