Technology Guides T1 Hardware T2 Software T3 Data and Databases T4 Telecommunications T5 The Internet and the Web T6 Technical View of System Analysis and Design TECHNOLOGY GUIDE 4 Telecommunications T4.1 Telecommunications Concepts T4.2 Communications Media (Channels) T4.3 Network Systems: Protocols, Standards, Interfaces, and Topologies T4.4 Network Architecture T4.1
T4.2 TECHNOLOGY GUIDES TELECOMMUNICATIONS T4.1 TELECOMMUNICATIONS CONCEPTS The term telecommunications generally refers to all types of long-distance communication that uses common carriers, including telephone, television, and radio. Data communications is the electronic collection, exchange, and processing of data or information, including text, pictures, and voice, that is digitally coded and intelligible to a variety of electronic devices. Today s computing environment is dispersed both geographically and organizationally, placing data communications in a strategic organizational role. Data communications is a subset of telecommunications and is achieved through the use of telecommunication technologies. In modern organizations, communications technologies are integrated. Businesses are finding electronic communications essential for minimizing time and distance limitations. Telecommunications plays a special role when customers, suppliers, vendors, and regulators are part of a multinational organization in a world that is continuously awake and doing business somewhere 24 hours a day, 7 days a week ( 24 7 ). Figure T4.1 represents a model of an integrated computer and telecommunications system common in today s business environment. Telecommunications System A telecommunications system is a collection of compatible hardware and software arranged to communicate information from one location to another. These systems can transmit text, data, graphics, voice, documents, or full-motion video information. A typical telecommunications system is shown in Figure T4.2. Such systems have two sides: the transmitter and the receiver. The major components are: 1. Hardware all types of computers (e.g., desktop, server, mainframe) and communications processors (such as a modems or small computers dedicated solely to communications). 2. Communications media the physical media through which electronic signals are transferred; includes both wireline and wireless media. 3. Communications networks the linkages among computers and communications devices. 4. Communications processors devices that perform specialized data communication functions; includes front-end processors, controllers, multiplexors, and modems. 5. Communications software software that controls the telecommunications system and the entire transmission process. 6. Data communications providers regulated utilities or private firms that provide data communications services. FIGURE T4.1 An integrated computer and telecommunications system. Computer Communications Processor Communications channels and media Communications Processor Computer Network Software
T4.1 TELECOMMUNICATIONS CONCEPTS T4.3 Host Computer Hardware Telecommunication media (channels) Host Computer PC or Terminal Multiplex Modem Modem Multiplex Front End Processor FIGURE T4.2 A telecommunications system. Front end processor Receiver 7. Communications protocols the rules for transferring information across the system. 8. Communications applications electronic data interchange (EDI), teleconferencing, videoconferencing, e-mail, facsimile, electronic funds transfer, and others. To transmit and receive information, a telecommunications system must perform the following separate functions that are transparent to the user: Transmit information. Establish the interface between the sender and the receiver. Route messages along the most efficient paths. Process the information to ensure that the right message gets to the right receiver. Check the message for errors and rearrange the format if necessary. Convert messages from one speed to that of another communications line or from one format to another. Control the flow of information by routing messages, polling receivers, and maintaining information about the network. Secure the information at all times. Electronic Signals Telecommunications media can carry two basic types of signals, analog and digital (see Figure T4.3). Analog signals are continuous waves that carry information by altering the amplitude and frequency of the waves. For example, sound is analog and travels to our ears in the form of waves the greater the height (amplitude) of the waves, the louder the sound; the more closely packed the waves (higher frequency), the higher the pitch. Radio, telephones, and recording equipment historically transmitted and received analog signals, but they are rapidly changing to digital signals. Digital signals are discrete on-off pulses that convey information in terms of 1 s and 0 s, just like the central processing unit in computers. Digital signals have several advantages over analog signals. First, digital signals tend to be less FIGURE T4.3 Analog vs. digital signals. Analog data transmission (wave signals) 0 1 0 1 0 1 0 1 0 Digital data transmission (pulse signals)
