If you experience problems using the fax feature on a VoIP network, ensure that all of the product cables
and settings are correct. Configuring the Fax Speed setting to Medium(V.17) or Slow(V.29) can also
improve your ability to send a fax over a VoIP network.
If you continue to have problems faxing, contact your VoIP provider.
The fax subsystem
The formatter, fax card, firmware, and software all contribute to the fax functionality. The designs of the
formatter and fax card, along with parameters in the firmware, determine the majority of the regulatory
requirements for telephony on the product.
The fax subsystem is designed to support V.34 fax transmission, lower speeds (such as V.17 fax), and
older fax machines.
Fax card in the fax subsystem
Two versions of the fax card are used in the product. One is used in the North American, South American,
and Asian countries/regions. The other is used primarily in European countries/regions.
The fax card contains the modem chipset (DSP and CODEC) that controls the basic fax functions of
tone generation and detection, along with channel control for fax transmissions. The CODEC and its
associated circuitry act as the third-generation silicon data access arrangement (DAA) to comply with
worldwide regulatory requirements.
The only difference between the two versions is that each version is compliant with the 2/4-wire phone
jack system from the respective country/region.
Safety isolation
The most important function of the fax card is the safety isolation between the high-voltage, transient-
prone environment of the telephone network (TNV [telephone network voltage]) and the low-voltage
analog and digital circuitry of the formatter (SELV [secondary extra-low voltage]). This safety isolation
provides both customer safety and product reliability in the telecom environment.
Any signals that cross the isolation barrier do so magnetically. The breakdown voltage rating of barrier-
critical components is greater than 5 kV.
Safety-protection circuitry
In addition to the safety barrier, the fax card protects against over-voltage and over-current events.
Telephone over-voltage events can be either differential mode or common mode. The event can be
transient in nature (a lightning-induced surge or ESD) or continuous (a power line crossed with a phone
line). The fax card protection circuitry provides margin against combinations of over-voltage and over-
current events.
Common mode protection is provided by the selection of high-voltage-barrier critical components
(transformer and relay). The safety barrier of the fax card PCB traces and the clearance between the
fax card and surrounding components also contribute to common mode protection.
A voltage suppressor (a crowbar-type SIDACTOR) provides differential protection. This product
becomes low impedance at approximately 300 V differential, and crowbars to a low voltage. A series
thermal switch works in conjunction with the crowbar for continuous telephone line events, such as
crossed power lines.
100 Chapter 5 Theory of operation ENWW
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