Chapter 6 Telecommunications and Networks
Communications • Communications • The message (data and information) is communicated via the signal • The transmission medium “carries” the signal Transmissionmedium Sender Receiver Signal
Communications ‘Discussion’ The transmission of data from one computer to another, or from one device to another. A communications device, therefore, is any machine that assists data transmission. For example, modems, cables, and ports are all communications devices. Communications software refers to programs that make it possible to transmit data.
Telecommunications • Telecommunications • The electronic transmission of signals for communications, including such means as: • Telephone • Radio • Television • Telecommunication medium • Anything that carries an electronic signal and interfaces between a sending device and a receiving device
Communications and Telecommunications • In human speech, the sender transmits a signal through the transmission medium of the air • In telecommunications, the sender transmits a signal through the transmission medium of a cable Schematic
Data Communications • Data communications • A specialized subset of telecommunications that refers to the electronic collection, processing, and distribution of data -- typically between computer system hardware devices
Elements of a Telecommunications System • Telecommunication devices • Relay signals between computer systems and transmission media Schematic
Computer Network • Computer network… • The communications media, devices, and software needed to connect two or more computer systems and/or devices • Used to share hardware, programs, and databases across the organization • Fosters teamwork, innovative ideas, and new business strategies
Types of Telecommunications Media (1) • Twisted pair wire cable • Insulated pairs of wires historically used in telephone service and to connect computer devices • Coaxial cable • Consists of an inner conductor wire surrounded by insulation, called the dielectric • The dielectric is surrounded by a conductive shield, which is surrounded by a non-conductive jacket. Coaxial cable has better data transmission rate than twisted pair
Twisted-pair ‘Discussion’ A type of cable that consists of two independently insulated wires twisted around one another. One wire carries the signal while the other wire is grounded and absorbs signal interference. Twisted-pair cable is used by older telephone networks and is the least expensive type of local-area network (LAN) cable. Other types of cables used for LANs include coaxial cables and fiber optic cables.
Coaxial Cable ‘Discussion’ A type of wire that consists of a centre wire surrounded by insulation and then a grounded shield of braided wire. The shield minimizes electrical and radio frequency interference. Coaxial cabling is the primary type of cabling used by the cable television industry and is also widely used for computer networks. Although more expensive than standard telephone wire, it is much less susceptible to interference and can carry much more data. Because the cable television industry has already connected millions of homes with coaxial cable, many analysts believe that they are the best positioned to capitalize on the much-heralded information highway.
Types of Telecommunications Media (2) • Fiber-optic Cable • Many extremely thin strands of glass or plastic bound together in a sheathing which transmits signals with light beams • Can be used for voice, data, and video
Fiber Optic ‘Discussion’ (1) A technology that uses glass (or plastic) threads (fibers) to transmit data. A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves. Fiber optics has several advantages over traditional metal communications lines: • Fiber optic cables have a much greater bandwidth than metal cables. This means that they can carry more data • Fiber optic cables are less susceptible than metal cables to interference • Fiber optic cables are much thinner and lighter than metal wires • Data can be transmitted digitally (the natural form for computer data) rather than analogically.
Fiber Optic ‘Discussion’ (2) The main disadvantage of fiber optics is that the cables are expensive to install. In addition, they are more fragile than wire and are difficult to split. Fiber optics is a particularly popular technology for local-area networks. In addition, telephone companies are steadily replacing traditional telephone lines with fiber optic cables. In the future, almost all communications will employ fiber optics.
Types of Telecommunications Media (3) • Microwave Communications • Line-of-sight devices which must be placed in relatively high locations • Microwave usage • Information is converted to a microwave signal, sent through the air to a receiver, and recovered Pretty picture
Types of Telecommunications Media (4) • Satellite transmission • Communications satellites are relay stations that receive signals from one earth station and rebroadcast them to another • They use microwave signals Pretty picture
Types of Telecommunications Media (5) • Cellular transmission • Signals from cells are transmitted to a receiver and integrated into the regular network Pretty picture
Cellular ‘Discussion’ Refers to communications systems, especially the Advance Mobile Phone Service (AMPS), that divide a geographic region into sections, called cells. The purpose of this division is to make the most use out of a limited number of transmission frequencies. Each connection, or conversation, requires its own dedicated frequency, and the total number of available frequencies is about 1,000. To support more than 1,000 simultaneous conversations, cellular systems allocate a set number of frequencies for each cell. Two cells can use the same frequency for different conversations so long as the cells are not adjacent to each other. For digital communications, several competing cellular systems exist, including GSM and CDMA.
Types of Telecommunications Media (6) • Infrared transmission • Involves sending signals through the air via light waves • Requires line-of-sight and short distances (a few hundred yards) • Used to connect various computing devices such as handheld computers Sorry, no pretty picture!
Terminology • Analog Signal • A continuous, curving signal • Digital Signal • A signal represented by bits • Modems • Devices that translate data from digital to analog and analog to digital
Analog ‘Discussion’ (1) Almost everything in the world can be described or represented in one of two forms: analog or digital. The principal feature of analog representations is that they are continuous. In contrast, digital representations consist of values measured at discrete intervals. Digital watches are called digital because they go from one value to the next without displaying all intermediate values. Consequently, they can display only a finite number of times of the day. In contrast, watches with hands are analog, because the hands move continuously around the clock face. As the minute hand goes
Analog ‘Discussion’ (1) around, it not only touches the numbers 1 through 12, but also the infinite number of points in between. Early attempts at building computers used analog techniques, but accuracy and reliability were not good enough. Today, almost all computers are digital.
