A Generic Algorithm for Midcall Audio Codec Switching

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Haytham Assem, Mohamed Adel, Brendan Jennings, David Malone, Jonathan Dunne and Pat

O’Sullivan

TSSG, Waterford Institute of Technology,Ireland

Hamilton Institute, National University of Ireland Maynooth, Ireland

IBM Software Lab, Dublin, Ireland

A Generic Algorithm for

Mid-call Audio Codec Switching

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Outline

• Voice-over-IP

• Measuring Quality-of-Experience of VoIP

• Impact of Codec Switching

• Codec Switching Algorithm

• Experimental Analysis

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Outline

• _{Voice-over-IP}

• Measuring Quality-of-Experience of VoIP

• Impact of Codec Switching

• Codec Switching Algorithm

• Experimental Analysis

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Voice-over-IP

• Rapid growth of IP networks provided unusual means of

communications.VoIP has evolved rapidly, providing real-time voice and video communications between users through the existing IP network in a cheap cost.

• VoIP has gained wide acceptance by general Internet users and is increasingly important in the enterprise communications sector.

• It has become essential to VoIP providers to have testing frameworks to assess users' QoE to provide the best possible service.

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Outline

• _{Voice-over-IP}

• Measuring Quality-of-Experience of VoIP

• Impact of Codec Switching

• Codec Switching Algorithm

• Experimental Analysis

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Quality-of-Experience of VoIP

• Achieving voice quality levels for VoIP remains a significant challenge, as IP networks typically do not guarantee delay, packet loss, jitter and bandwidth levels.

• Packet loss in the IP network is considered one of the most important factors that cause degradation in the overall voice call quality.

• The maximum quality that can be achieved differs from codec to another under different packet loss rates.

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Measuring Audio Call Quality

individually listening to an audio stream of several seconds and rate the audio quality on the scale of 1 (Poor) to 5 (Excellent), with these ratings being used to form a single Mean Opinion Score (MOS).

• Objective testing provides an alternative for measuring voice quality. It is classified into:

• Intrusive (e.g. PESQ)

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Deriving E-Model for non-ITU

codecs

• E-Model does not provide codec impairment values for widely used non-ITU codecs like SILK, ILBC, SPEEX, and GSM.

• We used PESQ to estimate MOS for the above codecs under different packet loss rates ranging from 0-20 %.

• Non-linear regression model, with parameters a, b, and c is driven for each codec.

I

e-effrepresents the codec impairment value.

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Measuring Audio Call Quality

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Outline

• _{Voice-over-IP}

• Measuring Quality-of-Experience of VoIP

• Impact of Codec Switching

• Codec Switching Algorithm

• Experimental Analysis

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Codecs' Performance

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Codecs' Performance

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Switch-over Gaps

switch-over gap. It the time taken between sending the RE-INVITE message from the sender side and receiving the ACK at the receiver side

indicating the start of transmission with the new codec.

• Switch-over gap indicates the response time to switch to another codec.

• At high packet loss rates, the RE-INVITE message will be at a higher probability of being lost, which will cause multiple retransmissions, and the same also will happen for the 200OK and ACK messages, therefore the switch-over gap will increase more.

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Switch-over Gaps

We focus our algorithm on switching codecs in that region (0-10% packet loss) in order to minimize the switch-over gap( avg of 0.5 sec) in order to increase the responsiveness of our algorithm.

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Frequent Codec Switching

• Frequent switching of codecs during a session could cause degradation in the overall call quality.

• In order to measure the only degradation in the call quality as a result of increasing the number of switches, we selected pairs of codecs which have nearly the same or almost same performance.

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Frequent Codec Switching

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Outline

• _{Voice-over-IP}

• Measuring Quality-of-Experience of VoIP

• Impact of Codec Switching

• Codec Switching Algorithm

• Experimental Analysis

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Codec Switching Algorithm

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Outline

• _{Voice-over-IP}

• Measuring Quality-of-Experience of VoIP

• Impact of Codec Switching

• Codec Switching Algorithm

• _{Experimental Analysis}

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Experimental Analysis

one default codec to start the call with.

• For our experiments we played a sample audio file for 3mins, with the potential for switching a codec being assessed every 10s.

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Experimental Analysis

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Outline

• _{Voice-over-IP}

• Measuring Quality-of-Experience of VoIP

• Impact of Codec Switching

• Codec Switching Algorithm

• _{Experimental Analysis}

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Conclusion & Future Work

• Switching codecs during an ongoing voice session can improve user’s perceived quality-of-experience due to the fact that different codecs behave differently under different packet loss conditions in the

network.

• Codec Switching has a slightly negative impact which has to be taken into account when designing codec switching algorithm.

• We intend to extend our algorithm to support wideband audio codecs by applying the newly developed POLQA objective testing method. • We are planning to improve our algorithm by studying loss patterns to

assess if the frequency and distribution of losses affect codecs’ quality differently.

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