- and packet-switched data transfer. Data Communications Another method for packet-switched data communications is connec- tionless datagram transmission in which routing devices perform routing procedures, and each packet contains a full destination address. We discuss this layer 3 (network layer) routing principle next. 6.2.3 Layer 3 Routing and Routers In the case of layer 3 routing every data packet carries the complete global destination information (network layer address of the destination) and all packets are routed independently. As a consequence, each packet may use a different route and arrive out of sequence. The operating principle of the Internet belongs to this category. The routing procedure is performed at the network layer, layer 3, and it requires analysis of each packet and the routing decision based on the desti- nation address on each router. Packets are stored and, when the route and the corresponding port of the router are defined, the packets are forwarded to the next router on the path to the destination. This operating principle make routers slower than the switching devices that operate at the data link layer, layer 2, and use virtual circuits. The operating principle of the data link layer switches is discussed next. 6.2.4 Switching and Routing Through Virtual Circuits The routing of packets is based on the virtual circuits in many public data networks, such as frame relay or ATM networks. Each frame or cell on a vir- tual circuit contains identifying information about the circuit to which it belongs. This identification has a different name in different networks but we call this identification the virtual circuit identifier (VCI). During the cir- cuit establishment phase, signaling messages are exchanged between user equipment and a network and end-to-end virtual circuits are set up in each node on the way through the network. Often a permanent virtual circuit is set up by the network operator when agreement is made about a data connec- tion between corporate sites. In the network eac