COMMUNICATIONS AND NETWORKS

Transcription

1 COMMUNICATIONS AND NETWORKS Data Communications Data communications describes a process in which one computer transfers data, instructions, and information to another computer. The basic model for data communications consists of: A sending device that initiates an instruction to transmit data, instruction, or information. e.g., Computer A, which sends out signals to another computer. A communication device that converts the data, instructions, or information from the sending device into signals that can be carried by a communications channel. e.g., Modem A, which converts the computer s digital signals into analog signals. A communications channel, or path, over which the signals are set. e.g., A standard telephone line, along which the analog signals are sent. A communications device that receives the signals from the communications channel and converts them into a form understood by the receiving device. e.g., Modem B, which converts the analog signals back into digital signals. A receiving device that accepts the data, instructions, or information. e.g., Computer B, which receive the signals from Computer A. Communication software, which consists of programs that manage the transmission of data, instructions, and information between computers. Uses of Communications (electronic mail) is the exchange of text messages and computer files transmitted via a communications network such as a local area network or the Internet. Using software, a user can retrieve, read, and reply to a message, or delete it from the mailbox. Voice Mail Voice mail, which functions much like an answering machine, allows callers to leave a voice message for a called party. Unlike an answering machine, a computer in the voice mail system converts an analog voice message into digital form, and then stores it in a voice mailbox, which is a storage location on a computer in the system. Page 1

2 Fax A fax (facsimile) machine is a device that sends and receives documents via telephone lines. Such a document is called a fax. A fax modem allows a user to send and receive faxes using a computer. Smart Phones Smart phones are cell phones that can track phone calls and their costs; include PDA functions; and use wireless technology that enables a user to fax, send and receive messages, and browse the Web. Instant Messaging Instant messaging (IM) is a communications service that notifies a user when one or more people are online and then allows the user to exchange messages or files with them or join a private chat room with them. Software called instant messenger has to be installed to use instant messaging. Telecommuting Telecommuting is a work arrangement in which employees work away from a company s standard workplace, but communicate with the office using some communications technology. Telecommuting benefits both employers and employees because it provides flexibility, thus, increases employee productivity and job satisfaction. It reduces the time used to commute to the office. It eliminates travel during unsafe weather conditions. It provides a convenient, comfortable work environment for employees. Employees may combine work and personal responsibilities, such as childcare. Telecommuting also reduces the air pollution caused by vehicles driven to and from the office. Videoconferencing Videoconferencing involves using video and computer technology to conduct a meeting between participants at two or more geographically separate locations. A conferencing system that uses Web browsers and Web servers to deliver its service is often called a Web conference. Groupware Groupware is a software application that helps groups of people work together on projects and share information over a network. Page 2

3 Groupware is a component of a broad concept called workgroup computing, which includes specific hardware and software that enables group members to communicate, manage projects, schedule meetings, and make group decisions. Global Positioning System A global positioning system (GPS) consists of one or more earth-based receivers that accept and analyze signals sent by satellites in order to determine the receiver s geographic location. A GPS is often used to locate a person or object; ascertain the best route between two points; monitor the movement of a person or object, or create a map. Many cars also use GPSs to provide directions to a destination or traffic and weather information. Bulletin Board System An electronic bulletin board system (BBS) is a computer that maintains a centralized collection of electronic messages. An individual called the system operator creates and updates the BBS. BBS usage is declining because the Internet can be used to access many of the same services. The Internet The Internet is a worldwide collection of networks that links together millions of business, government offices, educational institutions, and individuals via modems, telephone lines, satellites, and other communications devices and media. The World Wide Web (WWW), or Web, consists of a worldwide collection of electronic documents called Web pages that have built-in links to other related documents. E-commerce (electronic commerce) includes activities such as shopping, banking, investing, and any other venture that uses either electronic money (e-money) or electronic data interchange. E-money is a means of paying for goods and services over the Internet. Electronic data interchange (EDI) is the transmission of business documents or data over communications media. Internet telephony enables a user to talk to other people over the Web. Internet telephony uses the Internet instead of the public switched telephone network to connect a calling party to one or more called parties. Page 3

