The preferential treatment of data packets belonging to important applications 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.
The FTP transfer uses quite large data packets of 1500 byte, whereas, the Voice over IP connection sends packets of e.g. 24 byte net in relatively short intervals. If FTP packets are in the sending queue of the LANCOM just at the moment when a VoIP packet is to be transferred, then the VoIP packet can only be sent after the line is free again. Depending on the transfer rate of the connection, this may cause a noticeable delay of the speech transmission.
This annoying behavior can be compensated if all data packets, which are not belonging to the connection preferred by QoS, do not exceed a certain packet length. While doing so, the data packets of the FTP connection will be divided into such small sections that the time-critical VoIP connection is able to deliver the packets without noticeable delay within the required time slots. A resulting delay has no disadvantageous effect to the TCP-secured FTP transfer.
Two different procedures exist to influence the packet length:
- The LANCOM 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 connection
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.
- The LANCOM 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 occur 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.