WAVECOM Decoder W74PC, W-PCI/e, W-CODE, W-CLOUD Manual V9.1.0 Modem and FAX Modes
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The receiving section will attempt to cancel out any echo, then demodulate the incoming signal, recover
carrier and bit synchronism then finally de-frame and de-scramble the bit stream. The equalizer will at-
tempt to invert line characteristics, and if Trellis coding has been used a Trellis decoder is inserted.
Handshaking
Before user data can be transmitted, handshaking must take place between the originating and answering
modems or fax equipment. Using fax communication as an example, five distinct phases can be identified:
Phase A – Call establishment
Receiver number is dialed and a calling tone (CNG) is sent, and when the receiver goes off-hook it an-
swers with an answer tone.
Phase B – Pre-message procedure
Using V.21, 300 bps FSK, capabilities, configuration and finally rate test signals are exchanged in HDLC
frames.
Phase C – Message transmission
After the Confirmation-to-Receive signal has been received, the originator starts sending images.
Phase D – Post-message procedure
Originator sends a Return-To-Control signal switching both devices back to V.21, and the originator sends
end-of-message signals. The answering equipment sends a Message Confirmation signal indicating that
the message has been successfully received.
The illustration below is a time vs. frequency spectrum of a V.32 call.
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