Fax over IP is one of the great features of Kapanga Softphone. From time to time, our prospects asks us about what actual benefit our implementation of Fax over IP brings to the table. Our answer is simple: reliability.
There are two ways of sending faxes over IP; one is transporting the fax tones over RTP (Real-Time Transport Protocol) just like plugging your standard fax machine to the ATA device from your provider; the other is transporting the binary information the fax tones carry using IFP (Internet Fax Protocol). The latter method is standardized by the International Telecommunications Union (ITU) under the T.38 specification. We at Kapanga Softphone implemented T.38 support and built it into the softphone.
Why is then T.38 the recommended way to send faxes over IP? Here are a few key reasons:
1. Transporting fax tones (and tones in general) over RTP is unreliable when using voice low bit rate codecs. In other words, sending a fax using G.723 is virtually impossible due to the distortion introduced by the transcoding.
2. When a fax is traveling through RTP, even the slightest presence of echo prevents the end points from demodulating (=receiving) fax tones correctly.
3. T.38 is the preferred option for end points like softphones and media servers since generating and receiving fax tone signals in audio requires a lot of CPU power.
4. T.38 provides embedded redundancy and error correction methods, resulting in a more reliable way of transmitting fax over IP than RTP.
Thus fax sent using T.38 through an IP network will result in a very reliable solution for both sending and receiving fax .