T4.4 TECHNOLOGY GUIDES TELECOMMUNICATIONS affected by interference or noise. Noise (e.g., static ) can seriously alter the information-carrying characteristics of analog signals, whereas it is generally easier, in spite of noise, to distinguish between an on and an off. Consequently, digital signals can be repeatedly strengthened over long distances, minimizing the effect of any noise. Second, because computer-based systems process digitally, digital communications among computers require no conversion from digital to analog to digital. Communications Processors Communications processors are hardware devices that support data transmission and reception across a telecommunications system. These devices include modems, multiplexers, front-end processors, and concentrators. MODEM. A modem is a communications device that converts a computer s digital signals to analog signals before they are transmitted over standard telephone lines. The public telephone system (called POTS for Plain Old Telephone Service ) was designed as an analog network to carry voice signals or sounds in an analog wave format. In order for this type of circuit to carry digital information, that information must be converted into an analog wave pattern. The conversion from digital to analog is called modulation, and the reverse is demodulation. The device that performs these two processes is called a modem, a contraction of the terms modulate/demodulate (see Figure T4.4). Modems are always used in pairs. The unit at the sending end converts digital information from a computer into analog signals for transmission over analog lines; at the receiving end, another modem converts the analog signal back into digital signals for the receiving computer. Like most communications equipment, a modem s transmission speed is measured in bits per second (bps). Today, typical modem speeds range from 38,400 to 57,600 bps. There are various types of modems, as described below: External modem a stand-alone device, attaches to a special serial port on a computer, and a standard telephone cord connects to a telephone outlet. Internal modem a card that you can insert into an expansion slot on a computer s motherboard. The internal moderm has the same functions as those of an external modem. Digital modem one that sends and receives data and information to and from a digital telephone line such as IDSN or DSL (see below). Cable modem a modem that sends and receives data over the cable television (CATV) network. Direction of message Company A Company B FIGURE T4.4 A modem converts digital to analog signals and vice versa. (Source: Computing in the Information Age, Stern and Stern, 1993 John Wiley & Sons, Inc.) Terminal or PC Digital signal Modem modulates signal Analog transmission Modem demodulates signal Digital signal Central computer
T4.1 TELECOMMUNICATIONS CONCEPTS T4.5 The amount of data actually transferred from one system to another in a fixed length of time is only partially dependent on the transmission speed. Actual throughput speed, or the effective throughput speed (usually measured in characters per second), varies with factors such as the use of data compression or electrical noise interference. NEWER ALTERNATIVES TO ANALOG MODEMS. Digital subscriber line (DSL) service allows the installed base of twisted-pair wiring in the telecommunications system (see section T4.2) to be used for high-volume data transmission. DSL uses digital transmission techniques over copper wires to connect the subscribers to network equipment located at the telephone company central office. Asymmetric DSL (ADSL) is a variety of DSL that enables a person connecting from home to upload data at speeds from 16 to 640 Kbps and download data at 1.5 to 8 Mbps. Clearly, this is many times faster than an analog modem. However, where it is available, ADSL service currently costs about $50 per month (which usually includes Internet service). Voice-over-DSL (VoDSL) is a kind of telecommunication service replacing the traditional telephone system. It provides voice phone functions using DSL services. An ADSL circuit connects an ADSL modem on each end of a twisted-pair telephone line, creating three information channels a high-speed downstream channel; a medium-speed duplex channel, depending on the implementation of the ADSL architecture; and a POTS (Plain Old Telephone Service) or an ISDN channel. The POTS/ISDN channel is split off from the digital modem by filters, thus guaranteeing uninterrupted POTS/ISDN, even if ADSL fails. ADSL depends on advanced digital signal processing and creative algorithms to squeeze so much information through twisted-pair telephone lines. To create multiple channels, ADSL modems divide the available bandwidth of a telephone line in one of two ways frequency division multiplexing (FDM) or echo cancellation. FDM assigns one band for upstream data and another band for downstream data. Echo cancellation assigns the upstream band to overlap the downstream and separates the two by means of local echo cancellation, a technique well known in V.32 and V.34 modems. With either technique, ADSL splits off a 4-kHz region for POTS at the DC end of the band. An ADSL modem organizes the aggregate data stream created by multiplexing downstream channels, duplex channels, and maintenance channels together into blocks, and it attaches an error-correction code to each block. The receiver then corrects errors