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

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

• Masters Thesis Presentation 18.10.2005

Author Mika VäisänenSupervisor Prof. Raimo
KantolaInstructor Ph.D. Peter Jungner
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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

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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

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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,
  )

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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

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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

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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

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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

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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

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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

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Conclusions

• The method proven to be surprisingly accurate,
  despite its simple implementation
• PESQ-MOS differences lt 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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