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̈ Chapter 9: Quality of Service LANCOM Reference Manual LCOS 3.50
175
Quality of Service
standard DSL connection, the DSL interface is thus adjusted in the LANCOM
to the appropriate upstream rate (e.g. 128 kbps).
9.3.2 Queues for receiving direction
Apart from the data transfer rate in transmission direction, the same consid-
eration applies also to the receiving direction. Due to its 10 or 100 Mbps
Ethernet interface, the LANCOM’s WAN interface is fed by clearly fewer data
from the broadband modem than would actually be receiveable. All data
packets received on the WAN interface are transferred to the LAN with equal
rights.
In order to be able to prioritise incoming data as well, thus an artificial “brake”
must be added also in this direction. Like already incorporated for the
upstream direction, the data transfer rate of the interface is therefore adapted
to the provider’s offer in the downstream direction. For a standard DSL con-
nection thus e.g. a downstream rate of 768 kbps applies. Again, the gross
data rate can be entered here, if known.
Reducing the receiving bandwidth makes possible to treat received data pack-
ets suitably. Preferred data packets will be directly passed on to the LAN up to
the guaranteed minimum bandwidth, all remaining data packets are running
into congestion. This congestion produces generally a delayed confirmation of
the packets. For a TCP connection, the sending server will react to this delay
by reducing its sending frequency and adapting itself to the available band-
width.
The following queues operate on the receiving side:
̈ Deferred Acknowledge Queue
Each WAN interface contains additionally a QoS reception queue, which
takes up those packets that should be „slowed down“. The storage period
of each individual packet depends on its length and on the actual permit-
ted reception bandwidth on the receiving side. Packets with a minimum
reception bandwidth assigned by a QoS rule are passing through without
any further delay, as long as the minimum bandwidth is not exceeded.
Data rates indicated by providers are mostly likely net rates. The gross data
rate, which is available for the interface is a little bit higher than the net
data rate guaranteed by the provider. If you know the gross data rate of
your provider, you can enter this value for the interface and slightly
increase in this way the data throughput. However, with entering the net
data rate you play safe in any case!