More formally known as Digital Switched Access (DSA). A switched digital data service that operates much like the public switched telephone network (PSTN) operates for voice calls. Switched 56 service operates over a public data network (PDN) that actually is a physical and logical partition of the PSTN. Where the PDN supports out-of-band signaling and control, the service sometimes is known as Switched 64, as the full 64 kbps bandwidth of a DS-0 channel is available to support end user data transmission. Switched-1536 service supports a full ISDN PRI of 24 channels, each of which provides the full 64 kbps of DS-0 bandwidth, with all signaling and control taking place out-of-band on another PRI circuit through a technique known as non-facility associated signaling (NFAS). See also DS-0, ISDN, NFAS, out-of-band signaling and control, PDN, PRI, and PSTN.
A dial-up digital service provided by local and long distance telephone companies. There is a monthly fee and per-minute charge like the analog voice network. For connection, a DSU/CSU is used instead of a modem. Switched 56 uses a 64 Kbps channel, but one bit per byte is used for band signaling, leaving 56 Kbps for data.