The type of link used for a connection between two Data Terminal Equipment (DTE) devices is a Modem.
X.21: This is an interface specification for differential communications, primarily used to interface with WAN services.
Frame Relay: This is a standardized wide area network technology that specifies the physical and data link layers of digital telecommunications channels using a packet switching methodology.
Modem: This is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data.
ATM (Asynchronous Transfer Mode): This is a telecommunications concept defined by ANSI and ITU (formerly CCITT) standards for carriage of user traffic, including telephony (voice), data, and video signals.
