The communication between two fax machines is as follows:
1. The transmitter calls the receiving fax machine. The transmitter will generate the dialing signal and then continuously send out a CNG tone which is used to signal an auto receive fax machine that another fax is making the call. This CNG tone is also used with allot of line sharing devices today.
2. The phone line on the receiving side will ring and the receiving fax will pick up the line and send out a CED tone. This tone only tells the transmitter that it has connected to a fax machine.
3. The receiving fax then sends out a PREAMBLE signal. This signal is a series of flags that are sent out to condition the phone line and to synchronize the two modems. A PREAMBLE will be generated just before any binary information is sent over the phone line.
4. The next set of signals are sent by the receiver and are binary signals. The first one is NSF Non Standard Facilities. This signal informs the receiver that it is a Canon machine with proprietary modes. This signal is not needed for a communication to take place. The next signal is CSI Called Subscriber Identification. With this signal the phone number of the receiving fax machine is sent out. This phone number needs to be programmed into the fax machine before this signal will be sent. If it is not programmed then the signal will not be sent, therefore this signal is also optional and is not needed for a communication to take place. The final signal sent by the receiver before the transmitter can respond is DIS Digital Identification Signal. This signal sends out all of the receiving faxes capabilities such paper size, modem speeds, resolution modes, etc. This signal is needed for the communication to take place.
5. After the transmitter receives the DIS signal it has enough information to continue with the procedure. The next signals that are sent are from the transmitter which is responding to the information sent to it from the receiver. After the PREAMBLE is sent the first signal is NSS Non Standard Facilities Setup. This signal will only be sent if the NSF from the receiver indicated that it was a Canon fax machine. If it is a Canon then the transmitter will send the NSS signal which will tell the receiver which Canon features will be used in the transmission. The transmitter has compared it's own Canon features with that of the receivers and commands the receiver to receive a document using certain Canon modes. The next signal sent is TSI Transmitting Subscriber Identification. This signal informs the receiving fax machine the phone number of the transmitter. This phone number must be programmed into the fax machine or the signal will not be sent. Finally the transmitter will send DCS Digital Command Signal. The signal also commands the receiving fax to receive a document in a particular mode. This signal is needed for communication to take place. All other signals are optional.
6. Now that the transmitter has commanded the receiver on how a document will be sent it, it begins the TRAINING signal. The training signal is generated by the high speed modems. For example it would start sending a training signal of its highest communication speed, typically 14.4kbps or 9600 bps. All other previously sent signals were sent using a 300bps modem speed. After the TRAINING has been performed a TCF Training Check Flag is sent to the receiver. The TCF is a consecutive number of "0" set for 1.5 seconds.
7. If the receiver receives the TCF correctly then the CFR Conformation To Receive Signal is sent back to the transmitter informing it that everything up until this point is alright. If the TCF is not received properly then the receiving fax will send back FTT Failure To Train at which point the transmitter will fall back to a slower modem speed and then try again. If the phone line is really poor the transmitter may need to fall back many times. The Fall back modes are as follows: 14.4kbps (1 try), 9600bps (1 try), 7200bps (1 try), 4800bps (2 tries), 2400bps (2 tries) and then failure.
8. After the transmitter receives the CFR command it begins TRAINING again and then begins sending the picture information. Each line of data is checked for errors on the receive side. After all the picture data is sent a RTC Return To Control signal is sent which informs the receiving modem that the transmitting modem is going to return control to the slower 300bps modem and away from the high speed modem. After this PREAMBLE is sent and then EOP End of Procedures is sent. EOP signals the receiver that the full page has been sent. If there were more pages to be sent or if a change to the transmission mode was to take place then the transmitter would send EOM End of Message or MPS Multi page signal.
9. If all the data was received properly the receiving fax would send back a PREAMBLE and then a MCF Message Confirmation signal. If the data had errors in them and it exceeded the maximum allowed then the receiver would send back RTN Retrain Negative of RTP Retrain Positive. Both of these signals indicate that that the data received exceeded the maximum allowed errors.
10. If the communication was proper and the transmitter received the MCF signal then the transmitter would send out PREAMBLE and then a DCN Disconnection notice which informs the receiving fax that it is disconnecting the phone line at which point the transmitter and receiver hang up the phone line.
No comments:
Post a Comment