Quality of Service
BAT54-Rail/F..
Release
7.54
06/08
9.5
QoS parameters for Voice over IP applica-
323
D The time for handing over the packet to the interface is defined by the quo-
tient of packet size and available bandwidth:
A 512 byte packet of an FTP connection occupies the line at 128 Kbps
upstream for at least 32 ms.
Besides, the packets of the VoIP connection are often much larger than
the pure net payload. The additional headers of the IP and Ethernet
packets, as well eventual IPsec headers have to be added as well. The
net load results from the product of net data rate and sampling time of
the used codec. For all codecs, each 40 bytes UDP header and at least
20 bytes for the IPSec header must be added (RTP and IPSec headers
can be larger, depending on the configuration).
The following table is an overview of bit rates for various VoIP codecs for
voice connections over VPN:
D IP payload: Voice payload + 40 byte header (12 byte RTP; 8 byte UDP;
20 byte IP header)
D IPSec payload: IP paket + padding + 2 byte (padding length & next
header) = multiple of the IPSec initialization vector
Packet size in bytes
1 64 128 256 512 1024 1500
56 Kbps
0,14 9 18 36 73 146 215
64 Kbps
0,13 8 16 32 64 128 187
128 Kbps
0,06 4 8 16 32 64 93
256 Kbps
0,03 2 4 8 16 32 47
512 Kbps
0,016 1 2 4 8 16 23
768 Kbps
0,010 0,6 1,3 2,6 5 11 16
1536 Kbps
0,005 0,3 0,6 1,3 3 5 8
VoIP codec Packets/s Voice payload IP payload IPSec payload
kbps Bytes kbps Bytes kbps Bytes
G.729
30ms
33,38 30327036136
G.726
30ms
33,3 32 120 42,7 160 62 232
G.711
30ms
33,3 64 240 74,7 280 92 344
G.711
20ms
50 64 160 80,0 200 106 264
G.722
20ms
50 64 160 80,0 200 106 264
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