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
Asia Pacific 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.
All communications cross the isolation barrier magnetically. The breakdown voltage rating of barrier-critical
components is greater than 5 kV.
Data path
TIP and RING are the two-wire paths for all signals from the telephone network. All signaling and data
information comes across them, including fax tones and fax data.
ENWW Fax functions and operation 35
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