Image communication system, and image communication apparatus and modem used in the system

A data communication apparatus capable of performing communication by selecting one of a plurality of transmission speeds, comprising:

• reception means for receiving a check data for determining a transmission speed sent from a communication partner,
• measuring means for measuring a number of times or a period of receiving the check data received correctly by said reception means;
• and
• communication means for performing subsequent communication with the communication partner at the transmission speed based on the number of times or the period measured by said measuring means.

A data communication method for performing communication by selecting one of a plurality of transmission speeds, comprising the steps of:

• receiving a check data for determining a transmission speed sent from a communication partner;
• measuring a number of times or a period of receiving the check data correctly in the receiving step;
• and
• performing subsequent communication at the transmission speed based on the number of times or the period measured at the measuring step.

A data communication method for performing communication by selecting one of a plurality of transmission speeds, comprising the steps of:

• transmitting line check data for determining a transmission speed;
• receiving the line check data;
• confirming that the line check data is received satisfactorily a predetermined number of times;
• selecting a desired one of the plurality of transmission speeds in accordance with a number of times the line check data is received satisfactorily or a number of times the line check data is not received satisfactorily;
• transmitting a response for designating the selected transmission speed;
• receiving the response;
• and
• performing a transmission speed changing operation in accordance with the received response.

A data communication system capable of performing communication at a plurality of transmission speeds, comprising:

• a transmission-side apparatus that transmits first data at a first transmission speed, said first data including a unique signal and continuous "1" data;
• and
• a reception-side apparatus that instructs said transmission-side apparatus to transmit second data at a second transmission speed by falling back from the first transmission speed by a desired number of steps in response to a reception state of the first data and first threshold values;
• wherein said transmission-side apparatus retransmits the second data at a second transmission speed in accordance with the instruction from said reception-side apparatus,
• wherein said reception-side apparatus determines whether or not the second transmission speed is satisfactory on the basis of a reception state of the second data and second threshold values,
• and wherein said reception-side apparatus determines the second transmission speed on the basis of a time of the unique signal and a maximum reception time of the continuous "1" data, and determines the number of fall-back steps so that the transmission speed of the transmission side apparatus coincides with the determined second transmission speed or with the first transmission speed.

A data communication method capable of performing communication at a plurality of transmission speeds, comprising the steps of:

• receiving data transmitted at a first transmission speed;
• determining a desired second transmission speed from the plurality of transmission speeds on the basis of a reception state in the receiving step;
• and
• transmitting, when the transmission data cannot be satisfactorily received from a destination apparatus, a full-duplex instruction signal through a backward channel during the data reception in the receiving step to communicate the data at the second transmission speed.

A data communication method capable of performing communication at a plurality of transmission speeds, comprising the steps of:

• transmitting data at a first transmission speed to a destination apparatus;
• receiving a full-duplex response from the destination apparatus through a backward channel during the data transmission in the transmitting step;
• and
• performing data transmission at a desired second transmission speed of the plurality of transmission speeds corresponding to the full-duplex response received in the receiving step.

A data communication apparatus capable of performing communication at a plurality of transmission speeds, comprising:

• communication means for communicating information for specifying a first speed with a communication partner;
• selecting means for selecting threshold values corresponding to the first speed;
• arithmetic means for calculating an error during reception of data transmitted by the communication partner at the first speed;
• and
• determining means for determining a second transmission speed for communicating with the communication partner in accordance with the error calculated by said arithmetic means and the threshold values selected by said selecting means.

A data communication method capable of performing communication at plural transmission speeds, comprising the steps of:

• selecting a first speed from the plural transmissions speeds;
• selecting threshold values corresponding to the first speed;
• calculating an error during reception of predetermined data transmitted at the first speed;
• determining a second transmission speed based upon the error calculated in the calculating step and the selected threshold values.