4 Communications Channel A communications channel is the communications path between two devices. A communications channel is composed of one or more transmission media. A transmission media consists of materials or techniques capable of carrying one or more signals. Transmission media can be physical or wireless. Physical transmission media use wire, cable, and other tangible (touchable) materials to send communications signals. Wireless transmission media send communications signals through the air or space using radio, microwave, and infrared signals. Physical Transmission Media Twisted-Pair Cable Twisted-pair cable consists of one or more twisted-pair wires bundled together. Each twisted-pair wire consists of two separate insulated copper wires that are twisted together. The wires are twisted together to reduce noise, which is an electrical disturbance that can degrade communications. Shielded twisted-pair (STP) cable has a metal wrapper around each twisted-pair wire, which further reduces noise. STP cables are used in environment susceptible to noise, such as in a local area network. Unshielded twisted-pair (UTP) cable does not have this shielding. UTP cables are commonly used in telephone networks because it is inexpensive and easy-to-install. Coaxial Cable Coaxial cable consists of a single copper wire surrounded by three layers: (1) an insulating material, (2) a woven or braided metal, and (3) a plastic outer coating. Coaxial cable is insulated more heavily than twisted-pair cable. Cable television wiring often uses coaxial cable because it can be cabled over longer distances than twisted-pair cable. Fiber-Optic Cable Fiber-optic cable consists of dozens or hundreds of thin strands of glass or plastic that use light to transmit signals. Each strand, called an optical fiber, is as thin as a human hair. Each optical fiber is surrounded by an insulating glass cladding and a protective coating. Page 4

5 Advantages of fiber-optic cable include: May carry significantly more signals than wire cables. Faster data transmission. Less susceptible to noise (interference) from other devices. Better security for signals during transmission because they are less susceptible to noise. Smaller size (much thinner and lighter). Disadvantages of fiber-optic cable include: Cost more than twisted-pair or coaxial cable Difficult to install and modify. Fiber-optic cables are used by many local and long-distance telephone companies, cable TV, and in high-traffic networks or as the main cable in a network. Wireless Transmission Media WAP (Wireless Application Protocol) is a standard set of communications specifications that help to ensure successful wireless communications to the Internet. Broadcast Radio Broadcast radio is a wireless transmission medium that distributes radio signals through the air over long distances such as between cities, regions, and countries. A transmitter is needed to send the broadcast radio signal and a receiver is needed to accept the signal. Some networks use a transceiver, which both sends and receives signals from wireless devices. Bluetooth Bluetooth is a radio frequency (RF) specification that many portable devices use for short-range wireless communications. Each device must include a transceiver chip and be within a specified range (about 10 meters). Cellular Radio Cellular radio is a form of broadcast radio that is used widely for mobile communications, specifically cellular telephones. A cellular telephones is a telephone device that uses radio signals to transmit voice and digital data messages. Portable computers can be connected to cellular telephones to surf the Web, send and receive , enter a chat room, or access a network. Page 5

6 Personal Communications Services (PCS) is a set of technologies used for completely digital cellular devices, which can include handheld computers, cellular telephones, pagers, and fax machines. CDMA (Code Division Multiple Access) is the most popular PCS technology today because of its fast transmission speed and lower cost. Microwaves Microwaves are radio waves that provide a high-speed transmission. Microwave transmission involves sending signals from one microwave station to another. A microwave station is an earth-based reflective dish that contains the antenna, transceivers, and other equipment necessary for microwave communications. Microwaves are limited to line-of-sight transmission. Microwaves must be transmitted in a straight line with no obstructions between microwave antennas. Communications Satellite A communications satellite is a space station that receives microwave signals from an earth-based station, amplifies (strengthens) the signals, and broadcasts the signals back over a wide area to any number of earth-based stations. An uplink is a transmission from an earth station to a satellite. A downlink is a transmission from a satellite to an earth station. Communications satellites are usually placed about 22,300 miles above the earth s equator and move at the same rate as the earth. Infrared Infrared (IR) is a wireless transmission media that sends signals using infrared light waves. Infrared transmission also requires a line-of-sight transmission. Transmission Characteristics Signal Type: Analog or Digital An analog signal consists of a continuous electrical wave. Digital signals are individual electrical pulses that represent the bits grouped together into bytes. Telephone equipment and telephone lines were originally designed to carry only analog signals. Computers process data as digital signals. A modem converts between digital signals and analog signals so that the signals can be transmitted through telephone lines. Page 6

