Calculating the Total Bandwidth for a VoIP Call
Codec choice, data-link overhead, sample size, and RTP header compression have positive
and negative impacts on total bandwidth, as demonstrated in Table 2-6.
Table 2-6 Total Bandwidth Required
Codec Speed Sample Size Frame Relay Frame Relay Ethernet
Codec (bps) (Bytes) (bps) with cRTP (bps) (bps)
G.711 64,000 240 76,267 66,133 79,467
G.711 64,000 160 82,400 67,200 87,200
G.726r32 32,000 120 44,267 34,133 47,467
G.726r32 32,000 80 50,400 35,200 55,200
G.726r24 24,000 80 37,800 26,400 41,400
G.726r24 24,000 60 42,400 27,200 47,200
G.726r16 16,000 80 25,200 17,600 27,600
G.726r16 16,000 40 34,400 19,200 39,200
G.728 16,000 80 25,200 17,600 27,600
G.728 16,000 40 34,400 19,200 39,200
G.729 8000 40 17,200 9600 19,600
G.729 8000 20 26,400 11,200 31,200
G.723r63 6300 48 12,338 7350 13,913
G.723r63 6300 24 18,375 8400 21,525
G.723r53 5300 40 11,395 6360 12,985
G.723r53 5300 20 17,490 7420 20,670
To perform the calculations, you must consider these contributing factors as part of the
equation:
■ More bandwidth required for the codec requires more total bandwidth.
■ More overhead associated with the data link requires more total bandwidth.
■ Larger sample size requires less total bandwidth.
■ RTP header compression requires significantly less total bandwidth.
Consider a sample total bandwidth calculation. A company is implementing VoIP to carry
voice calls between all sites. WAN connections between sites will carry both data and
voice. To use bandwidth efficiently and keep costs to a minimum, voice traffic traversing
the WAN will be compressed using the G.729 codec with 20-byte voice samples. WAN
connectivity will be through a Frame Relay provider.
The following calculation is used to calculate total bandwidth required per call:
Total_Bandwidth = ([Layer_2_Overhead + IP_UDP_RTP Overhead + Sample_Size] /
Sample_Size) * codec_Speed
Calculation for the G.729 codec, 20-byte sample size, using Frame Relay without
Compressed RTP (cRTP) is as follows:
Total_Bandwidth = ([6 + 40 + 20] / 20) * 8000
Total_Bandwidth = 26,400 bps
Calculation for G.729 codec, 20-byte sample size, using Frame Relay with cRTP is as
follows:
Total_Bandwidth = ([6 + 2 + 20] / 20) * 8000
Total_Bandwidth = 11,200 bps