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LANCOM Reference Manual LCOS 3.50 ̈ Chapter 9: Quality of Service
182
Quality of Service
The transfer time of the packets to the interface (serialization) assumes a
PMTU of 512 bytes on a 128 Kbps connection. Therefore, for slower inter-
faces or other codecs it is eventually necessary to adjust jitter buffers and/
or PMTU values.
Please notice that the bandwidths are required in the sending and
receiving 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
according rule applies to the sending or to the receiving direction. But which
direction refers to sending and receiving for a given a data transfer depends
on the particular point of view. The following two variants apply:
̈ The direction corresponds to the logical connection setup
̈ The direction corresponds to the physical data transfer over the appropri-
ate interface
The differences are unveiled by looking at a FTP transfer. A client of the LAN
is connected to the Internet through a LANCOM.
̈ During an active FTP session, the client sends by the PORT command the
information to the server, on which port the DATA connection is expected.
As the result, the server establishes the connection to the client and sends
the data in the same direction. In this case, the logical connection as well
as the real data stream over the interface go from the server to the client,
and the LANCOM takes both as the receiving direction.
̈ Different is the case of a passive FTP session. Here the client itself estab-
lishes the connection to the server. The logical connection setup thus is
from client to server, but the data transmission over the physical interface
flows in the reverse direction from server to client.
With standard settings, a LANCOM assumes the sending or receiving direction
depending on the logical connection setup. Because such a point of view may
not be easy to follow in certain application scenarios, the point of view can
alternatively be changed to the flow of the physical data stream.
The differentiation between sending and receiving direction applies
only to the installation of maximum bandwidths. For a guaranteed
minimum bandwidth, as well as for fragmentation and PMTU reduc-