ices are widely deployed and very successful. Examples include frame relay, X.25 packet-based service, and ATM. All will be discussed in detail later in the book. Connectionless Networks The alternative to connection-oriented switching is connectionless switching, sometimes called datagram service. In connectionless net- 2The alternative is to have a network administrator manually preconfigure the routes from source to destination. This guarantees a great deal of control, but also obviates the need for an intelligent network. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols works there is no predetermined path from the source to the destination. There is no call setup packet; all data packets are treated independently, and the switches perceive no relationship between them as they arrive- hence the name, "connectionless." Every packet carries a complete desti- nation address, since it cannot rely on the existence of a preestablished path created by a call setup packet. When a packet arrives at the ingress switch of a connectionless net- work, the switch examines the packet's destination address. Based on what it knows about the topology of the network, congestion, cost of indi- vidual routes, distance (sometimes called hop count), and other factors that affect routing decisions, the switch will select an outbound route that optimizes whatever parameters the switch has been instructed to concern itself with. Each switch along the path does the same thing. For example, let's assume that the first packet of a message, upon arrival at the ingress switch, would normally be directed out physical port number 7 because, based upon current known network conditions, that port provides the shortest path (lowest hop count) to the destina- tion. However, upon closer examination, the switch realizes that while port 7 provides the shorte