Always connected, unless a system administrator or power failure takes the leased-line down. 

Initiated on demand, when a user tries to call a remote peer. 

Uses synchronous and asynchronous communications. For asynchronous communications, a long-haul modem is often used. 

Uses asynchronous communications. 

Rented from a provider. 

Uses existing telephone lines. 

Requires synchronous units. 

Uses less costly modems. 

Requires synchronous ports, which are common on most SPARC systems. However, synchronous ports are not common on x86 systems and newer SPARC systems. 

Uses standard serial interfaces that are included on most computers.