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What is FoIP?
FoIP is a deviation from voice over Internet Protocol (VoIP) as it makes use of a new protocol (T.38) instead of a voice codec. However, both VoIP and FoIP have session management features in common in that both have connection, disconnection and negotiation stages. In VoIP the data is audio and is sent over an audio compression codec for example G.117a which is a lossy compression scheme: a compression method whereby data that is compressed and then decompressed may well be different from the original, but is "close enough" to be useful in some way. In FoIP the data is T.30 fax data instead of audio and T.30 data does not make use of a lossy compression scheme. Since T.30 data is quite compressed in itself, there is no need for compression but mostly data integrity, and as illustrated in the diagram below, FoIP uses a protocol called T.38 to transfer fax data over IP.
The fax session
A fax session is a standardized way of transferring images via a communications medium in real-time. The real-time element is necessary to confirm the transfer of the images. Nowadays, this confirmation in conjunction with Error Correct Mode (ECM), makes a fax legally binding and is used for legal reasons.
Standardized fax data is called T.30. This data is the same for all types of fax sessions with the difference being that it is encoded in various ways depending on the communications medium used. The following illustrates a typical fax session over analog public switched telephone network (PSTN) telephone lines:
In the illustrations below, the communications medium used is analog PSTN telephone lines. These can only send audible data in the range of just below 4 KHz which is only good for voice. The T.30 fax data is encoded to audible data, sent to the recipient, decoded back to T.30 data and thus resulting in an image at the recipient's side.
Possibility of Fax over VoIP audio
It is possible to send a fax of normal audio VoIP but the success rates are very low. This method is referred to as Fax over Voice over IP.
VoIP uses a lossy codec that humans hear correctly. This codec eliminates certain parts of the audio that our ear cannot hear for further compression. To reduce the bandwidth used by the codec for audio data, silence is also not transmitted as part of the audible data. To make things worse, some VoIP codecs produce what is called 'comfort noise' so that the human caller hears silence as soothing noise. Fax machines do not like comfort noise since they can 'hear' better than humans do and need the audible data intact including the silent parts and also the non-human-audible parts. Therefore, in general Fax over Voice over IP is not a good option and will surely fail miserably.
Calling a third party with VoIP
There are many ways to call a third party using VoIP and all these methods are also relevant for FoIP. The various methods are described below.
The Direct IP method
Using a Registrar/Gatekeeper
This method builds on the Direct IP method described above, however, since IP addresses are not very easy to remember, it makes use of regular phone numbers to call a third party using VoIP. This method uses a Registrar (SIP) or a Gatekeeper (H.323), where both Registrar and Gatekeeper perform the same task but called differently depending on the protocol used.
This method works by registering both IP address and phone number with the Registrar or Gatekeeper. When a third party needs to be called, it is then sufficient to dial the phone number of the third party only and the Registrar or Gatekeeper are then queried for the IP address of the specific phone number to call.
The following examples describes the process:
1. Phone 10.1.1.1 registers with server having phone number 123
2. Phone 10.1.1.2 registers with server having phone number 456
3. Phone 10.1.1.1 needs to call phone number 456 so it queries the server for the IP address of the phone number 456.
4. The server replies saying that phone number 456 has IP 10.1.1.2
5. Phone 10.1.1.1 calls 10.1.1.2 directly.
The above steps are all done transparently from the user by the phone of VoIP application.
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