units at Layer 3 are inefficient or otherwise not wholly suitable for data known as packets, while the protocol data units at Layer traffic. 2 are called frames, at least when writers are precise. The Ultimately, dedicated circuits are a high-cost form of X.25 protocols include Layer 3 functions, while frame connectivity, and setting up a circuit via a switching relay, which is essentially X.25 with error correction and system designed for voice communications results in long flow control removed, remains at Layer 2.) circuit-initiation times, as well as high cost. Connectionless switching Packet switching Local Area Network (LAN), which people began to In the late 1960s the notion of packet switching was develop in the 1970s, is a form of connectionless com- developed (see Figure 3.1-2). The first commercial out- munication. When a Layer 2 bridge connects LANs, growth of this technology was the X.25 network. In spite a form of frame switching takes place. When a Layer nodes Figure 3.1-2 Packet switching. CHAPTER 3.1 Data and voice traffic 3 router connects LANs, a form of packet switching takes video, voice, etc.-will travel together across the net- place. IP and its precursors were the first wide-area work links. Each data flow is broken up into packets connectionless protocols used extensively. Each packet (variable-length chunks of data), each carrying its own includes its source and destination addresses and moves address, with the packets being forwarded by the net- independently through the network. With these work to their ultimate destination based on the connectionless switching systems, for the first time there information they carry with them. was no advance setup of the path needed. There was also Most importantly, the new network will separate no advance agreement on the part of the intermediate services and infrastructure. In effect, the network is stages committing to a specific level of service. There was moving from being the equivalent of a rail system, where no state maintained on the network'