Early telemedicine networks employed dedicated telecommunications circuits (e.g. leased digital lines) in which the sender and receiver were connected by a private circuit. More recently, the Internet has become widely available for general use, including telemedicine. The Internet was engineered to permit network paths to be shared by all users, so data transmission is fundamentally different from traditional, circuit-switched networks. Early telemedicine applications demonstrated the feasibility of Internet Protocol transmission. The basic performance criteria to use in evaluating newer digital communications technologies that carry both voice and data are: (1) bandwidth; (2) packet loss; (3) end-to-end delay; (4) jitter; (5) privacy and security. Network engineering involves performance trade-offs between the hardware, architecture, security and the budget available. A telemedicine application may be running over a network whose design is entirely under the user's control, or the application may employ some part of the Internet whose design is unknown to the user. If an application is not running to satisfaction, then a network engineer should be consulted.