Table of Contents:

Definition and Overview

1. Introduction

2. Why Fax Testing Is Necessary

3. Fax Operation Overview

4. Basic Fax System Structure and Process

5. Message Repetitions and Time-Outs

6. Fax System Test Issues

7. Network Test Issues

8. Common Deviations from ITU Standards

9. Fax Testing Customer Types

10. Fax Testing Application Areas

11. Conclusion

Self-Test

Glossary

PDF of this tutorial

Sending a fax is far more complex than just dialing a phone number and sending an image. The calling unit must first confirm that the call is being answered by an actual fax device rather than a data modem, answering machine, or human being. Once it is determined that two fax devices are talking to each other, these devices must then exchange information to find out what capabilities they both support (e.g., data transmission rate, image resolution, data compression schemes, paper size). They then must agree on a mutually supported subset of these capabilities. Next, the phone connection must be evaluated to determine the maximum practical data rate available. Finally, the fax image is sent. However, it is possible that noise or distortion will corrupt the image during its transit of the connecting network. To check for this possibility, the receiving fax device must evaluate the image and send an acknowledge message indicating whether the image was received correctly or whether it had an unacceptable number of errors.

The basic system structure of the typical fax device is shown in Figure 1.

Figure 1. Basic Fax System Structure and Process

The protocol for sending or receiving a fax image and exchanging associated messages is defined in the International Telecommunications Union (ITU) Recommendation T.30. The sequence proceeds as follows:

1. An image to be transmitted is first scanned to create a bit map in which black and white dots are represented by ones and zeros.
2. This bit map is compressed to eliminate redundant data and then passed to a T.30 protocol handler, a set of software routines that manages the exchange of capability information and image transmission.
3. The data is finally passed to a modem for transmission through the network.

In a similar manner, received images are extracted from the T.30 message sequences, decompressed, and printed or stored. The sequence of events in a simple call with no errors is shown as the following:

1. Calling unit dials phone number and waits for answer.
2. Answering unit detects RING and answers call.
3. Answering unit identifies itself as a fax device and lists its capabilities.
4. Calling unit responds with message specifying transmission parameters.
• data rate
• image resolution
• data compression method to be used
• image size, etc.
5. Calling unit sends sample data pattern to test phone-line connection.
6. Answering unit evaluates sample data pattern and either accepts proposed data rate or requests a lower data rate.
7. Calling unit sends image.
8. Answering unit acknowledges receipt of image.
9. Call ends.