IIntegrated Services Digital Network, or ISDN, is a dial-up connection carried over specially installed lines, while digital subscriber line, or DSL, is an always-on connection that does not need to dial over the lines that it uses. ISDN and DSL work over the same kind of network, but the main difference is that the ISDN has to actually dial out to achieve a connection for data to flow through. DSL does not need to dial out, and as a result, the user can make calls while connected to the Internet.
The telephone wires that carry signals for both systems are copper-based wires. The lines for ISDN have to be installed because they require an adapter installed at both ends of the line. This means that the service provider must have the adapter, and it must also be placed into the home or business where the service is being used.
Generally, DSL uses existing telephone lines along with a modem. A DSL connection does not need any additional installation of wires as long as there is already a telephone line in the home. Most DSL connections are actually ADSL, with the "A" standing for asymmetric. ADSL is better suited to a home or business that downloads data more than it uploads it.