elated structure dubbed Inter-networks. The Internet is a completely different structure with its own unique behavior in terms of how it moves a payload, otherwise called content or information. The concept is built around an idea that combines payload information with address information, hands it to the network, and the network not only carries the infor- mation, but when it gets to its destination, the network communicates that fact back to the sender. Overall result: A third network is built using the same kinds of standard layer 1 and layer 2 facilities as voice and data networks. Continuing to add communications facilities for separate network applications has led many to question and wonder if there may not be a better way to organize their traffic to get better use of all the facilities being paid for. Attempts to address these issues have led to solutions called converged or multi-service networks. A more appropri- ate label might start with the traffic whereby disparate traffic types are converged into a common access facility or across a common transport facility. Viewing the three layered models in a single con- text can be a constructive and instructive step to defining require- ments for a network capable of carrying disparate traffic. LINKING IN COMMUNICATIONS NETWORKS Linking involves two types of paths through the network: physical and virtual. Paths are created through the network when physical links are connected in series between two terminal sites. A receive path output from equipment in a terminal is connected to transmit path input on another facility. For a full-duplex or two-way path, the receive path from the opposite direction must be connected to the transmit path in the opposite direction. This type of linking process in digital networks can be extended many times without undue ser- vice impairment, except for the accumulation of transmission delay and errors. The latter can be mitigated by good link engineering practice common in radio and optical link budgeting. When any network is 