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VoIP Bandwidth Calculation
Voice over IP over Ethernet, Example 1: G.711
- Codec G.711 - 64 kbps, 20 ms sample period
- 1 frames per packet (20 ms)
- Standard IP headers
- Ethernet transmission medium
One packet is sent every 20 ms, 50 packets per second. Payload is 64,000 ÷ 50 = 1,280 bits (160 octets).
Fixed IP overhead 40 octets, fixed Ethernet overhead 38 octets.
Total size 238 octets. Bandwidth required is (160 + 40 + 38) x 50 x 8 = 95,200 bps.
Voice over IP over Ethernet, Example 2: G.729a
- Codec G.729a - 8 kbps, 10 ms sample period
- 2 frames per packet (20 ms)
- Standard IP headers
- Ethernet transmission medium
One packet is sent every 20 ms, 50 packets per second. Payload is 8,000 ÷ 50 = 160 bits (20 octets).
Fixed IP overhead 40 octets, fixed Ethernet overhead 38 octets.
Total size 98 octets. Bandwidth required is (20 + 40 + 38) x 50 x 8 = 39,200 bps.
Silence Suppression
Certain codecs support silence suppression. Voice Activity Detection (VAD) suppresses the transmission of data during silence periods. As only one person normally speaks at a time, this can reduce the demand for bandwidth by as much as 50 percent. The receiving codec will normally generate comfort noise during the silence periods.
Additional Codec Data
|
Codec
|
Bandwidth
|
Sample
period
|
Frame
size
|
Frames/
packet
|
Ethernet
Bandwidth
|
| G.711 (PCM) |
64 kbps |
20 ms |
160 |
1 |
95.2 kbps |
| G.723.1A (ACELP) |
5.3 kbps |
30 ms |
20 |
1 |
26.1 kbps |
| G.723.1A (MP-MLQ) |
6.4 kbps |
30 ms |
24 |
1 |
27.2 kbps |
| G.726 (ADPCM) |
32 kbps |
20 ms |
80 |
1 |
63.2 kbps |
| G.728 (LD-CELP) |
16 kbps |
2.5 ms |
5 |
4 |
78.4 kbps |
| G.729a (CS-CELP) |
8 kbps |
10 ms |
10 |
2 |
39.2 kbps |
| AMR (ACELP) |
4.75 kbps |
20 ms |
12 |
1 |
36.0 kbps |
| AMR (ACELP) |
7.4 kbps |
20 ms |
19 |
1 |
38.8 kbps |
| AMR (ACELP) |
12.2 kbps |
20 ms |
31 |
1 |
43.6 kbps |
| AMR-WB/G.722.2 (ACELP) |
6.6 kbps |
20 ms |
17 |
1 |
38.0 kbps |
You can explore the effects of sample periods and silence suppression on the bandwidth required in our on-line VoIP Bandwidth Calculator.
Summary
Although there are many factors that influence the amount of bandwidth required to transmit a voice call over an IP network, by approaching the problem one element at a time the final calculation becomes relatively simple. Other factors may influence the actual bandwidth used, such as RTP header compression, silence suppression and other techniques still under development.
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IPsec and VoIP
