Xerox Phaser 6128MFP Service Manual page 114

Theory of Operation
2-60
The following steps contains the line connection procedure between two Faxes.
1. When an AT command (a modem control command) is sent from the control
circuit to the modem, the hook switch is activated, and a state is obtained that
is identical to that when the handset of a telephone is lifted. A dial tone
(400Hz/48VDC) is sent from the local switch. The modem's speaker emits the
dial tone as an audible sound.
2. After image scanning, the telephone number (a previously stored number,
number entered by pressing phone buttons, etc.) is automatically dialed and
transmitted to the local switch.
3. The switch connects lines according to the transmitted number.
4. When a connection between local switches is established, the local switch of
the called party sends a ringing signal to the telephone of the called party.
The telephone that receives the ringing signal emits its ringtone. At the same
time, the called party's local switch sends a ring back tone (RBT) to the calling
party's telephone to indicate that a connection to the called party has been
established.
5. When the called party's handset is lifted, activating the hook switch, the local
switch on the called party side receives a response signal and stops sending
tones to the calling and called parties. This is when a communications path is
established between both parties.
6. The receiving party's Fax automatically answers when it receives the call
signal, and the hook switch is activated. The local switch on the receiving party
side receives a response signal and stops sending tones to the sending and
receiving parties, thereby establishing a communications path between both
parties.
In the case of a telephone call, only voice conversion between the two parties
follows. For Fax, preparation for delivery of image data is required that
includes the following types of exchanges:
a. The sending Fax indicates that the transmission is a Fax transmission.
b. The receiving Fax indicates that it is ready to receive and also its
communications capacity.
c. The sending Fax then sends data in accordance with the receiving Fax's
communications capacity.
Once the preparation is completed, image data sending and receiving is
started. Image data is modulated into an analog signal by the A/D converter
at the sending Fax, then sent from its NCU. Image data received by the NCU
of the receiving Fax is demodulated into a digital signal by its A/D converter
and then sent to the control circuit. When image data reception is completed,
the Fax automatically disconnects the line (hook is Off).
In summary, the NCU automatically executes a series of such operations from
hook switch On to hook switch Off.
The control circuit also retains other important functions such as data compression
and memory. With data compression, any part of the scanned imaged data
consisting of continuous or black pixels is encoded into a single element, thus
compressing the volume of data. Memory temporarily stores data during
transmission and reception.
Phaser 6128MFP Service Manual