that occur during transmission up to the limits implied by the code and the block length. The unit may, at the user s option, also create superblocks by interleaving data within subblocks; this allows the receiver to correct any combination of errors within a specific span of bits. This technique allows for effective transmission of both data and video signals alike. As noted above, cable modems are offered by cable television companies in many areas as a high-speed way to access a telecommunications network. These modems operate on one channel of the TV coaxial cable. Cost and transmission speed are comparable to that of an ADSL. A cable modem gives users high-speed Internet access through a cable TV network at more than 1 mbps (1 million bits per second), or about 20 times faster than a traditional dialup modem. When a cable modem unit is installed next to the computer, a splitter is placed on the side of the household. It separates the coaxial cable line serving the cable modem from the line that serves the TV sets. A separate coaxial
T4.6 TECHNOLOGY GUIDES TELECOMMUNICATIONS cable line is then run from the splitter to the cable modem. Cable modems typically connect to computers through a standard 10Base-T Ethernet interface. Data are transmitted between the cable modem and computer at 10 mbps. For details, see cable-modem.net/tt/primer.html. MULTIPLEXER. A multiplexer is an electronic device that allows a single communications channel (e.g., a telephone circuit) to carry data transmissions simultaneously from many sources. The objective of a multiplexer is to reduce communication costs by maximizing the use of a circuit by sharing it. A multiplexer merges the transmissions of several terminals at one end of the channel, while a similar unit separates the individual transmissions at the receiving end. This process is accomplished through frequency division multiplexing (FDM), time division multiplexing (TDM), or statistical time division multiplexing (STDM). FDM assigns each transmission a different frequency. TDM and STDM merge together many short time segments of transmissions from different sending devices. FRONT-END PROCESSOR. With most computers, the central processing unit (CPU) has to communicate with several devices or terminals at the same time. Routine communication tasks can absorb a large proportion of the CPU s processing time, leading to degraded performance on more important jobs. In order not to waste valuable CPU time, many computer systems have a small secondary computer dedicated solely to communication. Known as a front-end processor, this specialized computer manages all routing communications with peripheral devices. The functions of a front-end processor include coding and decoding data, error detection, recovery, recording, interpreting, and processing the control information that is transmitted. It can also poll remote terminals to determine if they have messages to send or are ready to receive a message. In addition, a front-end processor has the responsibility of controlling access to the network, assigning priorities to messages, logging all data communications activity, computing statistics on network activity, and routing and rerouting messages among alternative communication links and channels. T4.2 COMMUNICATIONS MEDIA (CHANNELS) For data to be communicated from one location to another, a physical pathway or medium must be used. These pathways are called communications media (channels) and can be either physical or wireless. The physical transmission use wire, cable, and other tangible materials; wireless transmission media send communications signals through the air or space. The physical transmission media are generally referred to as cable media (e.g., twisted pair wire, coaxial cable, and fiber optic cable). Wireless media include cellular radio, microwave transmission, satellite transmission, radio and infrared media. The advantages and disadvantages of various media are highlighted in Table T4.1. The essentials of these communications media are described below. Cable Media Cable media (also called wireline media) use physical wires or cables to transmit data and information. Twisted-pair wire and coaxial cable are made of copper, and fiber-optic cable is made of glass. However, with the exception of fiber-optic
T4.2 COMMUNICATIONS MEDIA (CHANNELS) T4.7 TABLE T4.1 Advantages and Disadvantages of Communications Channels Channel Advantages Disadvantages Twisted-pair Coaxial cable Fiber-optic cable Microwave Satellite Radio Cellular Radio Infrared Inexpensive Widely available Easy to work with Unobtrusive Higher bandwidth than twisted pair Less susceptible to electromagnetic interference Very high bandwidth Relatively inexpensive Difficult to tap (good security) High bandwidth Relatively inexpensive High bandwidth Large coverage area High bandwidth No wires needed Signals pass through walls Inexpensive and easy to install Low-to-medium bandwidth Signals pass through walls Low-to-medium bandwidth Slow (low bandwidth) Subject to interference Easily tapped (low security) Relatively expensive and inflexible Easily tapped (low-to-medium security) Somewhat difficult to work with Difficult to work with (difficult to splice) Must have unobstructed line of sight Susceptible to environmental