What is a wide area network that uses radio signals to transmit and receive data?

Navigation

• • Lesson 4 - NETWORKS

    Nội dung chính Show

    • Lesson 4 - NETWORKS
    • What device sends and receives radio signals?
    • Which of the following is a wireless transmission medium that distributes radio signals through the air over long distances?
    • Which device should ideally be used as the central communication point for the network?
    • Is RFID a network protocol?

Lesson 4 - NETWORKS

A network is a collection of computers and devices connected together via communications devices and transmission media. Many businesses network their computers together to facilitate communications, share hardware, share data and information, share software, and transfer funds.

A network can be internal to an organization or span the world by connecting to the Internet. Instead of using the Internet or an internal network, some companies hire a value-added network provider for network functions. A value-added network (VAN) is a third-party business that provides networking services for a fee.

Networks facilitate communications among users and allow users to share resources with other users. Some examples of resources are data, information, hardware, and software. 

LANs, MANs, and WANs

Networks usually are classified as a local area network, metropolitan area network, or wide area network. The main differentiation among these classifications is their area of coverage, as described in the following paragraphs.

LAN A local area network (LAN) is a network that connects computers and devices in a limited geographical area such as a home, school computer labora- tory, office building, or closely positioned group of buildings. Each computer or device on the network, called a node, often shares resources such as printers, large hard disks, and programs. Often, the nodes are connected via cables. A wireless LAN (WLAN) is a LAN that uses no physical wires. Very often, a WLAN com- municates with a wired LAN for access to its resources.

MAN A metropolitan area network (MAN) is a high-speed network that connects local area networks in a metropolitan area such as a city or town and handles the bulk of communications activity across that region. A MAN typically includes one or more LANs, but covers a smaller geographic area than a WAN.

A MAN usually is managed by a consortium of users or by a single network provider that sells the service to the users. Local and state governments, for example, regulate some MANs. Telephone companies, cable television operators, and other organizations provide users with connections to the MAN.

WAN A wide area network (WAN) is a network that covers a large geographic area (such as a city, country, or the world) using a communications channel that combines many types of media such as telephone lines, cables, and radio waves (Figure 8-8). A WAN can be one large network or can consist of two or more LANs connected together. The Internet is the world’s largest WAN. 

Network Architectures

The design of computers, devices, and media in a network, sometimes called the network architecture, is categorized as either client/server or peer-to-peer.

CLIENT/SERVER On a client/server network, one or more computers act as a server; the other computers on the network request services from the server (Figure 8-9). A server controls access to the hardware, software, and other resources on the net- work and provides a centralized storage area for programs, data, and information. The clients are other computers and mobile devices on the network that rely on the server for its resources. For example, a server might store a database of customers. Clients on the network (company employ- ees) access the customer database on the server.

Some servers, called dedicated servers, perform a specific task and can be placed with other dedicated servers to perform multiple tasks. For example, a file server stores and manages files. A print server manages printers and documents being printed. A database server stores and provides access to a database. A network server manages network traffic (activity).

A client/server network typically provides an efficient means to connect 10 or more com- puters. Most client/server networks require a person to serve as a network administrator because of the large size of the network. 

PEER-TO-PEER One type of peer-to-peer network is a simple, inexpensive network that typically connects fewer than 10 com- puters. Each computer, called a peer, has equal responsibilities and capabilities, shar- ing hardware (such as a printer), data, or information with other computers on the peer-to-peer network. Each computer stores files on its own storage devices. Thus, each computer on the network contains both the network operating system and application software. All computers on the network share any peripheral device(s) attached to any computer. For example, one computer may have a laser printer and a scanner, while another has an ink-jet printer and an external hard disk. Peer-to-peer net- works are ideal for very small businesses and home users. 

INTERNET PEER-TO-PEER Another type of peer-to-peer, called P2P, describes an Internet network, on which users access each other’s hard disks and exchange files directly (Figure 8-11). This type of peer-to-peer network sometimes is called a file sharing network because users with compatible soft- ware and an Internet connection copy files from someone else’s hard disk to their hard disks. As more users connect to the network, each user has access to shared files on other users’ hard disks. When users log off, others no longer have access to their hard disks.

Examples of networking software that support P2P are BitTorrent, Gnutella, Kazaa, and LimeWire which allow users to swap music and other files via the Web. 

Network Topologies

A network topology refers to the layout of the computers and devices in a communica- tions network. Three commonly used network topologies are bus, ring, and star. Networks usually use combinations of these topologies.

