This document discusses various packet loss recovery schemes for real-time applications: 1) Forward Error Correction (FEC) schemes send redundant encoded data to reconstruct lost packets, but introduce overhead as group size increases. 2) "Piggyback Lower Quality Stream" sends a lower quality redundant stream to recover from single losses with less overhead. 3) Interleaving divides data into smaller chunks and interleaves them to have partially filled chunks upon loss with no redundancy overhead but increased delay. 4) Receiver-based schemes like repetition or interpolation replace or interpolate lost packets.

1. PACKET LOSS RECOVERY SCHEMES
By:
Chinmaya Mahadev M N
Ece Sec-2, CIT

2. Problem: Packet loss
• Loss is in a broader sense:
Packet never arrives or arrives later than its scheduled playout time.
• Since retransmission is inappropriate for Real Time applications, FEC or
Interleaving are used to reduce loss impact.

3. Recovering From Packet Loss
Forward Error Correction
• Send redundant encoded chunk every n chunks (XOR original n chunks)
 If 1 packet in this group lost, can reconstruct
 If > I packets lost, cannot recover
• Disadvantages
 The smaller the group size, the larger the overhead
 Playout delay increased

4. Tradeoff:
 Increase n, less bandwidth waste
 Increase n, longer playout delay
 Increase n, higher probability that 2 or more chunks
will be lost.

5. 2nd FEC scheme “Piggyback Lower Quality Stream”
 Send lower resolution audio stream as redundant information
e.g., nominal stream PCM at 64 kbps and redundant stream GSM at
13 kbps.
 Whenever there is non-consecutive loss, receiver can conceal the
loss.
 With one redundant low quality chunk per chunk, scheme can
recover from single packet losses.

6. Recovering from packet loss Piggybacking Lo-fi stream

7. Recovering From Packet Loss Interleaving
• Divide 20 msec of audio data into smaller units of 5 msec
each and interleave.
• Upon loss, have a set of partially filled chunks.
• If packet lost, still have most of every chunk
• No redundancy overhead, but increases playout delay

8. Interleaving

9. Recovering From Packet Loss Receiver-based Repair
• The simplest form : Packet Repetition
 Replaces lost packets with copies of the packets that arrived immediately
before the loss.
• A more computationally intensive form: Interpolation.
 Uses Audio before and after the loss to interpolate a suitable packet to
cover the loss.

10. THANK YOU