interference Expensive Must have unobstructed line of sight Signals experience propagation delay Must use encryption for security Create electrical interference problems Susceptible to snooping unless encrypted Require construction of towers Susceptible to snooping unless encrypted Must have unobstructed line of sight Used only for short distances cables, cables present several problems, notably the expense of installation and change, as well as a fairly limited capacity. Several cable media exist, and in many systems a mix of media (e.g., fibercoax) can be found. The major cable media are as follows. TWISTED-PAIR WIRE. Twisted-pair wire is the most prevalent form of communications wiring, because it is used for almost all business telephone wiring. Twisted-pair wire consists of strands of insulated copper wire twisted in pairs to reduce the effect of electrical noise. Twisted-pair wire is relatively inexpensive, widely available, easy to work with, and can be made relatively unobtrusive by running it inside walls, floors, and ceilings. However, twisted-pair wire has some important disadvantages. It emits electromagnetic interference, is relatively slow for transmitting data, is subject to interference from other electrical sources, and can be easily tapped to gain unauthorized access to data. Twisted-pair cabling comes in two varieties: shielded and unshielded. Unshielded twisted pair (UTP) is more popular and is generally the better option for small networks. The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The support for transmission rate is higher depending on how tight the wires are twisted. The EIA/TIA (Electronic Industry Association/Telecommunication Industry Association) has established standards of UTP and rated five categories of wire. A disadvantage
T4.8 TECHNOLOGY GUIDES TELECOMMUNICATIONS of untwisted pair cable is that it may be susceptible to radio and electrical frequency interference. Shielded twisted pair is suitable for environments with electrical interference. Shielded twisted pair is often used on networks using IBM s Token Ring topology. COAXIAL CABLE. Coaxial cable consists of insulated copper wire surrounded by a solid or braided metallic shield and wrapped in a plastic cover. It is much less susceptible to electrical interference and can carry much more data than twisted-pair wire. For these reasons, it is commonly used to carry high-speed data traffic as well as television signals (i.e., in cable television). However, coaxial cable is 10 to 20 times more expensive, more difficult to work with, and relatively inflexible. Because of its inflexibility, it can increase the cost of installation or recabling when equipment must be moved. Data transmission over coaxial cable is divided into two basic types: Baseband. Transmission is analog, and each wire carries only one signal at a time. Broadband. Transmission is digital, and each wire can carry multiple signals simultaneously. Because broadband media can transmit multiple signals simultaneously, it is faster and better for high-volume use. Therefore, it is the most popular Internetaccess method. Broadband needs a network interface card (NIC), also called a LAN adapter, in order to run. An NIC is a card that is inserted into an expansion slot of computer or other device, enabling the device top connect to a network. Today, it can be in a form of USB type or PCMCIA type. FIBER OPTICS. Fiber-optic technology, combined with the invention of the semiconductor laser, provides the means to transmit information through clear glass fibers in the form of light waves, instead of electric current. Fiber-optic cables contain a core of dozen or thin strands of glass or plastic. Each strand is called an optical fiber and is thin as hair. These fibers can conduct light pulses generated by lasers at transmission frequencies that approach the speed of light. Advantages are: able to carry significantly more signals than wire, faster data transmission, less susceptible to noise from other devices, better security for signals during transmission, smaller size. Disadvantages are: costs more than wire, can be difficult to install and modify. Besides significant size and weight reductions over traditional cable media, fiber-optic cables provide increased speed, greater data-carrying capacity, and greater security from interferences and tapping. A single hairlike glass fiber can carry up to 50,000 simultaneous telephone calls, compared to about 5,500 calls on a standard copper coaxial cable. The capacity of fiber is doubling every 6 to 12 months. Optical fiber has reached data transmission rates of six trillion bits (terabits) per second in laboratories and, theoretically, fiber can carry up to 25 terabits per second. Until recently, the costs of fiber and difficulties in installing fiber-optic cable slowed its growth. The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon is called photonics. This science includes light emission, transmission, deflection, amplification, and detection by optical components and instruments, lasers and other light sources, fiber optics,
