ts. If a particular route becomes congested or has degraded operation, a node can send a packet on another route, and if that route becomes congested, possibly a third route will be available to forward the packet to its destination. At a data source, a file is segmented into comparatively short data packets, each of the same length and each with its own header and trailer. As we mentioned, these packets may take diverse routes through various nodes to their destination. The destination node is responsible for data message reassembly in its proper order. This is a short description of an idealized packet-switched network. We will run into some terminology typical of packet handling and switching of data. For example: A. There is connection-oriented and connectionless service. B. Packets may be acknowledged or unacknowledged. C. We can have PVCs or SVCs (permanent virtual circuits or switched virtual circuits). Our POTS is a typical connection-oriented service. In other words a fixed or virtual connection is set up at the beginning of a telephone call, is maintained in that condition throughout the call, and is taken down (terminated) at the end of the call. The circuit is then returned to the pool of idle circuits awaiting the next call assignment. Connection-oriented is sometimes called circuit-switched service. In ideal packet-switched data service the service is connectionless. The packet is released by the originator and placed on an idle or "free" outgoing data link to the next node. Each node ideally maintains a dynamically updated routing table. The rout- ing table will provide information on the appropriate outgoing port to direct the packet toward its destination and will reflect information regarding congestion and/or degrada- tion or outage on all possible outgoing routes. In the real world this dynamic updating is seldom achievable because we will tend to load circuits up with service messages rather than revenue-bearing traffic. A more common method is the virtual connection where a logical connectio