| 1.1 |
| 1 | 1.1 What Is a Network? Put simply, a network is a collection of devices that share a common communication protocol and a common communication medium (such as network cables, dial-up connections, and wireless links). We use the term devices in this definition rather than computers, even though most people think of a network as being a collection of computers; certainly the basic concept of a network in most peoples' mind is of an assembly of network servers and desktop machines. However, to say that networks are merely a collection of computers is to limit the range of hardware that can use them. For example, printers may be shared across a network, allowing more than one machine to gain access to their services. Other types of devices can also be connected to a network; these devices can provide access to information, or offer services that may be controlled remotely. Indeed, there is a growing movement toward connecting noncomputing devices to networks. While the technology is still evolving, we're moving toward a networkcentric as opposed to a computing-centric model. Services and devices can be distributed across a network rather than being bound to individual machines. In the same way, users can move from machine to machine, logging on as if they were sitting at their own familiar terminal. One fun and popular example from very early on in the history of networking is the soda machine connected to the Internet, allowing people around the world to see how many cans of a certain flavor of drink were available. While a trivial application, it served to demonstrate the power of networking devices. Indeed, as home networks become easier to use and more affordable, we may even see regular household appliances such as telephones, televisions, and home stereo systems connected to local networks or even to the Internet. Network and software standards such as Sun's Jini already exist to help devices and hardware talk to each other over networks and to allow instant plug-and-play functionality. Devices and services can be added and removed from the network (as, for example, when you unplug your printer and take it to the next room) without the need for complex administration and configuration. It is anticipated that over the course of the next few years, users will become just as comfortable and familiar with network-centric computing as they are with the Internet. In addition to devices that provide services are devices that keep the network going. Depending on the complexity of a network and its physical architecture, elements forming it may include network cards, routers, hubs, and gateways. These terms are defined below. • Network cards are hardware devices added to a computer to allow it to talk to a network. The most common network card in use today is the Ethernet card. Network cards usually connect to a network cable, which is the link to the network and the medium through which data is transmitted. However, other media exist, such as dial-up connections through a phone line, and wireless links. • Routers are machines that act as switches. These machines direct packets of data to the next "hop" in their journey across a network. 2 • Hubs provide connections that allow multiple computers to access a network (for example, allowing two desktop machines to access a local area network). • Gateways connect one network to another—for example, a local area network to the Internet. While routers and gateways are similar, a router does not have to bridge multiple networks. In some cases, routers are also gateways. While it is useful to understand such networking terminology as it is widely used in networking texts and protocol specifications, programmers do not generally need to be concerned with the implementation details of a network and its underlying architecture. However, it is important for programmers to be aware of the various elements making up the network. |
Комментарии