T4.2 COMMUNICATIONS MEDIA (CHANNELS) T4.9 electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and information processing. Photons are used to move data at gigabit-per-second speeds across more than 1,000 wavelengths per fiber strand. Since the most common method of increasing cable capacity is to send more wavelengths through each fiber, attenuation is a problem for fiber transmission. Attenuation is the reduction in the strength of a signal, whether analog or digital. Attenuation requires manufacturers to install equipment to receive the distorted or dirty signals and send them out clean. These signal regenerators can cost tens of thousands of dollars to install on land; those under water can cost one million dollars each. In addition, fiber absorbs some of the light passing through it, making it necessary to amplify optical signals every 50 to 75 miles or so along the route. (Boosting the strength of the light so it can travel farther without amplification increases interference, causing distortion.) Eliminating optical amplifiers could save millions of dollars. A recent advance has dramatically increased the capacity of fiber-optic cables. Scientists have replaced solid glass fibers with hollow glass tubes containing a vacuum. These tubes are lined with mirrors that reflect virtually 100 percent of the light beaming through the tube. This advance multiplies fiber capacity and reduces the need for expensive amplification equipment. A metropolitan area network (MAN) is a data network designed usually for a town or a city. The fiber optic and associated equipment that make up the MAN can be connected to the national communications backbone. Because the demand for high-speed data service is growing fast (even during an economic downturn), it is estimated that roughly 43.5 million high-speed access devices will be in use by 2005. The existing legacy system is called Synchronous Optical Networks (SONETs), and these networks are best suited for voice traffic. The MAN can be divided loosely into the metro core and the edge: The core connects to long-haul points-of-presence; the edge is the aggregator or collector networks, which interface with large customers. The approaches for building the MAN are either to improve exising SONETs by making next-generation SONET boxes or to develop multi-protocol Dense Wavelength Division Multiplexing (DWDM) devices. The DWDM devices sit on the fiber rings and allow each wavelength to act as a separate pipeline, while adding intelligence to make transport more efficient. Both approaches are being used. Wireless Media Cable media (with the exception of fiber-optic cables) present several problems, notably the expense of installation and change, as well as a fairly limited capacity. The alternative is wireless communication. Common uses of wireless data transmission include pagers, cellular telephones, microwave transmissions, communications satellites, mobile data networks, personal communications services, and personal digital assistants (PDAs). Table T4.2 shows comparisons among various communications mediums. MICROWAVE. Microwave systems are widely used for high-volume, longdistance, point-to-point communication. These systems were first used extensively to transmit very-high-frequency (up to 500 GHz) radio signals at the
T4.10 TECHNOLOGY GUIDES TELECOMMUNICATIONS TABLE T4.2 Comparisons among Various Communications Media Technology Capacity (Mbps) Advantage Limitations Fiber to Several hundred, Highest speed cost Home up to 1000 DSL Downstream: Uses existing Speed decreases 6 8; upstream: phone lines with distance, no up to 1.5 service past 18,000 ft Wireless Comparable to No cables Multipath interference, (terrestrial) DSL required weather and terrain problems, limited distance Wireless Varied No cable, no Limited data (satellite) antennas, best rates likely suited to broadcasts Cable Downstream: typical Uses existing Data rate drops with 1; upstream: coaxial cable number of users, 0.1 0.5 poor security, requires major upgrade speed of light in a line-of-sight path between relay stations spaced approximately 30 miles apart (due to the earth s curvature). To minimize line-of-sight problems, microwave antennas were usually placed on top of buildings, towers, and mountain peaks. Long-distance telephone carriers adopted microwave systems because they generally provide about 10 times the data-carrying capacity of a wire without the significant efforts necessary to string or bury wire. Compared to 30 miles of wire, microwave communications can be set up much more quickly (within a day) and at much lower cost. However, the fact that microwave requires line-of-sight transmission severely limits its usefulness as a practical large-scale solution to data communication needs, especially over very long distances. Additionally, microwave transmissions are susceptible to environmental interference during severe weather such as heavy rain or snowstorms. Although still fairly widely used, long distance microwave data communications systems have been largely replaced by satellite communications systems. SATELLITE. A satellite is a space station that receives microwave signals from an earth-based station, amplifies the signals, and broadcasts the signals back over a wide area to any number of earth-based stations. Transmission to a satellite is an uplink, whereas transmission from a satellite to an earth-based station is a downlink. A major advance in communications in recent years is the use of communications satellites for digital transmissions. Although the radio frequencies used by satellite data communication transponders are also line-of-sight, the enormous footprint of a satellite s coverage area from high altitudes overcomes the limitations of microwave data relay stations. For example, a network of just three evenly spaced communications satellites in stationary geosynchronous orbit 22,241 miles above the equator is sufficient to provide global coverage. The advantages of satellites include the following: The cost of transmission is the same regardless of the distance between the sending and receiving stations
