Fax Flow - HP FlexNetwork MSR Series Configuration Manual

Configuring fax over IP
Traditional fax machines transmit and receive faxes over PSTN. Fax has gained wide acceptance
due to its many advantages, such as high transmission speed and simple operations. By far, G3 fax
machines dominant fax communications. A G3 fax machine adopts the signal digitizing technology.
Image signals are digitized and compressed internally, then converted into analog signals through a
modem, and finally transmitted into the PSTN switch through common subscriber lines.
FoIP means sending and receiving faxes over the Internet. Routers can provide the FoIP function
after the FoIP feature is added, on the basis of the VoIP function. Because the FoIP is the
Internet-based fax service, sending national and international faxes costs less.
The network diagram for FoIP is similar to that for VoIP. You just replace the IP phone with a fax
machine to implement the fax function. As long as you can use IP phones, you can use the fax
function. This makes the fax function very simple.
The following figure illustrates an FoIP system structure.
Figure 88 FoIP system structure
PSTN
Fax
FoIP protocols and standards
IP real-time fax complies with the ITU-T T.30 and T.4 protocols on the PSTN side and the H.323 and
T.38 protocols on the IP network side.
•
T.30 protocol pertains to file and fax transmission over PSTN. It describes and regulates the
communication traffic of G3 fax machines over common telephone networks, signal format,
control signaling, and error correction to the full extent.
•
T.4 protocol is a standard protocol involving the G3 fax terminals for file transmission. It
provides a standard regulation for the G3 fax terminals on image encoding/decoding scheme,
signal modulation and speed, transmission duration, error correction, and file transmission
mode.
•
T.38 protocol pertains to the real-time G3 fax over IP networks. It describes and regulates the
communication mode, packet format, error correction and some communication flows of
real-time G3 fax over IP networks.

Fax flow

In FoIP, the call setup, handshake, rate training, packet transfer, and call release are always
real-time. From the perspective of users, FoIP is no different than faxing over PSTN.
Signals that a G3 fax machine receives and sends are modulated analog signals. Therefore the
router processes fax signals in a different way than it processes telephone signals. The router needs
to perform A/D or D/A conversion for fax signals (that is, the router demodulates analog signals from
PSTN into digital signals, or modulates digital signals from the IP network into analog signals), but
does not need to compress fax signals.
Internet
270
PSTN
Fax