 (1) communications parameters that are important to establishing and maintaining the connection and (2) how to provide Virtual Circuit Switch with an Internal Routing Table Trunk Figure 1.5 A virtual curcuit Chapter 1 Telecommunications Technology Fundamentals the proper performance for the application they are supporting. The types of com- munication parameters that could be included are message size, the path to be taken, how to deal with acknowledgments in the event of errors, flow control proce- dures, error control procedures, and policies or agreements negotiated between car- riers. The term virtual circuit is largely used to describe connections between two hosts in a packet-switching network, where the two hosts can communicate as though they have a dedicated connection, although the packets may be taking very different routes to arrive at their destination. Interplanetary Networking Protocols With the imminent onset of space colonization, Vinton Cerf, Adrian Hooke, and others are looking at ways to make interplanetary communications faster. An interesting article in Wired News ("Pushing the Internet into Space," at www.wired.com/news/technology/ 0,70377-0.html) talks about their new form of delay-tolerant networking, which involves "bundling," a sort of store-and-forward protocol that is really the opposite of packet switching. Rather than cutting up data into little bits, bundling builds large data collec- tions that hold themselves together and are not affected by the lengthy latencies (some- times hours) introduced by enormous distances (millions of miles). There are two types of virtual circuits: permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). The vast majority of implementations today involve PVCs. PVCs and SVCs are commonly used in packet-switching networks (e.g., X.25, Frame Relay, ATM). A PVC is a virtual circuit that is permanently available; that is, the connection always exists between the two locations or two devices in question. A PVC is man- ually configured by a ne