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vaisanen 181005

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Published on October 7, 2007

Author: FunnyGuy

Source: authorstream.com

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

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