Computer Networks and the Internet
STUDY COMPANION FOR COMPUTER NETWORKING, THIRD EDITION
Most Important Ideas and Concepts from Chapter 1
Nuts and bolts of computer networks. Computer networks consist of end systems, packet switches, and communication links. End systems—also called hosts—include desktop PCs, laptops, hand-held network devices (including cell phones, PDAs, and BlackBerries), sensors, and servers (such as Web and mail servers). Just as cities are interconnected by a network of roads and intersections, end systems of a computer network are interconnected by a network of communication links and packet switches. Communication links can be wired or wireless. Distributed applications. A computer network enables distributed applications. A distributed application runs on end systems and exchanges data via the computer network. Distributed applications include Web surfing, e-mail, instant messaging, Internet phone, distributed games, peer-to-peer file sharing, television distribution, and video conferencing. New distributed applications continue to be invented and deployed on the Internet. Packet switching. When one end system sends data to another end system, the sending end system breaks the data into chunks, called packets. Similar to the process of delivering post-office mail, the Internet transports each packet separately, routing a packet to its destination using a destination address that is written into the packet. When a packet switch receives a packet, it uses the packet’s destination address to determine on which link it should forward the packet. Thus, a packet switch performs “packet switching,” forwarding incoming packets to outgoing links packet by packet. Also, packet switches typically “store and forward” packets—that is, before a switch begins to forward a packet on an outgoing link, first it receives and stores the entire packet. Protocol. A protocol defines the format and order of messages exchanged between two or more communication entities, as well as the actions taken on the transmission and/or receipt of a message or other event. Computer networks make extensive use of protocols. Figure 1.2 (page 7 in your textbook) provides an analogy between a human protocol and a computer network protocol for messages exchanged between a Web browser and a Web server. In this example, first, the Web browser sends an introductory message to the server; next, the server responds with its own introductory message; then, the browser sends another message, requesting a specific Web page; and finally, the server sends a last message, which includes the requested Web page. Circuit-switching. Computer networks constitute one major class of communication networks. Another major class of communication networks is traditional digital telephone networks. Traditional digital telephone networks do not use packet switching to move data from a source to a destination; they use a technique known as circuit switching. In circuit switching, before transmitting data between two end systems, the network establishes a dedicated end-to-end connection between the end systems and reserves bandwidth in each link along the connection. The reserved connection bandwidth is “wasted” whenever the end systems are not sending data.
COMPUTER NETWORKS AND THE INTERNET
Physical media and access networks. The communication links in a computer network may have different physical media types. Dial-up modem links, DSL, and most Ethernet links are made of copper wire. Cable links are made of coaxial cable. Long-haul Internet backbone links are made of fiber optics. In addition to these wired links, there is a plethora of wireless links, including Wi-Fi, Bluetooth®, and satellite. An access link is a link that connects the end system to the Internet. Access links can be copper wire, coaxial cable, fiber optics or wireless. A tremendous variety of media types can be found on the...