T4.2 COMMUNICATIONS MEDIA (CHANNELS) T4.11 within the footprint of a satellite, and cost remains the same regardless of the number of stations receiving that transmission (simultaneous reception). Satellites have the ability to carry very large amounts of data. They can easily cross or span political borders, often with minimal government regulation. Transmission errors in a digital satellite signal occur almost completely at random; thus, statistical methods for error detection and correction can be applied efficiently and reliably. Finally, users can be highly mobile while sending and receiving signals. The disadvantages of satellites include the following: Any one-way transmission over a satellite link has an inherent propagation delay (approximately one-quarter of a second), which makes the use of satellite links inefficient for some data communications needs (voice communication and stepping-on each other s speech). Due to launch-weight limitations, satellites carry or generate very little electrical power, and this low power, coupled with distance, can result in extremely weak signals at the receiving earth station. Signals are inherently not secure because they are available to all receivers within the footprint intended or not. Some frequencies used are susceptible to interference from bad weather or ground-based microwave signals. Types of Orbits. Currently, there are three types of orbits in which satellites are placed: geostationary earth orbit, medium earth orbit, and low earth orbit. Geostationary earth orbit (GEO) satellites orbit 22,241 miles directly above the equator and maintain a fixed position above the earth s surface. These satellites are excellent for sending television programs to cable operators and broadcasting directly to homes. However, transmissions from GEO satellites take a quarter of a second to send and return (called propagation delay), making twoway telephone conversations difficult. Also, GEO satellites are large and expensive, and the equatorial orbit cannot hold many more GEO satellites than the 160 that now orbit there. In 2000, a system of eight GEO satellites was launched by Hughes Electronics at a cost of $3 billion. Medium earth orbit (MEO), also called intermediate circuit orbit (ICO), satellites are located 6,250 to 13,000 miles above the earth s surface, in orbits inclined to the equator. While fewer satellites are needed to cover the earth than in LEO orbits, telephones need more power to reach MEO satellites than to reach LEO satellites. Low earth orbit (LEO) satellites are located 500 to 1,500 miles above the earth s surface. Their closer proximity to the earth reduces or eliminates apparent signal delay. They can pick up signals from weak transmitters, meaning that cellular (wireless) telephones need less power and can use smaller batteries. LEO satellites consume less power and cost less to launch than GEO and MEO satellites. However, the footprints of LEO satellites are small, requiring many of them in order to cover the earth. Table T4.3 shows the differences among the three types of satellites. Satellite Networks. Multiple LEO satellites from one organization are referred to as constellations. Many companies are in the process of building constellations of satellites for commercial service. SkyBridge (skybridgesatellite.com) uses two constellations of 40 LEO satellites each, orbiting at an altitude of 1,469 kilometers, to cover the entire earth, except for the polar regions. Satellite networking technology has taken a great stride forward. Now, the satellite dish (which is a major part of the hardware) is just half the previous size, with uplink speeds increased 533 times (from 19.2 Kbps to 10 Mbps) as a result of better compression, higher-powered satellites, and improvements in satellite
T4.12 TECHNOLOGY GUIDES TELECOMMUNICATIONS TABLE T4.3 Three Basic Types of Telecommunications Satellites Type Considerations Orbit Number GEO Stellites remain stationary relative to point 22,282 8 on Earth miles Few satellites needed for global coverage Transmission delay (approximately.25 second) Most expensive to build and launch Longest orbital life (12+ years) MEO Satellites move relative to point on Earth 6,250 to 13,000 10 12 Moderate number needed for global coverage miles Require medium-powered transmitters Negligible transmission delay Less expensive to build and launch Moderate orbital life (6 12 years) LEO Satellites move rapidly relative to point 500 to 1,500 many on Earth miles Large number needed for global coverage Require only low-power transmitters Negligible transmission delay Least expensive to build and launch Shortest