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?

The explanations for each of the parts of the question are given below.

Explanation:

a)  FEC means Forward Correction Error

i)  For the FEC First scheme:

The FEC first scheme generates a redundant chunk for every four original chunks.

The additional bandwidth required =  1/4 =0.25 I.e. 25%.

The playback delay is increased by 25% with 5 packets.

ii) For the FEC Second scheme:

The transmission rate of low-bit encoding =  25% of the nominal stream transmission rate.

Therefore, the additional bandwidth = 25%

The play back delay is increased by 25% with 2 packets.

b)  In the second scheme, the loss of the first packet reflects in the redundant scheme immediately. The lost packet has a lower audio quality than the remaining packets

The first scheme has a higher audio quality than the second scheme.

c)  If the first packet is lost in every group of two packets, most of the original packets in the first scheme will be lost causing a great reduction in the audio quality.

In the second scheme,the audio qualities in the two packets are not the same, the second packet will be received by the receiver if the first is lost, so no part of the audio stream is lost. This will still give an audio quality that is acceptable.

The second scheme will have better audio quality.

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1. b

2. c

3. b

4. a

5. b