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̈ Chapter 9: Quality of Service LANCOM Reference Manual LCOS 3.50
181
Quality of Service
the IPSec header must be added (RTP and IPSec headers can be larger,
depending on the configuration).
Since packets encrypted with DES, 3DES, or AES, are only able to grow in
block sizes of 64 bytes, the IPSec packet for G.711 consists of 160 bytes
payload + 96 bytes up to the next block limit + 20 bytes IPsec header =
276 bytes.
A similar “quote of loss“ can also occur for the G.723 codec, if e.g. the
RTP header is longer than 12 bytes. Then, the IP packet will grow up to
the next block limit of 128 bytes; plus 20 bytes for the IPsec header cre-
ates packets of an overall length of 128 bytes, which means more than
the sixfold net load!
The required bandwidth for transmission results finally from the quotient
of packet size and sampling time.
̈ 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
remote station.
̈ 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
packets multiplied with the sampling time of the codec.
̈ 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:
Codec Net data
rate
Sampling Packets
per sec.
payload IP packet IPsec
packet
Band-
width
G.723.1 6,3 Kbit/s 30 ms 33,3 24 byte 64 byte 84 byte 22,3 Kbps
G.711 64 Kbit/s 20 ms 50 160 byte 200 byte 276 byte 110.4 Kbps
Codec Process-
ing
Serializa-
tion
Propga-
tion
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