PDF of this tutorialNew fax products must be designed to be compatible with the existing installed base of fax systems made by many different companies. They must be compatible with fax products from many suppliers having unique performance characteristics. They must adapt to changing fax technology over the years, and they must handle new enhanced services. Testing is a vital requirement for achieving and maintaining such compatibility. Because carriers and network operators must be able to move huge and growing volumes of fax traffic over increasingly complex networks, transmission and compatibility problems must be quickly analyzed and corrected. Transmission efficiency (throughput) must be continuously improved by using the most effective data compression schemes in order to reduce the cost per call, and all factors working to impede efficient transmission such as unnecessary image padding must be identified and compensated for immediately. Furthermore, transmission quality must be continually improved. Comprehensive testing on an ongoing basis is mandatory for achieving these goals.
With the advent of voice-over–IP (VoIP) networks, fax traffic detection in VoIP gateways and fax transmission via IP networks is an important component for the success of real-time, IP–based services. The transport and quality expectations of fax in IP networks are very different from voice. To accommodate fax, special IP–fax protocols have been developed for the store-and-forward (T.37) and real-time (T.38) transmission of fax-over–IP (FoIP) networks. Systems using these new protocols not only require thorough testing during development and implementation, but will also need ongoing testing during operation to guarantee acceptable quality of service (QoS). Testing issues that must be addressed include the following:
- interoperability problems between different vendor equipment on the IP side
- compatibility problems between new IP–fax protocols and conventional telephony networks using older standard fax protocols
- IP network impairments such as packet loss, jitter, and extensive delay that often must be compensated for by modification of the fax transmission in the telephone networks; these problems (mostly timing-related) are typically handled in network gateways via various spoofing techniques, including stuffing of fax frames with empty information or sending dummy frames to keep the receiving fax on-line; such techniques can have a severe impact on the length and completeness of the fax transmission