orbital life (as low as 5 years) modems. Downstream rates have been improved from 1 Mbps to 60 Mbps. One unique feature of satellite networks is that they can broadcast massive chunks of data to multiple points. For more on satellites, see farsite.co.uk/satellite_routers/ FarLinX_satellite_router.htm. GLOBAL POSITIONING SYSTEMS. A global positioning system (GPS) is a wireless system that uses satellites to enable users to determine their position anywhere on the earth. GPS is supported by 24 U.S. government satellites that are shared worldwide. Each satellite orbits the earth once in 12 hours, on a precise path at an altitude of 10,900 miles. At any point in time, the exact position of each satellite is known, because the satellite broadcasts its position and a time signal from its on-board atomic clock, accurate to 1-billionth of a second. Receivers also have accurate clocks that are synchronized with those of the satellites. Knowing the speed of signals (186,272 miles per second), it is possible to find the location of any receiving station (latitude and longitude) within an accuracy of 50 feet by triangulation, using the distance of three satellites for the computation. GPS software computes the latitude and longitude and converts it to an electronic map. Other countries, troubled that the Global Positioning System is run by the U.S. military and controlled by the U.S. government, are building independent satellite navigation networks. As a result, Europe is building a civil satellite system called Galileo, scheduled to be in operation by 2008. Mainland China and Russia are also constructing satellite systems for GPS uses. GPS equipment has been used extensively for navigation by commercial airlines and ships and for locating trucks. GPS is now also being added to many
T4.2 COMMUNICATIONS MEDIA (CHANNELS) T4.13 consumer-oriented electronic devices. The first dramatic use of GPS came during the Persian Gulf War, when troops relied on the technology to find their way in the Iraqi desert. GPS also played the key role in targeting for smart bombs. Since then, commercial use has become widespread, including navigation, mapping, and surveying, particularly in remote areas. For example, several car manufacturers (e.g., Toyota, Cadillac) provide built-in GPS navigation systems in their cars. GPSs are also available on cell phones, so you can know where the caller is located. As of October 2001, cell phones in the United States must have a GPS embedded in them so that the location of a caller to 911 can be detected immediately. GPSs are now available to hikers in the form of handheld devices costing less than $100. GPSs are also embedded in some PDAs. RADIO. Radio electromagnetic data communications do not have to depend on microwave or satellite links, especially for short ranges such as within an office setting. Broadcast radio is a wireless transmission medium that distributes radio signals through the air over both long distances and short distances. Radio is being used increasingly to connect computers and peripheral equipment or computers and local area networks. The greatest advantage of radio for data communications is that no wires need be installed. Radio waves tend to propagate easily through normal office walls. The devices are fairly inexpensive and easy to install. Radio also allows for high data transmission speeds. However, radio can create reciprocal electrical interference problems with other office electrical equipment, and from that equipment to the radio communication devices. Also, radio transmissions are susceptible to snooping by anyone similarly equipped and on the same frequency. (This limitation can be largely overcome by encrypting the data being transmitted.) INFRARED. Infrared (IR) light is light not visible to human eyes that can be modulated or pulsed for conveying information. IR requires a line-of-sight transmission. Many computers and devices have an IrDA (Infrared Data Association) port that enables the transfer of data using infrared light rays. IrDA is a standard defined by the IrDA Consortium. It specifies a way to transfer data wirelessly via infrared radiation. The most common application of infrared light is with television or videocassette recorder remote control units. With computers, infrared transmitters and receivers (or transceivers ) are being used for shortdistance connection between computers and peripheral equipment, or between computers and local area networks. Many mobile phones have a built-in infrared (IrDA) port that supports data transfer. Advantages of infrared light include no need to lay wire, equipment is highly mobile, no electrical interference problems, no Federal Communications Commission (FCC) permission required to operate an infrared transmitter, no certification needed before selling an infrared device, and fairly inexpensive devices with very high data rates. Disadvantages of infrared media include susceptibility to fog, smog, smoke, dust, rain, and air temperature fluctuations. For details, see hw.cz/english/docs/irda/irda.html. CELLULAR RADIO TECHNOLOGY. Mobile telephones, which are being used increasingly for data communications, are based on cellular radio technology, which is a form of broadcast radio that is widely used for mobile communications.