BUS NETWORK A bus network consists of a single central cable, to which all computers and other devices connect. The bus is the physical cable that connects the computers and other devices. The bus in a bus network transmits data, instructions, and information in both directions. When a sending device trans- mits data, the address of the receiving device is included with the transmission so that the data is routed to the appropriate receiving device.

Bus networks are popular on LANs because they are inexpensive and easy to install. One advantage of the bus network is that comput- ers and other devices can be attached and detached at any point on the bus without dis- turbing the rest of the network. Another advantage is that failure of one device usually does not affect the rest of the bus network. The greatest risk to a bus network is that the bus itself might become inoperable. If that hap- pens, the network remains inoperative until the bus is back in working order. 

RING NETWORK On a ring network, a cable forms a closed loop (ring) with all computers and devices arranged along the ring. Data transmitted on a ring net- work travels from device to device around the entire ring, in one direction. When a computer or device sends data, the data travels to each computer on the ring until it reaches its destination.

If a computer or device on a ring network fails, all devices before the failed device are unaffected, but those after the failed device cannot function. A ring network can span a larger distance than a bus network, but it is more difficult to install. The ring topology primarily is used for LANs, but also is used in WANs. 

    STAR NETWORK On a star network, all of the computers and devices (nodes) on the network connect to a central device, thus forming a star. Two types of devices that provide a common central connection point for nodes on the network are a hub and a switch. All data that transfers from one node to another passes through the hub/switch.

Star networks are fairly easy to install and maintain. Nodes can be added to and removed from the network with little or no disruption to the network.

On a star network, if one node fails, only that node is affected. The other nodes continue to operate normally. If the hub/switch fails, however, the entire network is inoperable until the device is repaired. 

Intranets

Recognizing the efficiency and power of the Internet, many organizations apply Internet and Web technologies to their own internal networks. An intranet (intra means within) is an internal network that uses Internet technolo- gies. Intranets generally make company information accessible to employees and facilitate working in groups.

Simple intranet applications include electronic publishing of organizational materials such as telephone directories, event calendars, procedure manuals, employee benefits information, and job postings. Additionally, an intranet typically includes a connection to the Internet. More sophis- ticated uses of intranets include groupware applications such as project management, chat rooms, newsgroups, group scheduling, and video conferencing.

An intranet essentially is a small version of the Internet that exists within an organization. Users update informa- tion on the intranet by creating and posting a Web page, using a method similar to that used on the Internet.

Sometimes a company uses an extranet, which allows customers or suppliers to access part of its intranet. Package shipping companies, for example, allow customers to access their intranet to print air bills, schedule pickups, and even track shipped packages as the packages travel to their destinations. 

Network Communications Standards

Today’s networks connect terminals, devices, and computers from many different manufacturers across many types of networks, such as wide area, local area, and wireless. For the different devices on various types of networks to be able to communicate, the network must use similar techniques of moving data through the network from one application to another.

To alleviate the problems of incompatibility and ensure that hardware and software components can be integrated into any network, various organizations such as ANSI and IEEE (pronounced I triple E) propose, develop, and approve network standards. A network standard defines guidelines that specify the way computers access the medium to which they are attached, the type(s) of medium used, the speeds used on different types of networks, and the type(s) of physical cable and/or the wireless tech- nology used. A standard that outlines characteristics of how two network devices communicate is called a protocol. Hardware and software manufacturers design their products to meet the guidelines specified in a particular standard, so that their devices can communicate with the network.

The following sections discuss some of the more widely used network communications standards for both wired and wireless networks including Ethernet, token ring, TCP/IP, 802.11 (Wi-Fi), Bluetooth, UWB, IrDA, RFID, WiMAX, and WAP.

ETHERNET Ethernet is a network standard that specifies no central computer or device on the network (nodes) should control when data can be transmitted; that is, each node attempts to trans- mit data when it determines the network is able to receive communications. If two computers on an Ethernet network attempt to send data at the same time, a collision occurs, and the computers must attempt to send their messages again.

Ethernet is based on a bus topology, but Ethernet networks can be wired in a star pattern. The Ethernet standard defines guidelines for the physical configuration of the network, e.g., cabling, network cards, and nodes. Today, Ethernet is the most popular LAN standard because it is relatively inexpensive and easy to install and maintain. Ethernet networks often use cables to transmit data.

TOKEN RING The token ring standard specifies that computers and devices on the network share or pass a special signal, called a token, in a unidirectional manner and in a preset order. A token is a special series of bits that function like a ticket. The device with the token can transmit data over the network. Only one token exists per network. This ensures that only one computer transmits data at a time. Token ring is based on a ring topology (although it can use a star topology). The token ring standard defines guidelines for the physical configuration of a network. Some token ring networks connect up to 72 devices. Others use a special type of wiring that allows up to 260 connections.

