Abstract: A digital trunk interface unit for interfacing with a digital trunk of a PSTN in the RAS for connecting the PSTN to a different network by a T1 or E1 trunk, comprises a plurality of line card slots; a plurality of E1 line cards inserted into the line card slots to provide an interface of an E1 trunk; a plurality of T1 line cards inserted into the line card slots to provide an interface of a T1 trunk; a digital switch; an R2 signaling part; a tone generator; a processor; a clock selector; a PLL; a frequency-divider; and a clock converter. Therefore, personal computers of the PSTN connected to the RAS by the trunk interface unit, can be connected to a packet network by a modem unit and a gateway, thereby being provided information services from information providers.