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TECHNOLOGIES BEHIND VoIP:



There are a number of recent technological advances that made Voice over IP a practical solution for today's small and medium size businesses. The most significant of those advances are discussed below:
The Internet
Internet provided a ubiquitous, relatively low-cost data network making the VoIP infrastructure possible. CLECs are already taking advantage of the Internet in providing low-cost calling card products that bypasses the traditional long-distance carriers. More and more private corporations are realizing that they too can deploy VoIP of their own.

H.323 Standard
The H.323 standard enabled companies to build customized solutions using products from many vendors, and run them over a multiservice network without compatibility problems.

High Powered DSPs
Recent introduction of high powered, lowered cost DSP chips have improved VoIP quality significantly while reducing the overall cost of technologies. It is now reasonable to assume that VoIP investments can be recovered within one year with long-distance savings alone.

QoS Policy Enforcement
Quality of Service is an important factor in the delivery of real time voice packets. The QoS is now more measurable and controllable, thanks to new updated IOS software.
EFFICIENT CODECs

The quality of the voice call is directly related the CODECs (Coder/DECoder) that the voice devices uses. Pulse code modulation (PCM) and adaptive differential PCM (ADPCM) are examples of "waveform" CODEC techniques. Waveform CODECs are compression techniques that exploit the redundant characteristics of the waveform itself.

In addition to waveform CODECs, there are source CODECs that compress speech by sending only simplified parametric information about voice transmission; these CODECs require less bandwidth. Source CODECs include linear predictive coding (LPC), code-excited linear prediction (CELP) and multipulse-multilevel quantization (MP-MLQ).

Coding techniques for telephony and voice packet are standardized by the ITU-T in its G-series recommendations. The Dataframe Voice capable products use the following coding standards:

G.711-Describes the 64-kbps PCM voice coding technique. In G.711, encoded voice is already in the correct format for digital voice delivery in the PSTN or through PBXs.
G.729-Describes CELP compression where voice is coded into 8-kbps streams. There are two variations of this standard (G.729 and G.729 Annex A) that differ mainly in computational complexity; both provide speech quality similar to 32-kbps ADPCM.

Mean Opinion Score
A wide range of listeners’ judge the quality of a voice sample (corresponding to a particular CODEC) on a scale of 1 (bad) to 5 (excellent). The scores give a good idea on how quality relates the use of CODECs.


 Compression Methods    Bit Rate (kbps)    Framing Size (ms)    MOS-Score
 G.711 PCM
64
0.125
4.1
 G.729 CS-ACELP1
8
10
3.92
 G.729 x 2 Encodings
8
10
3.27
 G.729 x 3 Encodings
8
10
2.68
 G.729a CS-ACELP
8
10
3.7