In this study, we realized the digital TETRA(Terrestial Trunked Radio) Modem and Gateway based upon the USN Network. Also, the digital TETRA Modem and Terminal is made of the SSPD(Single Slot Packet Data) method and MSPD(Multi Slot Packet Data) method that has the function of PEI, DMO, UMAC and MLE. The H/W design of digital TETRA Modem and Terminal has RF transmitter/receiver part ,baseband part, DSP subsystem, OMAP5910 MCU subsystem, 25KHz RF channel bandwidth and the S/W design has TETRA Protocol Stack Layer 1, TETRA Protocol Stack Layer 2(LLC and MAC), TETRA Protocol Stack Layer 3(MLE). USN Part is realized by using the new manager part and agent part and is suggested the new data frame.