You can use digital service and Digital Data Services (DDS) with Point-to-Point Protocol (PPP).
With digital service, data travels all the way from the computer of the sender to the central office of the telephone company, to the long distance provider, to the central office, and then to the computer of the receiver in digital form. Digital signaling offers much more bandwidth and higher reliability than analog signaling. A digital signaling system eliminates many of the problems that analog modems must deal with, such as noise, variable line quality, and signal attenuation.
Digital Data Services (DDS) is the most basic of digital services. DDS links are leased, permanent connections, running at fixed rates of up to 56 kbps. This service is also commonly designated as DS0.
You can connect to DDS using a special box called channel service unit/data service unit (CSU/DSU), which replaces the modem in an analog scenario. DDS has physical limitations that are primarily related to the distance between the CSU/DSU and the telephone company central office. DDS works best when distance is less than 9000 m (30 000 ft). Telephone companies can accommodate longer distances with signal extenders, but this service comes at a higher cost. DDS is best suited for connecting two sites that are served by the same central office. For long distance connections that span different central offices, mileage charges can quickly add up to make DDS impractical. In such cases, Switched-56 might be a better solution. Typically, you can connect to a DDS CSU/DSU over V.35, RS449, or X.21 serial interface with synchronous protocol at rates up to 56 kbps.