What is the most standards compliant way to send a fax from an application?

From what I understand 开发者_开发问答fax over VoIP is very unreliable due to the real-time requirements of the fax protocol. Simply using VoIP to send fax message to POTS-connected fax machines is hence not possible.

Besides from proprietary fax API:s (such as sending fax messages via a company specific SOAP API), what standards based protocols are there for sending fax messages programmatically over IP?

There's a fax-over-IP protocol.

If you don't control the QoS (e.g. delays) of the network, then a more reliable way is to use a store-and-forward mechanism (e.g. email) to route the image though the (non-real-time) cloud, to a fax server on the other side, which then sends the fax over a real-time Plain Old Telephone line.

If you are looking for Fax over SIP then mostly sip providers adopt one on these two methods mentioed below

1. Fax transmission using T.38 protocol

2. G.711 pass through

T.38 is more reliable therefore preferred for fax communication and when there is non availability or compatibility issues then G.711 fax pass through is option for fax transmission

Following is an open source project based on freeswitch that deploy both stratgies

http://www.ictfax.org