Sometimes sending faxes fails although T.38 UDP is used. (kb2280)

Le informazioni contenute nel presente articolo riguardano i seguenti prodotti:

  • SwyxWare from version 4

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Why does sending faxes via T.38 UDP sometimes fail?


The UDP network protocol does not implement a retransmission mechanism for lost packets, which are not received at the receivers side. In case you try to send faxes via IP links/networks with packet loss (e.g. WAN links) it may happen, that faxtransmissions fail, or the received faxes contain failures.

Since SwyxWare version 4.02 it is possible to increase the redundancy of the transmitted packets for T.38 UDP. With the help of a registry key it is possible to switch T.38 into a mode, where each transmittet packet does contain n of its predecessors. This will increase the reliability of faxtransmission significantly, but at the same time will increase the required bandwidth.

The appropriate registry key is entered this way:

Location: HKEY_USERS\<SID of the user account the SwyxWare service is running with>\Software\Swyx\IpPbxGate\CurrentVersion\Options
Name: T38UdpErrRecoveryPackets
Value: Number of preceded pakets, which should be attached to the current transmitted one. If the registry key is not present, a default of 0 is used. This means the default is to use no redundancy at all. To increase reliability with network connections where packet loss occurs, choose value 1.


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