Digital ‘Discussion’ (1) Describes any system based on discontinuous data or events. Computers are digital machines because at their most basic level they can distinguish between just two values, 0 and 1, or off and on. There is no simple way to represent all the values in between, such as 0.25. All data that a computer processes must be encoded digitally, as a series of zeroes and ones. The opposite of digital is analog. A typical analog device is a clock in which the hands move continuously around the face. Such a clock is capable of indicating every possible time of day. In contrast, a
Digital ‘Discussion’ (2) digital clock is capable of representing only a finite number of times (every tenth of a second, for example). In general, humans experience the world analogically. Vision, for example, is an analog experience because we perceive infinitely smooth gradations of shapes and colors. Most analog events, however, can be simulated digitally. Photographs in newspapers, for instance, consist of an array of dots that are either black or white. From afar, the viewer does not see the dots (the digital form), but only lines and shading, which appear to be continuous. Although
Digital ‘Discussion’ (3) digital representations are approximations of analog events, they are useful because they are relatively easy to store and manipulate electronically. The trick is in converting from analog to digital, and back again. This is the principle behind compact discs (CDs). The music itself exists in an analog form, as waves in the air, but these sounds are then translated into a digital form that is encoded onto the disk. When you play a compact disc, the CD player reads the digital data, translates it back into its original analog form, and sends it to the
Digital ‘Discussion’ (4) amplifier and eventually the speakers. Internally, computers are digital because they consist of discrete units called bits that are either on or off. But by combining many bits in complex ways, computers simulate analog events. In one sense, this is what computer science is all about.
How a Modem Works • Modem • Modulates a digital signal into an analog signal for transmission via analog medium, then demodulates the signal into digital for receiving Pretty picture
Modem ‘Discussion’ (1) Acronym for modulator-demodulator. A modem is a device or program that enables a computer to transmit data over telephone lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms. Fortunately, there is one standard interface for connecting external modems to computers called RS-232. Consequently, any external modem can be attached to any computer that has an RS-232 port, which almost all personal computers have. There are also modems
Modem ‘Discussion’ (2) that come as an expansion board that you can insert into a vacant expansion slot. These are sometimes called onboard or internal modems.
Multiplexer • Multiplexer • Allows several telecommunications signals to be transmitted over a single communications medium at the same time Pretty picture
Communicationslink Multiplexor Front-endprocessor Hostcomputer
Multiplexor ‘Discussion’ A communications device that multiplexes (combines) several signals for transmission over a single medium. A demultiplexor completes the process by separating multiplexed signals from a transmission line. Frequently a multiplexor and demultiplexor are combined into a single device capable of processing both outgoing and incoming signals. A multiplexor is sometimes called a mux.
Front-End Processor • Front-end processor… • Special purpose computers that manage communication to and from a computer system Pretty picture
Job 1 Incoming and outgoing jobs Job 2 Job 3 Front-endprocessor Mainsystem Job 4
Carriers and Services (1) • Carriers • Organizations that take the responsibility of ensuring telecommunications can effectively take place between enterprises • Common carriers • Long-distance telephone companies • Value-added carriers • Companies that have developed private telecommunications systems and offer their services for a fee • Switched lines • Lines that use switching equipment to allow one transmission device to be connected to other transmission devices (e.g., standard telephone line) • Dedicated line • A line that provides constant connection between two points. No switching or dialing is needed
Carriers and Services (2) • Private branch exchange (PBX) • Communication system that can manage both voice and data transfer within a location (e.g. a building) and to outside lines • Wide area telecommunication service (WATS) • Billing method for heavy users of voice services • Phone and dialing services • Includes automatic number identification (a.k.a. caller ID) • Integration of telephones and personal computers • Access code screening • Call priorities • One number portability (use anywhere) • Intelligent dialing (auto re-dial for a busy number)
Carriers and Services (3) • ISDN • ISDN = Integrated Services Digital Network • Technology that uses existing common-carrier lines to simultaneously transmit voice, video, and image data in digital form Pretty picture
Carriers and Services (4) • T1 carriers • An expensive service developed by AT&T to increase the number of voice calls that could be handled through existing cables • Digital subscriber lines (DSL) • Uses existing phone wires going into today’s homes and businesses to provide transmission speeds exceeding 500 Kbps at a cost of $100 - $300 per month
Networks and Distributed Processing • Centralized processing • Data processing that occurs in a single location or facility • Decentralized processing • Data processing that occurs when devices are placed at various remote locations • Distributed processing • Data processing that occurs when computers are placed at remote locations but are connected to each other via telecommunications devices
Network Concepts and Considerations • Network Topology • A logical model that describes how networks are structured or configured • Topologies… • Ring (see chapter 1) • Bus (see chapter 1) • Star (see chapter 1) • Hierarchical • Hybrid
Hierarchical • Uses treelike structures with messages passed along the branches of the hierarchy • Hybrid • Network made up of various types of topologies
Network Types • Local area network (LAN) • Connects computer systems and devices in the same geographic area (can be Ring, Bus, Hierarchical, Star, Hybrid) • Wide area network (WAN) • Ties together large geographic regions using microwave and satellite transmission or telephone lines • International network • Links systems between countries
Terminal-to-Host Connection • Applications and databases reside on the same host computer • User interacts with the application using a “dumb terminal” Target PC “dumb terminal” Hostcomputer