Switching to VoIP

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Chapter 6: Replacing the Voice Circuit with VoIP

process is fairly simple, because variations in DAC techniques are driven by require-

ments of different data links and devices and by regional standards variations.

The DAC processes employed in Voice over IP aren’t tied to the data link, so they

can vary greatly: different DAC, digitizing, and compression techniques are used in

different circumstances. Sometimes the data link’s properties, like bandwidth capac-

ity and latency, are factors in the selection of these techniques, but not always. DAC

is required in all telephony environments, even where VoIP isn’t used, because just

about every traditional telephony system employs digital carriers and analog sound

reproduction devices like speakers and transducers.

DAC includes of quantizing or digital “sampling” of sounds, filtering for bandwidth

preservation, and signal compression for bandwidth efficiency. Pulse code modula-

tion (PCM) is the most common sampling technique used to turn audible sounds

into digital signals. We’ll deal with DAC subjects in greater detail later.

The 64 kbps channel

To connect a phone call, a traditional telephone, whether analog or digital, requires a

loop with enough quality for 64 kilobits per second of digital throughput. In fact, 64

kbps is the fixed ...