TCP/IP Short for Transmission Control Protocol/Internet Protocol, TCP/IP is a network standard, specifically a protocol, that defines how messages (data) are routed from one end of a network to the other. TCP/IP describes rules for dividing messages into small pieces, called packets; providing addresses for each packet; checking for and detecting errors; sequencing packets; and regulating the flow of messages along the network.

TCP/IP has been adopted as a network standard for Internet communications. Thus, all hosts on the Internet follow the rules defined in this standard. Internet communications also use other standards, such as the Ethernet standard, as data is routed to its destination.

When a computer sends data over the Internet, the data is divided into packets. Each packet contains the data, as well as the recipient (destination), the origin (sender), and the sequence information used to reassemble the data at the destination. Each packet travels along the fastest individual available path to the recipient’s computer via communications devices called routers.

802.11 (WI-FI) Developed by IEEE, 802.11 also known as Wi-Fi (wireless fidelity) and wireless Ethernet, is a series of network standards that specifies how two wireless devices communicate over the air with each other. Using Wi-Fi, computers or devices that have the appropriate wireless capa- bility communicate via radio waves with other computers or devices. The Wi-Fi standard uses tech- niques similar to the Ethernet standard to specify how physically to configure a wireless network. Most of today’s computers and many personal mobile devices, such as smart phones and handheld game consoles, are Wi-Fi enabled. 

One popular use of the Wi-Fi standard is in hot spots that offer mobile users the ability to connect to the Internet with their Wi-Fi enabled wireless computers and devices. Many homes and small businesses also use Wi-Fi to network computers and devices together wirelessly.

BLUETOOTH Bluetooth is a standard, specifically a protocol, that defines how two Bluetooth devices use short-range radio waves to transmit data. To communicate with each other, Bluetooth devices often must be within about 10 meters (about 33 feet) but can be extended to 100 meters with additional equip- ment. Examples of Bluetooth devices can include desktop computers, notebook computers, handheld computers, smart phones, PDAs, headsets, microphones, digital cameras, and printers.

UWB UWB, which stands for ultra-wideband, is a network standard that specifies how two UWB devices use short-range radio waves to communicate at high speeds with each other. For optimal com- munications, the devices should be within 2 to 10 meters (about 6.5 to 33 feet) of each other. Examples of UWB uses include wirelessly transferring video from a digital video camera, printing pictures from a digital camera, downloading media to a portable media player, or displaying a slide show on a projector.

IRDA Some computers and devices use the IrDA specification to transmit data wirelessly to each other via infrared (IR) light waves. Infrared requires a line-of-sight transmission; that is, the sending device and the receiving device must be in line with each other so that nothing obstructs the path of the infrared light wave.

RFID RFID (radio frequency identification) is a standard, specifically a protocol, that defines how a network uses radio signals to communicate with a tag placed in or attached to an object, an animal, or a person. The tag consists of an antenna and a memory chip that contains the information to be transmitted via radio waves. Through an antenna, an RFID reader reads the radio signals and trans- fers the information to a computer or computing device. Readers can be handheld or embedded in an object such as a doorway or tollbooth.

WIMAX WiMAX (Worldwide Interoperability for Microwave Access), also known as 802.16, is a newer network standard developed by IEEE that specifies how wireless devices communicate over the air in a wide area. Using the WiMAX standard, computers or devices with the appropriate WiMAX wireless capability communicate via radio waves with other computers or devices via a WiMAX tower. The WiMAX tower, which can cover up to a 30-mile radius, connects to the Internet or to another WiMAX tower. 

Two types of WiMAX specifications are fixed wireless and mobile wireless. With fixed wire- less WiMAX, a customer accesses the Internet from a desktop computer at home or other permanent location. Mobile wireless WiMAX, by contrast, enables users to access the WiMAX network with mobile computers and mobile devices such as smart phones.

The WiMAX standard provides wireless broadband Internet access at a reasonable cost over long distances to business and home users. The WiMAX standard, similar to the Wi-Fi stan- dard, connects mobile users to the Internet via hot spots. The next generation of game consoles also plans to support the WiMAX standard. 

WAP The Wireless Application Protocol (WAP) is a standard, specifically a protocol, that specifies how some mobile devices such as smart phones can display the content of Internet services such as the Web, e-mail, and chat rooms. For example, users can check weather, sports scores, and headline news from their WAP-enabled smart phone. To display a Web page on a smart phone, the phone should contain a microbrowser. WAP uses a client/server network. The wireless device contains the client software, which connects to the Internet access provider’s server.