PURPOSE: To realize a function revising a modulo number accordingly by judging statistically transmission quality of a transmission medium in the data communication equipment capable of HDLC communication.

CONSTITUTION: The communication equipment is provided with an HDLC control means 7 implementing transmission reception in an HDLC format, a transmission quality calculation means 9 calculating the transmission quality of a transmission medium and a storage means 11 storing the calculated result, and a control means 17 controlling the HDLC control means 7, the transmission quality calculation means 9 and the storage means 11 and discriminating statistically the content of the storage means 11 and implements data communication with a best modulo number at all times. Thus, a packet of the modulo number is sent consecutively in a conventional method and when a first packet is in error in the transmission medium, a disadvantage has been caused conventionally that it is required to send again the data from the first packet and the HDLC communication section implements useless processing, the system cannot implement other processing in the system and the performance has been deteriorated, but in this invention, when the transmission quality state of the transmission medium is deteriorated, the modulo number is reduced to save the useless processing.