Frame header based speech quality analysis method in a circuit switched media gateway
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Frame Header Based Speech Quality Analysis Method in a Circuit-Switched Media Gateway. Master’s Thesis Presentation 18.10.2005. Author: Mika Väisänen Supervisor: Prof. Raimo Kantola Instructor: Ph.D. Peter Jungner. Contents. Introduction Circuit-Switched Media Gateway Speech Coding

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Frame header based speech quality analysis method in a circuit switched media gateway

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


Contents
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


Introduction
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


Circuit switched media gateway cs mgw
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, …)


Speech coding
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


Iu and nb user plane protocols
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


Speech quality measurement
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


Estimation method development
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


Analysis of the method
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


Analysis of the method1
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


Conclusions
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