Quality of Service
320
9.5
QoS parameters for Voice over IP applica-
BAT54-Rail/F..
Release
7.54
06/08
Two different procedures exist to influence the packet length:
D The BAT can inform the peers of a data connection that they should only
send data packets up to a certain length. Thereby, an appropriate PMTU
(Path Maximum Transmission Unit) is enforced on the sending side. This
procedure is called PMTU reduction”.
The PMTU reduction can be used for sending as well as for receiving
direction. For the sending direction, the data source of the own LAN is
adjusted with the PMTU reduction to a smaller packet size, for the
receiving direction the data source of the WAN, e.g. web or FTP servers
in the Internet.
Provided that the data connection already exists when the VoIP connec-
tion is started, the senders regulate packet lengths very quickly to the
permitted value. When setting up new data connections while a VoIP
connection is already established, the maximum permitted packet length
is negotiated directly during the connection phase.
Note: The reduced packet length on the data connection still remains also
after terminating the VoIP connection, as long as the sender checks the
PMTU value again.
D The BAT is able to split packets to be sent above an adjustable maximum
size (e.g. 256 byte) into smaller units itself. But such a procedure called
”fragmentation” is not supported by all servers of the Internet, because
dealing with fragmented packets is considered as a security risk, and
therefore is turned off by many servers. That’s why disturbances can oc-
cur e.g. while downloading or while transmitting web pages.
Thus, this procedure is recommended only for connections without
involving unknown servers, e.g. for a direct connection of branches to
their head 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 exam-
ined in extensive tests, what human beings perceive as sufficient voice qual-
ity, and has published as the result in the ITU G.114 recommendation.
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