Codecs
When designing a VoIP system, the choice of a speech codec is function of a number of network factors, such as delay and the available processing power, the user requirement of service quality and expectation of speech quality. The attributes of a speech codecs include bit rate, complexity, delay, and quality.
The commonly used codecs such as G.723, G.729, and G.728 were developed with specific requirements and priorities in mind; as such, they provide different levels of compromises along these four dimensions. The total characteristic of more used codecs is summarized in table 2.
Table 2
| Codecs | Useful load of packet, bytes | Transmission bitrate, kbps | The algorithmic delay, milliseconds | Occupied stream, kbps |
| IP-packets | Ethernet-frames |
| G.711 | 160 | 64 | 20 | 64,8 | 80 |
| G.723.1 (6.3) | 24 | 6,3 | 37,5 | 6,9 | 17,1 |
| G.723.1 (5.3) | 20 | 5,3 | 37,5 | 5,9 | 16 |
| G.726-32 | 160 | 32 | 20 | 32,8 | 42,7 |
| G.726-24 | 160 | 24 | 20 | 24,8 | 34,7 |
| G.726-16 | 160 | 16 | 20 | 16,8 | 26,7 |
| G.729 (8) | 20 | 8 | 25 | 8,8 | 18,7 |
| G.729 (6.4) | 16 | 6,4 | 25 | 7,2 | 17,1 |
For IP video telephony exist following codecs:
Table 3
| H.261 | H.263 |
| Transmission bitrate | From 40 kbit/s | from 28,8 kbits/s |
| Supports a picture sizes | - 176x144
- 352x288
- 704x576 (added in a later revision in 1993)
| - 128x96
- 176x144
- 352x288
- 704x576
- 1408x1152
|
| Sampling scheme | 4:2:0 | 4:2:0 |
| Frame rate | from 10 frames/s to 30 frames/s | from 10 frames/s to 30 frames/s |
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