Fax functions and operation
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.
●
The internal firmware cannot be modified through the fax connection.
●
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 finally 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.
Receive faxes when you hear fax tones
In general, incoming faxes to the product are automatically received. However, if other devices are
connected to the same phone line, the product might not be set to answer automatically.
70 Chapter 1 Theory of operation ENWW
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