7 Baseband vs. Broadband Baseband transmission transmits only one signal at a time. Broadband transmission can transmit multiple signals simultaneously. Broadband transmission transmits signals at a much faster speed. Two widespread applications of broadband transmission are digital subscriber lines (DSL) and cable television networks. Transmission Mode: Asynchronous and Synchronous With asynchronous transmission, transmissions are not synchronized that is, transmission does not occur at predetermined or regular intervals. A sending device thus can transmit bytes at any time, and the receiving device must be ready to accept them as they arrive. A start bit marks the beginning of a byte and a stop bit marks the end of the byte. An additional bit called a parity bit is sometimes included at the end of each byte to allow for error checking. Asynchronous transmission usually involves communications in which data can be transmitted intermittently instead of in a steady stream. Asynchronous transmission is relatively slow. Synchronous transmission involves sending large blocks of bytes at regular intervals without any start/stop signals. Synchronous transmission requires that both the sending and receiving devices be synchronized before any bytes are transmitted. Synchronous transmission requires more expensive equipment but provides greater speed and accuracy than asynchronous transmission. Transmission Direction: Simplex, Half-Duplex, and Full-Duplex In simplex transmission, data flow only in one direction from the sending device to the receiving device. Simplex transmission is used only when the sending device does not require a response from the receiving device. Security systems and fire alarms that contain a sensor use simplex transmission. In half-duplex transmission, data can blow in either direction from sender to receiver and back but only in one direction at a time. Many fax machines, credit card verification systems, and automatic teller machines use half-duplex transmission. Page 7

8 In full-duplex transmission, data can flow in both directions at the same time. A regular telephone line supports full-duplex transmission so that both parties can talk at the same time. Transfer Rates The speed with which a transmission medium carries data is its transfer rate. Transfer rate is usually expressed as bits per second (bps). The transfer rate of a transmission medium depends on the medium s bandwidth and its speed. Bandwidth is the range of frequencies that a transmission medium can carry in a given period of time. The higher the bandwidth of a medium, the more frequencies it can transmit, and the more data can be transmitted at one time. For analog signals, bandwidth is expressed in hertz (Hz), or cycles per second (cps). For digital signals, bandwidth is expressed in bits per second, or bps. The Telephone Network The public switched telephone network (PSTN) is the worldwide telephone system that handles regular voice telephone calls. The PSTN uses a variety of transmission media. However, the final link from the local telephone company to a home or office often uses analog twisted-pair cable. Data, instructions, an information can be sent over the PSTN using dial-up lines or dedicated lines. Dial-up Lines A dial-up line is a temporary connection that uses one ore more analog telephone lines for communications. Each time a call is placed, the telephone company switching office select the line to use to establish the connection. Advantages of a dial-up line include: It costs no more than making a regular telephone call. Computers at any two locations can establish a connection using modems and the PSTN. A disadvantage of dial-up line is that a user cannot control the quality of the connection because the telephone company s switching office randomly selects the line for connection. Page 8

