Modem having speed notifying means

A modem (2) includes a data transmission and reception part (30) for making data transmission and reception with a terminal equipment, a modem part (50) for modulating a transmission signal with a predetermined modulation system before transmitting the same to another modem (4) and for demodulating a reception signal which is received from the other modem, a data compression and decompression part (40) for compressing serial data from the data transmission and reception part at a predetermined compression rate before supplying the same to the modem part and for decompressing the compressed serial data from the modem part before supplying the same to the data transmission and reception part, and a speed determining part (60), notified of a communication speed between the two modems determined from the modem system as well as a compression rate determined from a data compression technique of the data compression and decompression part, for determining a optimum communication speed when the connection between the two modems is established.