Information on the impact of IP-based connections on fax use and resulting error symptoms

Due to the ongoing ISDN switch-off and migration from analogue connections to All-IP connections, we would like to take this opportunity to point out the associated system-related (qualitative) effects, especially on the use of fax in IP environments, which are often mistakenly perceived by customers as errors in the SwyxWare/NetPhone solution.

For this reason - and to create a better understanding - we would like to go into the possible causes in more detail below.

Effects on fax usage due to the change from analogue to IP-based connection/telephone system:

The fax protocol was developed for a classic circuit-switched service. There are no packet losses or different transit times. However, these factors are a fundamental nature of IP.

When switching from analogue/line-switched to IP-based connection, on the other hand, both voice and fax data are transmitted as IP packets containing approximately 20 milliseconds of user data.

However, due to the system, individual packets may be lost if the connection is poor or fully loaded. A voice connection does not suffer too much if a packet is lost. The missing information of a fortieth of a second is noticeable as a small "crackle". Only when numerous packets in succession do not reach the recipient does the recipient notice this as a short interruption. A fax modem, however, reacts much more sensitively to such time-delayed dropouts, because then it loses synchronisation and the connection is interrupted. Therefore, the longer a transmission takes, the greater the chance that the connection will fail.

In addition to packet loss, there may also be a situation where two fax machines in direct (analogue, delay-free) communication with each other can fax successfully, but not with a VoIP link in between (and this even though there is no packet loss).

The T.30 standard, which describes the protocol for fax transmission between analogue fax machines, also defines timers and tolerances for protocol handshakes. Manufacturers of analogue fax machines sometimes do not adhere to the timers and tolerance times specified in the standard. This is not a problem for data transmissions without delay (and thus for analogue connections). However, if delays occur, and these are unavoidable due to packet transmission via VoIP, this lower, non-standard tolerance of the fax machine firmware can cause the fax transmission to fail. In this case, therefore, the manufacturer of the analogue fax machines is typically called upon to update the firmware.

Even the use of the IP-FAX protocol T.38 does not change this much. Especially when it comes to fax machines that logically do not have this protocol implemented, it can lead to pages being lost. This is also not necessarily reproducible, as the respective utilisation of the IP route comes into play here. These system-related "disadvantages" of VoIP with regard to fax are also described in the current issue of the magazine c't Netzwerke 2016 on page 19.

As with the changeover from analogue (FM) broadcasting to digital broadcasting (DAB), where there is only good or no reception and no longer "bad noisy reception", it is also the case with the IP changeover in the case of FAX that, as a "side effect" of digitisation, there are more frequent no connections or dropouts.

Since these problems are system-related, they can occur anywhere where there is a change to IP - be it in the system or at the connection. This also means that it is not a manufacturer-specific problem but a problem of the technology change from TDM to IP.

In the following you will find a rough overview of categories of fax-related error patterns and Swyx' approach to them:

1) Swyx Fax Server:
Errors concerning the Swyx Fax Server or fax transmission within a SwyxWare solution will be analysed by Swyx and the aim is to fix them (if reproducible and validated) in a future release.

2) Externally connected fax devices
a. External fax devices connected to routers/gateways:
Errors which occur in connection with the connection of external (analogue) fax devices to a router (for example: Lancom, Zyxcel, Bintec etc.) to which a SwyxWare is also connected, will not be classified as errors by Swyx and therefore will not be validated and fixed. Swyx has no control over the SW/HW of the router/gateway, the fax and the (quality of the) connection/line.

3) Change of telephone system:
a. Update of SwyxWare:
Errors occurring after an update or configuration change of an existing SwyxWare installation will be analysed by Swyx and the aim is to fix them (if reproducible and validated) in a future release.

b. Change from a PBX of another manufacturer to SwyxWare:
In case of errors reported in this context, it has to be considered that a direct comparison with the behaviour of the old system is not suitable to determine a misbehaviour of SwyxWare as cause. Due to the IP changeover, considerable system-related quality restrictions apply to fax, which are explained above. Based on these limitations the cause of reported errors is not given in the SwyxWare and consequently they are not classified as errors by Swyx and therefore not validated or fixed.