Quality of Service
318
9.4
Reducing the packet length
BAT54-Rail/F..
Release
7.54
06/08
In order to be able to prioritize incoming data as well, thus an artificial “brake”
must be added also in this direction. Like already incorporated for the up-
stream 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
packets 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 con-
firmation 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 avail-
able bandwidth.
The following queues operate on the receiving side:
D 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
permitted reception bandwidth on the receiving side. Packets with a min-
imum reception bandwidth assigned by a QoS rule are passing through
without any further delay, as long as the minimum bandwidth is not
exceeded.
D Standard reception queue
All packets that do not need special treatment because of an active QoS
rule on the receiving side end up here. Packets of this queue are directly
passed on resp. confirmed without consideration of maximum band-
widths.
9.4 Reducing the packet length
The preferential treatment of data packets belonging to important applica-
tions can be endangered - depending on the situation - by very long data
packets of other applications. This is the case e.g. when IP telephony and a
FTP data transfer are simultaneously active on the WAN connection.
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