 but on usage: Billing is based on the volume of packets or the amount of bandwidth subscribed to. Distance insensitivity is a part of the packet-switched networking environment. Generations of Packet Switches Similar to circuit switches, packet switches have gone through several basic generations: X.25 switches (first generation), rout- ers (second generation), Frame Relay and cell switches (third generation), and Establishing Connections: Networking Modes and Switching Modes MPLS and tag-switched switches (fourth generation). Each generation of packet switching has increased the efficiency of packet processing and the speed of interfaces it supports. In effect, the size of the pipes and the size of the interfaces dictate how effectively the packet-switched network performs. In packet switching, the processing is being pushed outside the network to the end nodes, so you need to have more intelligent software at the end nodes that get involved in the session setup, maintenance, and teardown, as well as flow control from end to end. Besides X.25 switches, routers, and Frame Relay switches, packet switches include ATM switches and a new breed, called Tbps (terabits per second) switch routers. A large number of vendors sell these packet switches, and it seems that more companies jump on the bandwagon each day. With packet switches, the ratio of performance to cost doubles every 20 to 10 months, SO we see the evolution of new entries in the product line much more rap- idly in this environment than in the circuit-switched world. However, again we rely on expensive end stations-PCs or other computers-to finish the job of commu- nication in packet switching. These end stations have to rely on protocols such as Transmission Control Protocol/Internet Protocol (TCP/IP), an open standard for internetworking that performs the equivalent of call setup/teardown and correct receipt of data. (TCP/IP is discussed in Chapter 8.) These end stations also have to ensure that all the data has been received and that it has been receive