T.38 is a standard by International Telecommunication Union (ITU) for real-time fax communication over IP networks. Under the T.38 protocol, fax signals and data from the local fax terminal are packed as Internet Fax Protocol (IFP) packets at the local gateway to be sent over the IP network. The gateway at the remote end of the IP network, unpacks the IFP payload relayed over network and either regenerates the fax signals for transmission to the remote fax machine, or directly processes the signals as an Internet Aware Fax (IAF).
Figure 1 shows the network topology for a T.38 connected network. Each T.38 device on the IP network acts as either a gateway, or an IAF device. Gateways may connect to fax machines directly, or over the PSTN. The T.38 and fax module in this document will operate in connection with a gateway and act together as an IAF such that no fax machine is required.
As shown in Figure 4, the interface with the IP network is provided by the local gateway and is mostly independent of the T.38 solution. The standard transport layer protocol that will be used by the gateway is either UDP + RTP, UDP + UDPTL or TCP.
In a TCP session, the GAO T.38 solution will exchange IFP packets with the application that is the TCP payload as shown in Figure 2. It is the responsibility of the TCP stack to create or remove the TCP and IP headers, and to ensure proper delivery and error correction of the TCP packet.
In a UDP session, the GAO T.38 solution will exchange packets with the application that contains UDPTL headers, the IFP packet and either UDPTL redundancy or UDPTL forward error correction. These packets are the UDP payload as shown in Figure 3. It is the responsibility of the UDP stack to create or remove the UDP and IP headers, and to handle any other UDP based protocols such as RTP. The GAO T.38 solution will handle packet re-ordering and error detection or correction in the UDPTL module as per the T.38 standard.
Session initiation is not included in T.38, and must be provided external to the GAO T.38 solution. Standard session initiation protocols include SIP + SDP, and H.323. The GAO T.38 solution has an interface that allows SDP to be configured for initiating a T.38 session with SIP + SDP according to T.38 Annex D. The SIP + SDP stack works along side of the T.38 network stack to manage the session.
The image data will be supplied from a specified TIFF Class F (TIFF-F) file formatted with T.4 (MH or MR) or T.6 (MMR) data. The GAO IAF solution can convert the TIFF file between the formats to match the capabilities of the remote fax machine.