Integrated Services Digital Network (ISDN)
Sites that use routers often need some sort of permanent, always-on WAN connectivity to other sites in the network. They also require a back-up system, usually some sort of a dial (circuit-switched) technology that can be used should that permanent connectivity fail. Other sites that need only occasional WAN connectivity can also implement a dialled circuit, for which a service provider charges a small fee per call and per minute, which is usually cheaper than using a leased line or packet-switched service. For these reasons, dialed links are still popular although they are not the typical choice for WAN connectivity.
Of all the circuit-switched "dial" options, Integrated Services Digital Network (ISDN) is the most popular choice for connectivity between routers. ISDN uses digital signals, which allows for faster speeds than analog lines. These speeds are in increments of 64 kbps. For Internet access, ISDN has been usurped by competing technologies such as Digital Subscriber Line (DSL), Asymmetric Digital Subscriber Line (ADSL) cable modems, and simply faster analog modems. ISDN, however, remains a popular option for temporary connectivity between routers and is frequently used to create a backup link when the primary leased line or Frame Relay connection is lost.