er A or B, while the capacity of other lines would be used on a time-shared basis among customers D, E, and F. Time multiplexing tech- nologies and pulse code modulation (PCM) transmission techniques enabled such an arrangement. Time multiplexing was the first major carrier technology that enabled such an economic model. Time slots were created per units of time and a certain number of them were allocated to a specific cus- tomer. Traffic to and from this customer would be transmitted only inside the allocated slots and the carrier would charge the customer at the end of the month appropriately. At the same time, two significant steps occurred almost simultaneously in the evolution of com- munications. One was the introduction and eventual global acceptance of the seven-layer Open System Interface (OSI) model, which profoundly shook the structure of systems development (although lay- ering was not a new concept since IBM had established it with its SNA years before). The other was the invention of packetized transmission, a radical departure from the previously accepted model of sequential transmission and permanent connection. This invention was going to become the beginning of all subsequent packet-based technologies, and it was originally epitomized in the introduction of the X.25 network. A permanent circuit would no longer need to be connected between two endpoints while a communication session was active. Routes (circuits) were switched at exchange locations, originally by giant racks of mechanical relays and then by solid-state electronics switches. With X.25, no precious switched resources would have to be reserved for a communications circuit that was only used part of the time. The transmitted information would be broken up into structured chunks (also known as packets, frames, and messages). Then some meaningful tags would be generated and prefixed or suffixed to each packet-for example, the sender's address, destination address, cyclic redundancy check (CRC), the number of packets being sent, and the 