<< Click to Display Table of Contents >>



Fax over IP (FoIP)



FoIP (Fax over Internet Protocol) is a new technology that allows faxes to be sent over an IP network rather than the traditional PSTN network thus reducing the cost per document sent. With FoIP the user experience remains largely the same, if transmitting IP fax to another IP fax users then you can still input a phone number however it is just an alias for the IP address of the IP fax server. Zetafax works in conjunction with the Zetafax FoIP Connector to achieve this and comes in two options the Zetafax FoIP Connector Standard and the Zetafax FoIP Connector SR140.

Features of Software IP faxing

Reduced complexity, lower maintenance costs

No boards to install/maintain, no T1/E1 lines to configure

Works just like traditional, real-time faxing

Delivers the same level of quality and real-time capability and confirmation you get from your fax machine

Reliable Faxing



Fax Background

When sending a fax message the “caller” and “called” fax machines have to communicate and understand each other, the way they do this is by using the T.30 protocol, much like 2 people talk having a conversation in the same language.



Standard Faxing Diagram

The T.30 protocol is made up of 5 different phases which fall into 2 categories; call control and image transfer. The call control handles various different factors of the call control, such as what speed to transmit at, whether to use error correction etc. The image transfer handles the transfer of document which has been converted into a TIFF image.
T.30 relies on a permanent connection between the 2 devices to ensure the fax is transmitted correctly. If the connection is lost or delayed for a long period of time (above 6 seconds) the fax fails. The network that connects the 2 devices is known as the PSTN network – the telephone network.


The Future

As more and more businesses migrate to VoIP solutions, the infrastructure that once catered for faxing is no longer available, as VoIP doesn’t require a separate legacy network (PSTN) to carry analogue traffic.
VoIP solutions make use of networks that are already in place - computer networks. These networks operate differently from telephone networks, they do not create permanent connections between devices to communicate, instead they transmit packets that could take any path to find its destination.
With this new challenge, faxing had to find a way to send faxes over an IP network. This was achieved by implementing a new protocol, which is called T.38.

T.38 allows fax messages to be sent over an IP network, the messages are sent to some sort of gateway (a gateway connects 2 different types of network together – in this case an IP network to a PSTN network).



T.38 diagram


It is important to remember that the T.38 protocol doesn’t replace T.30, it sits on top of it, and makes it compatible for transmission over a computer network. Once the data reaches the gateway, the message is transmitted in the T.30 protocol, which means normal fax machines are still able to communicate to the new IP fax machines/servers.

Zetafax supports both of Eicons implementation of Soft IP (H323 and SIP), and Brooktrout’s TR1034 FoIP enabled fax board, which means that businesses planning to migrate to VoIP, or were cautious of doing so due to faxing compatibility are now able to take advantage of this new technology.



H.323 – IP Telephony

H323 was designed by the International Telecommunications Union; it was the first of the 2 technologies.
It is a collection of different standards that make VoIP possible, these include:

H.225 Registration, Admission, and Status

H.225 Call Signaling

Real-Time Transport Protocol

Real-Time Transport Control Protocol


H.323 Components

Terminals – real time bidirectional multimedia communications (PC or H323 Application)
Gateways – connects 2 different networks i.e. SIP compatible LAN à PSTN
Gatekeepers – Addressing, authorizing & authentication



H323 diagram


Gatekeeper’s Role:

Address Translation

Admissions Control

Bandwidth Control

Zone Management

Call Control Signalling

Call Authorization

Call Management



SIP – Session Initiation Protocol (Defined by the Internet Engineers Task Force)


SIP is the newer of the 2 IP telephony technologies; it takes a top down approach for the communication between the 2 end users. SIP creates a session over the IP network and uses SIP commands to setup the call.


SIP Details

Session Initiation Protocol, defined by the IETF: RCF2543

Call control (signalling) protocol

Creates a session over packet networks for media

Independent of lower-layer transport protocols

Series of ASCII/Text messages similar to HTTP

SIP call control protocol establishes session connection

SDP – Session Description Protocol – describes the media that will be established

  SIP sessions use T.38 protocol to carry fax media - SIP & T.38 FoIP is defined in ITU-T T.38 Anex D



User Agent - Endpoint Entity which issues/accepts SIP commands (Software or hardware phone)

Registrar (optional but recommended) - Accepts REGISTER requests, Updates location database with the contact information for the user

Proxy Server (optional) –Makes requests on behalf of clients, Address lookup

Redirect Server (optional) –Accepts a SIP request, maps the SIP address of the called party to a new address and returns it to the client



SIP diagram


Registrar’s role

Dynamically build’s a table of registered User Agents




IP – Internet Protocol

VoIP – Voice over Internet Protocol

FoIP – Fax over Internet Protocol

PSTN – Public switched telephone network

LAN – Local Area Network

T.30 – Analogue protocol used to control and transmit faxes

T.38 – Digital protocol that allows faxes to be passed over a data network



Related topics

Installing the Brooktrout SR140

Installing the Brooktrout devices and drivers

Configuring Brooktrout devices for use in Zetafax Server.