atency and packet loss can considerably degrade real- time (i.e., time-sensitive) applications. In fact, the first generation of packet switch- ing, X.25, dealt with data only. It could not handle voice or video. As discussed later in this chapter, newer generations can handle data because we have found ways to design the network to handle the requirements of voice and video applications. In general, in the traditional mode, packet switching offered no QoS guaran- tees; QoS simply was not a design objective. It did, however, offer the knowledge that packets would usually make it to their destination because they could be rerouted around trouble points. But because they could be rerouted around trouble points, which might mean congestion points or failed points, there could be no guarantees about the latencies or losses that you would experience. Therefore, it is a relatively new concept to try to build in QoS as a metric in packet-switched net- Packet-Switched Networks works. New packet networks can provision QoS by means of resource management and allocation techniques, such as RSVP-TE. (QoS techniques are discussed in more detail in Chapters 8 and 10.) A packet-switched network is a data-centric environment, and instead of switching millions of physical circuits, as happens in the circuit-switched environ- ment, the data-centric network switches packets, or switched virtual circuits. Cir- cuit switching makes individual paths from the source to the destination, and in the case of the telephone network, millions of individual paths are created. Packet switching provides a dynamically determined route (hopefully a good route) for each packet presented to the network from the source to the destination. In Figure 7.12, multiple packets are statistically multiplexed as they come in through the packet switch, a routing table is consulted, an appropriate path is selected, and the packets are sent over the correct virtual circuit, leading to the next most logical stop in the network. The capacity of the transmission fa