Networking Basics (لاتین كامل)

دسته: علوم انسانی
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This lesson introduces the basic building blocks of network communications and some of the structures used to construct data networks. There are many different kinds of data networks- from enterprise networks used by large corporations to a simple two- node local area network (LAN) used in a private home. However , many of the same of the same principles apply to all networks, regardless of size or complexity.

After this lesson , you will be able to

• List the services provided by network protocols.
• Describe how protocols enable networked computers to communicate
• Distinguish a LAN from a wide area network (WAN)
• Understand the difference between baseband and broadband networks
• Identify and distinguish the characteristics of a packet- switched network and a circuit- switched network
• Understand full- duplex and half- duplex communications
• Describe the basic segment and backbone design of an enterprise network
• Distinguish a server- based network from a peer- to- peer network

Estimated lesson time: 30 minutes

When you connect two or more computers so they can communicate with each other, you create a date network. This is true whether you connect the computers using a cable , a wireless technology such as infrared or radio waves , or even modems and telephone line. The technology that connects the computers together , no matter what form it tacks, is called the network medium , and for this reason the term network cable is often used to refer to any kind of network medium.

Signals and Protocols

Computers can communicate over a network in many ways and for many reasons , but a great deal that goes on in the networking process is unconcerned with the nature of the data passing over the network medium. By the time the data generated by the transmitting computer reaches the cable or other medium, it has been reduced to signals that are native to that medium. These might be electrical voltages for a copper cable network, pulses of light for fiber optic , or infrared or radio waves. These signals form a code that the network interface in each receiving computer converts back into the binary data understood by the software running on that computer. The computer then interprets the binary code into information it can use in a variety of ways. Of course there is a great deal more to this process than this description indicates , and there is a lot going on to make it possible for the e- mail you just sent to your mother to get reduced to electrical voltages , transmitted halfway across the country , and then reconstituted into text on her computer.

In some cases , a network consists of identical computers running the same version of the same operating system and using all the same applications , whereas other networks consist of many different computing platforms running entirely different software. It might seem that it would be easier for the identical computers to communicate than it would be easier for the different ones, and in some ways it is. But no matter what kind of computers the network uses and what software the computers are running , they most have a common language to understand each other. These common languages are called protocols, and computers use many of them during even the simplest exchanges of network data. Just as two people must speak a common language to communicate , two computers must have one or more protocols in common to exchange data.

A network protocol can be relatively simple or highly complex. In some cases , a protocol is simply a code  - such as a pattern of electrical voltage - that defines the binary value of a bit of data: 0 or 1. The concept is the same as that of Morse code, in which a pattern of dots and dashes represents a latter of the alphabet.

More complicated networking protocols can provide a variety of services , including the following:

• Packet acknowledgment. This is the transmission of a return message by the recipient to verify the receipt of a packet or packets. A packet is the fundamental unit of data transmitted over a LAN.
• Segmentation. This is the division of a lengthy data stream into segments sufficiently small for transmission over the network inside packets.
• Flow control. This is the generation by a receiving system of messages that instruct the sending system to speed up or slow down its rate of transmission.
• Error detection. This is the inclusion of special codes in a packet that the receiving system uses to verify that the content of the packet wasn't damaged in transit.
• Error correction. This is the generation by a receiving system of messages that informs the sender that specific packets were damaged and must be retransmitted.
• Data compression. This is a mechanism for reducing the amount of data transmitted over a network by eliminating redundant information.
• Data encryption. This is a mechanism for protecting the data transmitted over a network by encrypting it using a key already known by the receiving system.

In most cases , protocols are based on public standards developed by an independent committee, not a single manufacturer or developer. These public standards ensure that different types of systems can use them without incurring any obligation to a particular company. There are still a few protocols , however , that are proprietary , having been developed by a single company and never released into the public domain.

One of the most important things to remember about networking is that every computer on a network uses many different protocols during the communications process. The functions provided by the various protocols are divided into the layers that make up the Open Systems Interconnection (OSI) reference model , described in Lesson 2 , later in this chapter. You might see references to Ethernet networks in books and articles , for example. Ethernet is a protocol running on those networks , but it is not the only protocol running on them. Ethernet is, however , the only protocol running at one particular layer (called the data- link layer). Some layers , however , can have multiple protocols running on them simultaneously.

Local Area Network and Wide Area Network

A LAN is a group of computers located in a relatively small area and connected by a common medium. Each of the computer and other communicating devices on the LAN is called a node. A LAN is characterized by three primary attributes: its topology , its medium , and its protocols. The topology is the pattern used to connect the computers together. With a bus topology , a network cable connects each computer to the next one, forming a china. With a star topology , each of the computers is connected to a central nexus called a hub or switch. A ring topology is essentially a bus network with the two ends joined together. You will learn more about network topologies in Chapter 2, "Network Hardware."

The network medium, as defined earlier , is the actual physical connection between the networked computers. The topology and the medium used on a particular network are specified by the protocol operating at the data- link layer of the OSI model , such as Ethernet or Token Ring. Ethernet , for example , supports several different topologies and media. When you select one combination of topology and medium for a LAN , such as unshielded twisted pair (UTP) cable in a star topology , you must (in most cases) use the same topology and medium for all of the computers on that LAN. There are some hardware products that enable you connect computers to the same LAN with different media, but this is only true for closely related technologies. You can't connect a bus Ethernet computer to a star Ethernet computer and have both systems be part of the same LAN.

In the same way, all of the computers on a LAN must share common protocols. You can't connect an Ethernet computer to Token Ring computer on the same LAN, for example. The same is true for the protocols operating at the other layers of the OSI model. If the systems on the LAN don't have common protocols at every layer of the stack, communication among them is not possible.

In most cases , a LAN is confined to a room , a floor , or perhaps a building. To expand the network beyond these limits, you can connect multiple LANs together using devices called routers. This forms an internetwork , which is essentially a network of networks. A computer on one LAN can communicate with the systems on another LAN because they are all interconnected. By connecting LANs in this way, you can build an internetwork as large as you need.

Many sources use the term network when describing a LAN , but just as many use the same term when referring to an internetwork.

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