9 Dedicated Lines A dedicated line is a connection that is always established between two communications devices. The quality and consistency of the connection is better than a dial-up line because dedicated lines provide a constant connection. A leased line is a dedicated line leased from a telephone or communications service company. Leased lines can either be analog or digital. Analog leased lines require modems at both the sending and receiving ends. Digital leased lines may use twisted-pair cable, coaxial cable, fiber-optic cable, microwaves, or infrared as transmission media. Digital leased lines provide faster transmission rates than analog leased lines. Four popular types of digital leased lines are: ISDN Lines ISDN (Integrated Services Digital Network) is a set of standards for digital transmission of data over analog telephone lines. With ISDN, the same twisted-pair telephone line that could normally carry only one computer signal, can now carry three to twenty-four signals at once, through the same line, using a technique called multiplexing. ISDN requires that both ends of the connection have an ISDN adapter installed. ISDN lines also require a special ISDN telephone for voice communications. The most affordable ISDN line, called a Basic Rate Interface (BRI), carries three signals and has transmission rates up to 128 Kbps. An ISDN line that carries 24 signals, called a Primary Rate Interface (PRI), is comparable in speed to a T-1 line. Digital Subscriber Lines A digital subscriber line (DSL) uses broadband to transmit a greater number of bytes on a standard twisted-pair cable. DSL provides slightly higher transfer rates than ISDN lines and thus is slightly more expensive. Page 9

10 An asymmetric digital subscriber line (ADSL) is a type of DSL that supports faster transmission rates when receiving data (range from 1.54 Mbps to 8.45 Mbps) than when sending data (range from 128 Kbps to 640 Kbps). To use ADSL, the computer must have a Universal ADSL card, or some other special modem card. Symmetric DSL (SDSL) is a better choice for longer distances. VDSL (Very high-rate DSL) is extremely fast for short distances. T-Carrier Lines A T-carrier line is any of several types of digital lines that carry multiple signals over a single communications line. Digital T-carrier lines use a technique called multiplexing so that multiple signals can share the telephone line. A T-1 line can carry 24 separate signals at a transmission rate of 64 Kbps each for a total transmission rate of Mbps. A T-3 line is equal to twenty-eight T-1 lines, and can carry 672 individual signals at a transmission rate of 64 Kbps each for a total transmission rate of 43 Mbps. The Internet backbone also uses T-3 lines. Asynchronous Transfer Mode Asynchronous transfer mode (ATM) is a service designed to carry voice, data, video, and multimedia at high speeds currently up to 622 Mbps. Communications Channel Example When a typical communications channel sends a request over the Internet: 1. A personal computer s request for information travels to the ISP. 2. When the request leaves the ISP, it travels over T-1 lines, microwave stations, and communications satellites until it reaches the Internet backbone. 3. The request travels over T-3 lines along the Internet backbone. 4. The request travels over T-1 lines until it reaches the destination network server. Page 10

11 Although many media and devices are involved, the entire communication process could take less than one second. Communications Software Communications software consists of programs that manage the transmission of data, instructions, and information between computers. For two computers to communicate, they must have compatible communications software. Communications software usually includes one or more of the following features: The dialing feature allows a user to store, review, select, and dial telephone numbers to connect to another computer. The file transfer feature allows a user to send one or more files from one computer to another. The terminal emulation feature allows a personal computer to act as a specific type of terminal, so that the user can connect to and access data and resources on a minicomputer or mainframe. The Internet access feature allows a user to use the computer to connect to the Internet so send , participate in chat rooms, visit World Wide Web sites, and so on. Communications Devices A communications device is any type of hardware capable of transmitting data, instructions, and information between a sending device (sender) and a receiving device (receiver). Modem A modem is a communications device that converts digital and analog signals. The word, Modem, is derived from a combination of the words, modulation and demodulation. Modulation is to convert digital signals into analog signals. Demodulation is to convert analog signals into digital signals. An external modem is a stand-alone device that attaches to a serial port on a computer with a cable and to a telephone outlet with a standard telephone cord. An internal modem is an expansion card that can be inserted into an expansion slot on a computer s motherboard; the modem then attaches to a telephone outlet with a standard telephone cord. Page 11

