Question
Recall the two FEC schemes for VoIP described in Section 7.3. Suppose the first scheme generates a redundant chunk for every four original chunks. Suppose the second scheme uses a low-bit rate encoding whose transmission rate is 25 percent of the transmission rate of the nominal stream.
a. How much additional bandwidth does each scheme require? How much playback delay does each scheme add?
b. How do the two schemes perform if the first packet is lost in every group of five packets? Which scheme will have better audio quality?
c. How do the two schemes perform if the first packet is lost in every group of two packets? Which scheme will have better audio quality?
Answer
a)
FEC First scheme:
In FEC (Forward Error Correction) first scheme, after sending all the chunks (say N chunks), an encoded redundant chunk is sent. The redundant chunk can be obtained by performing exclusive OR on all the N chunks. In this way, the lost packet can be reconstructed.
Consider the FEC first scheme generates a redundant chunk for every four original chunks.
If FEC first scheme generates one redundant chunk for every four original chunks, the additional bandwidth required is 1/4 =0.25 or 25%.
The scheme increase its playback delay by 25% with 5 packets.
FEC Second scheme:
In FEC second scheme, low-resolution, low-bit audio streams are sent as redundant information along with nominal audio stream.
The transmission rate of low-bit encoding is 25% of the nominal stream transmission rate.
So, the second scheme require 25% additional bandwidth.
The play back delay is increased by 25% with delay of 2 packets.
b)
In the FEC first scheme, if the first chunk is lost, then it can be reconstructed after receiving all the 4 chunks and the redundant chunk by performing exclusive OR.
In the FEC second scheme, if the first packet is lost, the loss is immediately reflected in the redundant stream as the quality of the lost packet will be lower than the other packets.
The second scheme gives lower quality audio stream than the first scheme.
c)
In the first scheme, most of the original packets will be lost, if the first packet in each group of two packets is lost.
Therefore, the quality of the audio stream will be very poor.
In the second scheme, one packet will be of high quality and the second will be of low quality. Even though the first packet is lost, the second will be received by the receiver. So, the receiver receives every part of the audio stream.
Therefore, audio quality will be acceptable.
Among the two schemes, the second scheme give better audio quality than the first one.