Information Technology Reference
In-Depth Information
The receiving device can then optionally send either a Called Subscriber Information (CSI) message that provides information
as to the identity of the receiving device, or a Non-Standard Facilities (NSF) message that informs the sending device about
additional features.
Next, the sending device can optionally send a Transmitting Subscriber Identification (TSI) message, or respond to an NSF
message with a Non-Standard facilities Setup (NSS) message. The NSS can be used to select additional parameters.
The sending device then sends a Training Check (TCF) message, and the receiving device responds either with a Failure to
Train (FTT) if the modulation speed is not acceptable or a Confirmation to Receive (CFR) if the speed is acceptable.
Phase C:
Message transmission.
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During this phase, the page data (T.4 data) is sent from the sending device to the receiving device.
The end of phase C is indicated by the sending device transmitting six End of Line (EOL) messages as T.4 data. These EOL
messages are seen as a Return to Control (RTC) message and confirm the end of phase C.
Note that if Error Correction Mode (ECN) is requested during phase B then T.4 page data that is not received error free then a
Partial Page Request (PPR) message can be sent to indicate that frames should be re-sent.
Phase D:
Post-message procedure.
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The sending device sends either a Partial Page Signal (PPS) or an End Of Procedure (EOP) message.
PPS is used if ECN is being used and this is acknowledged using a Message Confirmation (MCF) from the receiving device.
The EOP message indicates that pages have been sent, and the receiving device acknowledges with an MCF.
Phase E:
Call release.
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At this stage either the sending or receiving device can terminate the call using a Disconnect (DCN) message.
Fax can be transported over IP networks either in real time or using store-and-forward.
