Publisher Summary This chapter describes packet switching. Packet switching is a mature, secure method of transferring data across a network. It divides data into segments, each of which is wrapped in an envelope to form a packet; a typical message comprises one or more packets. Each packet contains the actual user data plus information helpful to its movement across the network, such as addressing, sequencing, and error control. The physical layer describes the physical connection—the electrical interface and procedures involved in establishing a communication path. The aim of the data link layer is to ensure that data passing between devices is error free and in the correct order. While the data link layer sorted out an error free link between two connected devices, the network layer provides communications between devices. The network layer can support multiplexing, sustaining many active virtual calls simultaneously. Communicating devices in a datagram network are not connected to each other as in a virtual circuit. Dynamic routing is a complex solution favored in academic or networks in which security or the lack of commercial demands favor more esoteric methods. A version of dynamic routing assigns a cost value to each circuit based on available bandwidth, link loading, transit delay, and throughput.