Fax functions and operation
The following sections describe the product fax capabilities.
Computer and network security features
The product can send and receive fax data over telephone lines that conform to public switch telephone
network (PSTN) standards. The secure fax protocols make it impossible for computer viruses to be
transferred from the telephone line to a computer or network.
The following product features prevent virus transmission:
●
No direct connection exists between the fax line and any devices that are connected to the USB or
Ethernet ports.
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The internal firmware cannot be modified through the fax connection.
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All fax communications go through the fax subsystem, which does not use Internet data-exchange
protocols.
PSTN operation
The PSTN operates through a central office (CO) that generates a constant voltage on the TIP and RING wires
(48 V, usually). A device goes on-hook by connecting impedance (such as 600 ohms for the U.S.) across the
TIP and RING so that a line current can flow. The CO can detect this current and can send impulses like dial
tones. The product generates more signaling tones, such as dialing digits, to tell the CO how to connect the
call. The product can also detect tones, such as a busy tone from the CO, that tell it how to behave.
When the call is connected, the CO behaves like a piece of wire connecting the sender and receiver. This is the
period during which all of the fax signaling and data transfer occurs. When a call is completed, the circuit
opens again and the line-current flow ceases, removing the CO connection from both the sender and the
receiver.
On most phone systems, the TIP and RING wires appear on pins 3 and 4 of the RJ-11 modular jack (the one on
the fax card). These two wires do not have to be polarized because all the equipment works with either TIP or
RING on pin 3 and the other wire on pin 4. This means that cables of either polarity can interconnect and will
still work.
These basic functions of PSTN operation are assumed in the design of the fax subsystem. The product
generates and detects the signaling tones, currents, and data signals that are required to transmit and
receive faxes on the PSTN.
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
Three versions of the fax card are used in the product. One for Asia Pacific Countries/Regions and the United
States, one for Europe, and one for Brazil. Each version is compliant with the 2/4-wire phone jack system
from the respective country/region.
ENWW Fax functions and operation 27
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