12 Most personal computer modems transmit data between 28.8 Kbps and 56 Kbps. Many modems today are called fax modems because they can send computer prepared documents as faxes and also receive faxes. A DSL modem is required to access the Internet using digital subscriber line. Cable Modem A cable modem is a modem that sends and receives data over the cable television (CATV) network, which consists largely of coaxial cable. Cable modems currently can transmit data at speeds of 500 Kbps to over 2 Mbps. An external cable modem is a stand-alone device that a user connects with a cable to a port on the computer s network interface card. An internal cable modem is an expansion card that is inserted into an expansion slot on a computer or other device. A cable modem can also be integrated with a set-top box to provide faster viewing of multimedia Web sites. A set-top box is a low-cost network computer placed on top of the television set that allows a user to access the Internet and navigate Web pages using a device that looks like a remote control. Multiplexers A multiplexer (MUX) is a device that combines two or more input signals from various devices into a single stream of data and then transmits it over a single transmission medium. By combining the separate data streams into one, a multiplexer increases the efficiency of communications and reduces the need for, and the cost of, using separate transmission media. Both the sending and receiving devices need a multiplexer for data transmission to occur. At the sending end, a multiplexer combines separate data transmission into a single data stream. It then compresses the data and sends it over the communications channel. At the receiving end, the multiplexer separates the single stream into its original parts. Network Interface Card A network interface card, or NIC, or LAN adapter, is an expansion card that is inserted into an expansion slot of a personal computer or other device, enabling the device to connect to a network. Page 12

13 Devices Used to Interconnect Networks Hub A hub, also called a concentrator or multistation access unit (MAU) is a device that provides a central point for cables in a network. Repeater A repeater is a device that accepts a signal from a transmission medium, amplifies it, and retransmits it over the medium. As a signal travels over a long distance, it undergoes a reduction in strength, an occurrence called attenuation. Bridge A bridge is a device that connects two LANs using the same protocol, such as Ethernet. Sometimes, it is more efficient and economical to use a bridge to connect two separate LANs, instead of creating one large LAN that combines the two separate LANs. Gateway A gateway is a combination of hardware and software that connects networks that use different protocols. Router A router is an intelligent communications device that sends (routes) communications traffic to the appropriate network using the fastest available path. In case of a partial network failure, routers can determine alternate paths over which to send data. A router is a device that connects multiple networks including those with different protocols. Networks A network is a collection of computers and devices connected by communications channels that allows users to share data, information, hardware, and software with other users. Networks promote: Hardware sharing Data and information sharing Software sharing Facilitated communications Networks exist in a range of sizes. Two widely used types of networks are local area networks and wide area networks. Page 13

14 Local Area Network A local area network (LAN) is a network that connects computers in a limited geographic area, such as a school computer laboratory, office, or group of buildings. A local area network that uses wireless transmission media such as radio or infrared is called a wireless LAN (WLAN). A network operating system (NOS) is the system software that organizes and coordinates the activities on a local area network. In most networks, a NOS is a set of programs separate from the operating systems. The NOS is either installed on each computer in addition to the current operating system or installed on the network server to which each computer connects. Some operating systems, such as Windows NT and Windows 2000, include networking capabilities that enable them to be considered a network operating system. Tasks performed by a NOS include Administration File management Printer management Security Two popular types of LANs are peer-to-peer and client/server. The major difference between them lies in how the data and information is stored. Peer-to-peer Network A peer-to-peer network is a simple, inexpensive network designed to connect less than ten computers together using twisted-pair cable or coaxial cable. Each computer, or peer, on the network can share the hardware, data, or information located on any other computer in the network. Each computer stores files on its own storage devices. A NOS and application software must be installed on each computer in the network. However, only one computer on the network needs to connect to peripherals, such as a printer or a scanner. Page 14

15 Client/Server Network A client/server network is a network in which one or more computers are designated as a server(s) and the other computers on the network, called clients, can request services from the server. A server controls access to the hardware and software on the network and provides a centralized storage for programs, data, and information. Some servers are dedicated to performing a specific task. A file server stores and manages files. A print server manages printers and print jobs. A database server stores and provides access to a database. A thin server provides access to the Internet for the home user. Clients rely on the servers for resources such as files, devices, processing power, and storage. A network administrator is the person in charge of operations of the network. Wide Area Network A wide area network (WAN) is a network that covers a large geographical area such as a city or a country. Computers are often connected to a WAN via public networks such as the telephone system or by dedicated lines or satellites. The Internet is the world s largest wide area network. A metropolitan area network (MAN) is a backbone network that connects local area networks in a metropolitan area such as a city or town. The network topologies or network architecture is the configuration, or physical arrangement, of the devices in a communications network. Three commonly used network topologies are bus, ring, and star. Networks can also 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. Page 15

