The Network Edge

The Network Edge

In the preceding section we presented a high-level outline of the Internet and networking protocols. We are now going to look into a bit more deeply into the components of a computer network (and the Internet, particularly). We begin in this section at the edge of a network and look at the elements with which we are most familiar namely, the computers, PDAs, cell phones and other devices that we use on a daily basis. In the next section we'll move from the network edge to the network core and study switching and routing in computer networks.

Recall from the preceding section that in computer networking terminology, the computers and other devices attached to the Internet are frequently referred to as end systems. They are referred to as end systems because they sit at the edge of the Internet, as shown in the following figure. The Internet's end systems include desktop computers (e.g., desktop PCs, Macs, and Linux boxes), servers (e.g., Web and e-mail servers), and mobile computers (e.g., portable computers, PDAs, and phones with wireless Internet connections). Furthermore, an increasing number of alternative devices are being connected to the Internet as end systems.

End-system interaction

End systems are also referred to as hosts because they host (that is, run) application programs such as a Web browser program, a Web server program, an e-mail reader program, or an e-mail server program. Throughout this blog we will use the terms hosts and end systems interchangeably; that is, host = end system. Hosts are sometimes further divided into two categories: clients and servers. Informally, clients tend to be desktop and mobile PCs, PDAs, and so on, whereas servers tend to be more powerful machines that store and distribute Web pages, stream video, relay e-mail, and so on.


Not too long ago, the end-system devices attached to the Internet were primarily traditional computers such as desktop machines and powerful servers. Beginning in the late 1990s and continuing today, a wide range of interesting devices of increasing diversity are being connected to the internet. These devices share the common attribute of needing to send and receive digital data to and from other devices. Given the Internets ubiquity, its well-defined (standardized) protocols, and the availability of Internet-ready commodity hardware, its natural to use Internet technology to attach these devices together.

Some of these devices seem to have been created purely for fun. A desktop IP capable picture frame [Ceiva 2009] downloads digital photos from a remote server and displays them in a device that look like a traditional picture frame; an Internet toaster downloads meteorological information from a server and burns an image of the days forecast (e.g., mixed clouds and sun) on your morning toast [BBC 2001]. Other devices provide useful information Web cams display current traffic and weather conditions or monitor a location of interest; Internet-connected home appliances (including washing machines, refrigerators, and stoves) have Web browser interfaces for remote monitoring and control. IP-enabled cell phones with GPS capabilities (such as Apples new iPhone) put Web browsing, e-mail and location-dependent services at your fingertips. A new class of networked sensor systems promises to revolutionize how we observe and interact with our environment. Net-worked sensors that are embedded into the physical environment allow monitoring of building, bridges, seismic activity, wildlife habitats, river estuaries, and the lower layers of the atmosphere [CENS 2009, CASA 2009]. Biomedical devices can be embedded and networked, raising numerous security and privacy issues [Halperin 2008]. An RFID tag or a tiny embedded sensor affixed to any object can make information about/from that object available on the Internet, leading to an "Internet of things" [ITU 2005].


network core, computer networking, end systems

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