Page 1 The department of Information & Communications Engineering Dong-uk, kim [email protected] A...

The department of Information & Communications Engineering

Dong-uk, [email protected]

A Survey of Packet Loss Recovery A Survey of Packet Loss Recovery Techniques for Streaming AudioTechniques for Streaming Audio- Colin Perkins, Orion hodson, and Vicky hardman - - Colin Perkins, Orion hodson, and Vicky hardman - Univ. college LondonUniv. college London IEEE Networks, Sep/Oct, 1998IEEE Networks, Sep/Oct, 1998

Table of Contents (1/1)

Overview

Multicast Channel Characteristics

Loss Repair techniques

Conclusion (Recommends)

Table of Contents (1/2)

The quality / complexity trade off

1. Splicing2. Silence substitution3. Noise substitution4. Repetition5. Repetition with Fading

The use of packet repetition with fading is recommended as offering a good compromise between achieved quality and excessive complexity

6. Waveform Substitution7. Pitch waveform substitution8. Time-scale modification9. Interpolation of Transmitted

state10.Model Based Recovery

Multicast Channel characteristics (1/2)

Indirection – Sender does not know the set of hosts which will receive a

packet

Multicast Channel characteristics (2/2)

Relatively high latency

Univ. Origon Univ. London

Forward Error Correction (Media-Independent)

Parity Code

Reed Solomon Code– original message is split into fixed length blocks – each block is split into m bit symbols

• RS(n,k)• encoder takes k data symbols of m bits each, appends n - k parity

symbols, and produces a code word of n symbols ( each of m bits).

Forward Error Correction (Media-Specific)

Primary encoding– First transmitted copy of the audio data

Secondary encoding– Subsequent transmissions

Congestion Control

The FEC repair data to a media stream is an effective means by which that stream maybe protected against packet loss – Should aware that

• Addition of large amounts of repair data will increase network congestion and hence packet loss

Network heterogeneity cause – Different sets of receivers

• Low-capa regions : Congestions• High-capa regions: underutilized

Need layered encoding

Interleaving

Retransmission

SRM (Scalable reliable multicast)– A member of an session detects loss, will wait a random time

(determined by distance from sender) and then request repair packet

Are not generally suitable for streaming media such as audio– They do not bound the transmission dealy – TCP has similar reasons

Error Concealment

Producing a replacement for a lost packet which is similar to the original– These technique work for

• Relatively small loss rates ( < 15 percent)• Small packets (4~40ms)• Loss length approaches the length of phoneme(5~100ms)

– Insertion based

– Interpolation based

– Regeneration based

Insertion based Repair

Splicing– No gap is left due to a missing packet– Low loss rates and short clipping lengths (4~16ms) faired best– But Performs poorly

Silence Substitution– Fill the gap with silence– Short packet length (4ms) and low loss rate (2 percent)

making suitable for interleaved audio over low-loss path Noise Substitution

– Filling in the gap left by a lost packet with silence, background noise is inserted instead.

Repetition– Replaces lost units with copies of the unit that arrived

immediately before the lost– Subjective quality can be improved by gradually fading

repeated units

Interpolation Based Repair

Recommendations (1/3)

Non-Interactive Applications– One –to- many transmissions (Radio broadcast)

• Latency is of considerably less importance then quality• Receiver set is likely to be diverse and the group may include

members behind low-speed links

– Interleaving is compatible• Some form of error concealment will still be need to compensate

– Repetition with fading is acceptable– Retransmission-based repair is not appropriate

• A media independent FEC scheme will perform better than a retransmission based repair scheme


Interactive Applications– IP Telephony

• The principal concern is minimizing end – end delay• Such delay imposed by use the

– Interleaving– Retransmission– Media-independent FEC

not acceptable for interactive applications

– Recommends media-specific FEC • Tunable bandwidth overhead• Repair is approximate due to the use of low-rate secondary

encodings


Error concealment– Receivers must be prepared to accept some loss in an audio

stream• Current(1998) use silence substitution to fill the gaps left by

packet loss– Does not provide acceptable quality

• Recommends “Packet repetition “– Simple to implement and having low computational overhead

• Other schemes discussed provide incremental improvements

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