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Network traffic

Data impact calculation

Each IP frame carrying voice data has the following overhead while on an ethernet LAN link:

Table 31. Data Overhead for IP Frame
Ethernet Header
14 bytes
IP Header
20 bytes
UDP Header
8 bytes
RTP Header
12 bytes
Ethernet Trailer
4 bytes
Total
58 bytes

If layer 2 (the data link layer) is to be included in the header calculation, increase this result by 8 bytes. If 802.1 p/Q fields are included, increase by another 4 bytes. The one-way voice data rate for each of the codecs is given in kilobits per second (kbps). To convert this to bytes per 30 ms, multiply by 30/8. Add the result to the header/trailer overhead (58) to get the total frame size. For example, a G.711 64 kbps voice sample is 64 x 30/8 = 240 bytes. Dividing by the compression ratios for the other codecs yields their data bytes per 30 ms frame. Adding on 58 bytes for the header/trailer yields the total 30 ms frame size.

Table 32. Calculation of 30 ms Voice Sample IP Frames 
Codec
Compression Ratio
Data Bytes' 30 ms Frame
Total Frame Size (bytes)
G.711
1
240
298
G.729
8
30
88
G.723a
10
24
82
G.723b
12
20
78

To calculate the data rate of these frames, multiply by 8/30. For example, the G.711 result is

298 x 8/30 = 79.5 kbps

To determine the data rates for other frame sizes, convert the 30 ms data bytes per frame to the appropriate result, add on the header, and convert to kbps. For example, to calculate the G.711 rate for 10 ms frames, divide the 30 ms result (240) by 3, add 58, and multiply by 8/10. The result is 110.4 kbps. G.711 and G.729 frames are multiples of 10 ms. G.723 frames are multiples of 30 ms. For voice quality reasons, frames should not exceed 60 ms.

Table 33. LAN Impact of One-Way Call (kps)
Codec
Voice Sample Size

10 ms
20 ms
30 ms
40 ms
50 ms
60 ms
G.711
110.4
87.2
79.5
75.6
73.3
71.1
G.729
54.4
31.2
23.5
19.6
17.3
16.7
G.723a
N/A
N/A
21.9
N/A
N/A
14.1
G.723b
N/A
N/A
20.8
N/A
N/A
13.1


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