If the fax transmission takes place over two or more IP-hops for some (mainly older) fax machines the roundtrip delay is too high if T.38 is used. There are two categories of errors. In the first case the negotiation fails and the fax transmission doesn’t start at all. In the second case the first page is transmitted and received correctly but it is not acknowledged in time. This causes the fax machine to display a communication error and a second page is not transmitted. T.38 can also be used directly from the beginning of a SIP connection (meaning from the initial SIP invite).

There will be no solution with the original fax machines.

- T.38 configuration/usage requires end-to-end DMC between HP4K gateways (minimized IP hops) because of timing requirements according to T.38 standard. In case of multiple IP hops the fax transmission could fail with some fax machines, e.g. Canon FAX-L100 Canon FAX-L120 Panasonic Fax UF-595 Panasonic Fax UF-495

Resolution for affected fax machines: replacement of old fax machines or usage of G711 transparent mode instead of T.38

- If T.38 is supposed to be used with AP1120, G.711 must be configured as voice codec only, i.e. always in case a modem or fax is connected to an AP1120 the only allowed codec in the AP1120 has to set to G.711 (no option for compression).