When you do not need a full-time connection, you can save money by using switched digital service, which is generally called Switch-56 (SW56).
An SW56 link is similar to Digital Data Services (DDS) setup in that the data terminal equipment (DTE) connects to the digital service by way of channel service unit/data service unit (CSU/DSU). An SW56 CSU/DSU, however, includes a dialing pad from which you enter the telephone number of the remote host. You can use SW56 to make dial-up digital connections to any other SW56 subscriber anywhere in the country or across international borders.
An SW56 call is carried over the long distance digital network just like a digitized voice call. SW56 uses the same telephone numbers as the local telephone system, and usage charges are the same as those for business voice calls.
SW56 is only in North American networks, and it is limited to single channels that can only carry data. SW56 is an alternative for locations where ISDN is unavailable.
Typically, you can connect to an SW56 CSU/DSU over V.35 or RS 449 serial interface with synchronous protocol at rates up to 56 kbps. With a V.25bis call/answer unit, data and call control flow over a single serial interface.