CCIE Collaboration Exam 400-051: iLBC Codec Bit Rate Calculation

# iLBC Codec Bit Rate Calculation

### Question

The iLBC codec operates at 38 bytes per sample per 20-millisecond interval.

What is its codec bit rate in kilobits per second?

### Explanations

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A. B. C. D. E.

C.

The internet Low Bit Rate Codec (iLBC) is designed for narrow band speech and results in a payload bit rate of 13.33 kbits per second for 30-millisecond (ms) frames and 15.20 kbits per second for 20 ms frames.

When the codec operates at block lengths of 20 ms, it produces 304 bits per block, which is packetized as defined in RFC 3952

Similarly, for block lengths of 30 ms it produces 400 bits per block, which is packetized as defined in RFC 3952.The iLBC has built-in error correction functionality to provide better performance in networks with higher packet loss.

To calculate the codec bit rate of iLBC, we need to use the following formula:

codec bit rate (kbps) = (sample size in bytes / sample interval in seconds) * 8

We know from the question that the iLBC codec operates at 38 bytes per sample per 20-millisecond interval.

First, we need to convert the sample interval from milliseconds to seconds by dividing by 1000:

20 milliseconds / 1000 = 0.02 seconds

Next, we can substitute the values into the formula:

codec bit rate (kbps) = (38 bytes / 0.02 seconds) * 8

Simplifying the expression inside the parentheses:

38 bytes / 0.02 seconds = 1900 bytes per second

Multiplying by 8 to convert from bytes to bits:

1900 bytes per second * 8 = 15200 bits per second

Converting from bits per second to kilobits per second:

15200 bits per second / 1000 = 15.2 kilobits per second

Therefore, the answer is C. 15.2.