34.5 FAX OVER IP

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Fax is used extensively because of its reliability and ease of use. Fax messages are transmitted using the PSTN and hence using the circuit switching operation. As in voice, we need to pay for long distance calls for sending messages outside the local area. If fax transmission is done using the IP networks, we can save a lot of money on fax calls. Original fax data is in digital form, and hence if packet transmission is used, there is no need to convert into analog form as is done for transmission over PSTN.

To transmit fax messages over IP networks, there are two approaches: the store-and-forward method and the real-time method.

In the store-and-forward method, the fax message is stored at intermediate servers and finally reaches the destination. It is similar to the store-and-forward mechanism used in e-mail except that the message is in fax format. It is easy to implement this method over the IP networks. ITU standard T.37 defines the store-and-forward method used for fax over IP.

Note

In the store-and-forward method of fax transmission over an IP network, the fax message is stored at a server and then relayed to the destination. ITU standard T.37 defines the protocols used for this method.

ITU standard T.38 defines the real-time transmission of fax over IP; this standard is a part of the H.323 series standards. Delivering a fax message in real time is a challenging task because of the inherent problems in the IP networks. The problems are:

Delay: This can be network delay or processing delay. Both are likely to create problems for fax transmissions—if the calling fax machine does not receive a response in a fixed time, it may time out and disconnect the call. Spoofing is used to overcome the problem of delay. In spoofing, a local response is generated without waiting for the response from the called machine.
Jitter: In IP networks, we cannot guarantee that all the packets will arrive at the destination with the same delay. The variation in the delay causes problems in reassembling the packets at the receiving end. Generally, packets are time stamped to overcome this problem.
Packet loss: In voice over IP, the packet loss can be compensated for by replaying the earlier packet or sometimes by ignoring that packet. However, we cannot use the same trick here—information in the fax message is lost if packets are ignored. The packet loss can be overcome using one of the following methods: (a) repeating the information in subsequent packets so that errors can be corrected at the receiving end or (b) using an error detection and correction protocol such as TCP, though it introduces additional delay.

In real-time transmission of fax over IP networks, the issues to be addressed are delay, jitter, and packet loss. ITU standard T.38 defines the protocol for real-time transmission of fax over IP networks. This standard is a part of the H.323 series of recommendations.

We need to devise methods to overcome these problems. Before we study the fax over IP software architecture, an understanding of fax transmission over the PSTN is required.

34.5.1 Fax Transmission over PSTN

For fax transmission, there are two standards defined by ITU. T.30 defines the mechanism for fax transmission, and T.4 defines the format for image data. For transmission over PSTN, the calling fax has to dial the called fax, transmit the data and then disconnect the call, just like a voice call. The fax call is divided into five stages:

Establishment of a fax call
Negotiation of capabilities
Page transfer
End of page and multipage signaling
Disconnection of the call

Establishment of fax a call: A fax call can be established either manually or automatically. In manual call establishment, the user dials the destination fax number and then switches to the fax mode. In automatic calling, the calling fax machine dials automatically. The fax call is indicated with a calling tone (CNG), a short periodic tone we can hear. The called fax machine sends an answer tone called station identification (CID), a high-pitched tone that can be heard (an audible indication that things are going fine).

Negotiation of capabilities: In this stage, the capabilities, particularly the speed of transmission, are negotiated. This negotiation takes place at 300bps data rate because the called fax machine's capabilities are not known. The called machine starts the negotiation procedure by sending a digital identification signal (DIS) containing its capabilities (such as the data rate it supports). Also, called subscriber information (CSI) and non-standard facilities (NSF) messages are sent (which are optional). The calling machine responds with a digital command signal (DCS) to define the conditions for the call, after examining its own capabilities table. Then, the calling fax machine sends a training check field (TCF) to verify that the channel is suitable for the transmission at the accepted data rate. The called machine responds with confirmation to receive (CTR) to confirm the capabilities and speed. This process is known as high speed modem training.

Page transfer: In this stage, the scanned page data is compressed and sent over the channel. T.4 protocol standard is used to format the page data.

Fax transmission over the PSTN is done in five stages: establishment of fax call, negotiation of capabilities of the fax machines, page transfer, end-of-page and multipage signaling, and disconnection of the call.

End-of-page and multipage signaling: At the end of the page transmission, if there are no further pages, an end of procedure (EOP) message is sent by the calling machine to indicate that the call is complete. If additional pages are to be sent, a multipage signal (MPS) is sent. The called machine responds with a message confirmation (MCF) to indicate its readiness to receive the next page.

Disconnection of the call: The calling machine sends the disconnect message (DCN) to indicate the end of the call. However, the calling fax machine may disconnect without sending this message.

Essentially, the same procedure is followed for fax over IP as well. However, the problems inherent in the IP networks such as, delay, jitter, and packet loss need to be taken care. Let us see how we can overcome these problems.

34.5.2 Fax over IP

To transmit fax over IP networks (and in general, over the packet networks), an inter working function (IWF) or gateway is required. The function of this IWF is to carry out protocol conversion between the fax machine protocols and the IP-based network protocols. The IWF is implemented as a software solution with necessary hardware interfaces using a fax interface unit (FIU). FIU acts as the interface between the IP network and the fax machine. The block diagram of FIU is shown in Figure 34.9.

click to expand
Figure 34.9: Fax interface unit.

Note

For fax over IP, an inter working function or gateway is required which carries out the protocol conversion between the PSTN protocols and the IP network protocols.

The functions of the FIU are:

Demodulate the analog voice band signal from the fax machine and convert it into digital format suitable for an IP-based packet network.
Modulate data received from the packet network to be sent over the analog interface to the fax machine.
Convert between analog fax protocols and digital fax protocols used in the packet networks.

As shown in Figure 34.9, the FIU contains three modules: fax modem module, fax protocol converter module, and fax network driver.

Fax modem module: This module has to support the functionality of the analog fax protocols, which include the following:

CED detection and generation
CNG detection and generation
300bps V.21 channel 2 binary signaling modulation and demodulation
HDLC framing
High speed modulation and demodulation per the following standards: V.27 (2400, 4800 bps), V.29 (7200, 9600 bps), V.17 (7200, 9600, 12000, 14400 bps) and V.33 (12000, 14400 bps).

Fax protocol converter module: This module compensates for the effects of delay and lost packets. It prevents the local fax machine from timing out if the response is not received from the called fax machine due to network delay. This is done by generating the HDLC flags. If the response is not received even after time out, it sends a command repeat (CRP) to resend the frame. The other function of this module is to carry out protocol processing.

Fax network driver: This module assembles and disassembles fax packets for transmission over packet networks. The functions of this module are to format the control information, format the fax data, remove the jitter by delaying the data through elastic buffering, and compensate for lost packets by repeating information of a packet in subsequent packets so that errors can be corrected using the information in the subsequent packets.

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