FAX OVER IP FoIP V17 V.17 G3 14400baud V34 33600baud V.34 SuperG3 T38


Phase A: Call establishment
CNG: comfort noise generation
 FAX generates it to identify itself as a FAX rather than a human
CED: constant/flatline tone pulse - determine G3 speed
ANSAM: SuperG3 capable, negotiation..
Phase 1: Network Interaction: V.34 or not?
Phase 2: Line probing & Ranging: Excnahge capab, round-trip delay,
 determine bandwidth and SNR
Phase 3: Primary Channel Equalizer Training: goes back to Phase 2
 if modulate and demodulate
 signals responding as expected
Phase B: pre-message procedures:
Called fax:
CSI: called subscriber ID (name of company, called number, et cetera..)
DIS: digital identification signal: the capabilities: compression rates, speeds..
Calling fax:
TSI: transmitting subscriber ID (name of company, called number, et cetera..)
DCS: digital command signal: based on the DIS this is what we will use
TCF: training check (not used in V.34), "quality of sound sent across"
Called fax:
CFR: confirmation to receive - TCF success, everything is okay
FTT: failure to train - TCF has failed..
Phase C: message transmission
Sending device starts short training (resnyc) then (w/ or w/o ECM error correction mode)
sending FAX image data FCD FCD FCD .. FCD (frames called Fax Code Data) then when all sent
RCP RCP RCP (exactly 3 RCP frames - return to control for partial page) "I finished with this section"
Phase D: post-message procedure
Sending side:
PPS - partial page signal: chacklist of how many frames has been sent
 PC page count, BC block count, FC frame count
Receiving side:
MCF - Message confirmation: PC BC FC actually transferred
PPR - if something went wrong.. Partial page request (missing frames?)
Phase C & D without ECM:
Sending side:
short training (resync) -> T4-non-ecm-data -> MPS (partial page signal)
short training (resync) -> T4-non-ecm-data -> repeat until end of data then
EOP end of procedure
Receiving side:
MCF - Message confirmation: PC BC FC actually transferred
RTP - if something went wrong.. and Retrain positive RTP have some concerns go back to Ph.B/Retrain negative RTN did not work at all
Phase E: call release
Sending side:
DCN disconnect
Training check frame (TCF)
The training check frame is the phase of a fax call when the sending fax transmits a sequence of zeros to the receiving fax machine at the highest common data rate negotiated during prior phases. This check determines whether the line quality is adequate to pass information at the desired rate. If the line quality is good, the receiving fax machine receives this sequence of zeros without error. If line quality is not good, bit errors will occur during reception, and the total amount of zeros is not received.
The duration of the zero sequence sent by the transmitting fax is 1.5 seconds. The T.30 protocol states that to successfully receive this training data, 1.5 seconds +/- 10% of these zero symbols should be received. If successful, the receiving fax sends back a confirm signal (CFR) to the transmitting fax. If the standard is not met, the receiving fax sends back a fail to train (FTT) signal, indicating to the transmitting fax that a lower data rate should be tried.
NaturalFax reports the results of training information. This information can be used to determine the quality of the training event, and therefore the quality of the line. This metric can be used to determine whether a receive problem is the result of poor line quality, insufficient training, or a compatibility issue. The rx_training_zeros metric reports the length of zeros received in tens of milliseconds. A good value for the TCF is greater than 1.35 seconds. If values less than this are reported, the receiver is not able to demodulate the incoming signal appropriately. This results in bit errors.


Leave a Reply

Your email address will not be published. Required fields are marked *