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LANCOM Reference Manual LCOS 3.50 ̈ Chapter 9: Quality of Service
178
Quality of Service
office via VPN connection, over which the Internet traffic is not running
simultaneously.
9.5 QoS parameters for Voice over IP applications
An important task when configuring VoIP systems is to guarantee a sufficient
voice quality. Two factors considerably influence the voice quality of a VoIP
connection: The voice delay on its way from sender to addressee, as well as
the loss of data packets, which do not arrive or do not arrive in time at the
addressee. The “International Telecommunications Union” (ITU) has examined
in extensive tests, what human beings perceive as sufficient voice quality, and
has published as the result in the ITU G.114 recommendation.
In case of a delay of not more than 100 ms, and a packet loss of less than 5%,
the quality is felt like a “normal” telephone connection. In case of more than
150 ms delay and less than 10% packet loss, the telephone user perceives still
a very good quality. Up to 300 ms and 20%, some listeners feel this quality
like still suitable, beyond that the connection is considered as no more suita-
ble for voice transmission.
Apart from the average delay time, also a variation in this delay is perceived
by the human ear. Delay differences of the voice information from sender to
addressee (jitter) are still tolerated up to 10 ms, and values beyond considered
as irritating.
Packet loss Jitter
Delay300 ms150 ms100 ms
20 %
5 %
10 %
10 ms