Quality of Service
324
9.6
QoS in sending or receiving direction
BAT54-Rail/F..
Release
7.54
06/08
Caution: The values in the table apply to the use of AES. With other en-
cryption methods the resulting package may vary on a minor degree.
Note: Further information on bandwidth requirements for Voice over IP with
IPSec is available in the BAT techpaper Performance Analysis of BAT
Routers.
D The time for transmission via Internet depends on the distance (about 1
ms per 200 km), and on the thereby passed routers (about 1 ms per hop).
This time can be approximated by the half average ping time to the re-
mote station.
D The jitter buffer can be adjusted directly at many IP telephones, e.g. as
fixed number of packets, which should be used for buffering. The tele-
phones load then up to 50% of the adjusted packets and begin afterwards
to replay. The jitter buffer correspond therefore to half of the entered pack-
ets multiplied with the sampling time of the codec.
D Conclusion: The total delay is composed as follows for the according
bandwidth, a ping time of 100 ms to the remote station and a jitter buffer
of 4 packets for both codecs in this example:
The transfer time of the packets to the interface (serialization) assumes a
PMTU of 512 bytes on a 128 Kbps connection. Therefore, for slower
interfaces or other codecs it is eventually necessary to adjust jitter buff-
ers and/or PMTU values.
Note: Please notice that the bandwidths are required in the sending and re-
ceiving direction, as well as just for one single connection.
9.6 QoS in sending or receiving
direction
For controlling data transfer by means of QoS one can select whether the ac-
cording rule applies to the sending or to the receiving direction. But which di-
rection refers to sending and receiving for a given a data transfer depends on
the particular point of view. The following two variants apply:
D The direction corresponds to the logical connection setup
D The direction corresponds to the physical data transfer over the appropri-
ate interface
Codec Processing Serialization Propagation Jitter buffer Sum
G.723.1 30 ms 32 ms 50 ms 60 ms 172 ms
G.711 20 ms 32 ms 50 ms 40 ms 142 ms
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