16 The bus in a bus network can transmit data, instructions, and information in both directions. However, only one device, such as a computer, can transfer items at one time. The bus topology is used primarily for local area networks. Bus networks are flexible because computers and other devices can be attached or detached from the network at any point without disturbing the rest of the network. Failure of one device also does not affect the rest of the bus network. The transmission simply bypasses the failed device. However, if the bus itself become inoperable, the network remains inoperative until the bus is back in working order. Ring Network A ring network is designed so a cable forms a closed ring, or loop, with all computers and devices arranged along the ring. Data transmitted on a ring network travels from device to device around the entire ring, in one direction. When a computer sends data, the data travels to each computer on the ring until it reaches its destination. If a device on a ring network fails, all devices before the failed device are unaffected, but those after the failed device cannot function. The ring topology is primarily used for LANs, but is also used to connect a mainframe to a WAN. Star Network In a star network, all of the devices in the network connect to a central computer, thus forming a star. The central computer that provides a common connection point for devices in the network is called the hub. All data that transfer from one computer to another passes through the hub. Because each device connects directly to the hub, if one device fails, only that device is affected. If the hub fails, however, the entire network is inoperable. Communications Protocols A protocol is a set of rules and procedures for exchanging information among computers. Page 16

17 Protocols define how the communications channel is established, how information is transmitted, and how errors are detected and corrected. Using the same protocol, different kinds of computers can communicate with each other. Two more widely used protocols for LANs are Ethernet and token ring. Ethernet Ethernet is a LAN protocol that allows personal computers to contend for access to the network. Ethernet was the first industry standard LAN protocol developed by Xerox in It is relatively inexpensive and easy to install and maintain. A network that uses the Ethernet protocol is sometimes called an Ethernet network. Ethernet is based on a bus topology, but Ethernet networks can be wired in a star pattern by using a hub. Ethernet networks use the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) access-control technique to detect possible collisions if two computers attempt to send data at the same time. If a computer with data to transmit senses that data is already on the network, it waits a random amount of time before attempting to retransmit the data. The maximum transmission rate on a standard Ethernet network is 10 Mbps. Another type of protocol, called Fast Ethernet, transmits at 100 Mbps. Gigabit Ethernet provides an even higher speed of transmission, 1000 Mbps. Token Ring A token ring protocol controls access to the network by requiring that a special signal, called a token, is shared or passed among network devices. Only the device with the token can transmit data over the network. If a device has nothing to transmit, it passes the token to the next device. Each device has an equal chance to transmit. Page 17

18 Token ring is based on a ring topology, although it can use a star topology. Networks using a token ring protocol are called token ring networks. A token ring network can operate at speeds of 4 Mbps using unshielded twisted-pair cables or 16 Mbps using shielded twisted-pair cables. TCP/IP TCP/IP (Transmission control protocol/internet protocol) is a set of protocols widely used on the Internet. TCP/IP manages the transmission of data by breaking it up into packets. The TCP/IP protocol defines how to break the message into packets, provides routing information for message deliver, and reassembles the message at the receiving end. TCP/IP was developed in 1973 for use in the ARPANET, and was adopted as the Internet standard in Intranets Intranets are internal networks that use Internet and Web technologies. Intranet is a small version of the Internet used within an organization. It uses TCP/IP protocols; supports multimedia Web pages coded in HTML, and is accessible via a Web browser. Intranet generally makes company information accessible to employees and facilitates working in groups. An intranet that extends to authorized users outside the company is called an extranet. To prevent unauthorized access to data and information, an intranet or extranet is often protected by a firewall. A firewall is a general term that refers to both hardware and software used to restrict access to data and information on a network. Home Network A home network connects multiple computers in a user s home or home office. Advantages of home network include: All computers in the house can be connected to the Internet at the same time using a single telephone line. Each computer can access files on the other computers. All computers can share peripherals, such as a scanner or a printer. Page 18