An elusive problem

Identifying fax transmission problems can be difficult. For example, two network operators were receiving customer complaints about fax call failures on their networks. The transmission medium was an international satellite connection with echo cancelers deployed at both switch sites.

Customers from one country were trying to send multiple-page faxes to the second country, and the machines would consistently fail to transmit all the pages successfully. Further investigation showed that in the majority of cases, the fax transmissions were failing at page boundaries.

The network operators could not identify the problem for some time because the circuits involved had echo cancellation. Detailed investigations revealed that signal level fluctuation and incompatible machines at each end were contributing factors. The receive and transmission of the timing signals were set in such a way that they became intolerant of echo whenever a burst of echo occurred.

Solving the problem involved applying a number of complementary "fixes." The existing echo cancelers were replaced with later generation units equipped with VFax software from Coherent Communications Systems, which offered improved tolerance to high signal levels. Additionally, the output signal levels of the machines were adjusted down.

Both operators now take great pains when planning fax routes, ensuring that the equipment is optimized to handle the transmission variables.