Usage of Fax T.38 via TCP instead of UDP (kb2291)

The information in this article applies to:

  • SwyxWare v4.02
  • SwyxWare from version 4

[ Question | Answer ]


How to switch from T.38 UDP transmission to T.38 TCP transmission ?


Since SwyxWare version 4.02 it's possible to switch from T.38 UDP faxtransmission to T.38 TCP. The advantage to use TCP is, that the network layer itself will retransmit lost packets (e.g. on using WAN links). Using T.38 TCP will minimize the probability of broken faxtransmissions. The only problem on using T.38 TCP is, that many third party equipment does not support it (e.g. Cisco gateways). In case you are using Swyxequipment only or third party products supporting T.38 TCP you can switch to TCP usage as default with the help of a registry key. Please add the following registry key to all machines running a SwyxServer, a SwyxGate or a SwyxLinkManager:

Value:Please enter the string "TCPUni" as value. This means, that T.38 faxtransmission will be done by one TCP channel for each direction. If no registry key is set, UDP will be used for transmission.

Please note the following article also:


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