T4.14 TECHNOLOGY GUIDES TELECOMMUNICATIONS The basic concept behind this technology is relatively simple: The Federal Communication Commission (FCC) has defined geographic cellular service areas; each area is subdivided into hexagonal cells that fit together like a honeycomb to form the backbone of that area s cellular radio system. Located at the center of each cell is a radio transceiver and a computerized cell-site controller that handles all cell-site control functions. All the cell sites are connected to a mobile telephone switching office that provides the connections from the cellular system to a wired telephone network and transfers calls from one cell to another as a user travels out of the cell serving one area and into another. The cellular telephone infrastructure has primarily been used for voice transmission, but recent development of a transmission standard called cellular digital packet data (CDPD) has made it possible for the infrastructure to support two-way digital transmission. The evolution of cellular transmission from analog to digital is described below. First-generation (1G) and second-generation (2G) cellular data transmission. 1G technology was characterized by bulky handsets and adjustable antenna, and was based on analog technology. 1G allowed only limited roaming. Second-generation (2G) cellular data transmission. 2G technology provides digital wireless transmission. 2G increases the voice capacity of earlier analog systems, and provides greater security, voice clarity, and global roaming. 2.5-generation (2.5G) cellular data transmission. 2.5G technology, usually associated with General Pocket Radio Service (GPRS), extends the 2G digital cellular standard and is installed as an upgrade to an existing 2G network. Third-generation (3G) technologies. 3G technology offers increased efficiency and capacity; new services, such as wide-area networks for PCs and multimedia; seamless roaming across dissimilar networks; integration of satellite and fixed wireless access services into cellular networks; and greater bandwidth. MOBILE COMPUTING. Mobile computing refers to the use of portable computer devices in multiple locations. It occurs on radio-based networks that transmit data to and from mobile computers. Computers can be connected to the network through wired ports or through wireless connections. Mobile computing provides for many applications, including m-commerce (see Chapter 6). Another type of mobile data network is based on a series of radio towers constructed specifically to transmit text and data. BellSouth Mobile Data and Ardis (formerly owned by IBM and Motorola) are two privately owned networks that use these media for national two-way data transmission. PERSONAL COMMUNICATION SERVICE. Personal communication service (PCS) uses lower-power, higher-frequency radio waves than does cellular technology. It is a set of technologies used for completely digital cellular devices, including handheld computers, cellular telephones, pagers, and fax machines. The cellular devices have wireless modems, allowing you Internet access and e-mail capabilities. The lower power means that PCS cells are smaller and must be more numerous and closer together. The higher frequency means that PCS devices are effective in many places where cellular telephones are not, such as in tunnels and inside office buildings. PCS telephones need less power, are smaller, and are less expensive than cellular telephones. They also operate at higher,
T4.2 COMMUNICATIONS MEDIA (CHANNELS) T4.15 less-crowded frequencies than cellular telephones, meaning that they will have the bandwidth necessary to provide video and multimedia communications. PERSONAL DIGITAL ASSISTANTS. Personal digital assistants (PDAs) are small, handheld computers capable of entirely digital communications transmission (see discussion in Technology Guide 1). They have built-in wireless telecommunications capabilities. Applications include Internet access, e-mail, fax, electronic scheduler, calendar, and notepad software. UPS s Delivery Information Acquisition Device (DIAD) is a hand-held electronic data collector that UPS drivers use to record and store information, thus helping UPS to keep track of packages and gather delivery information within UPS s nationwide, mobile cellular network. It digitally captures customers package information, thus enabling UPS to keep accurate delivery records. Drivers insert the DIAD into a DIAD vehicle adapter (DVA) in their delivery vehicles to transmit over UPS s nationwide cellular network for immediate customer use. It contains 1.5MB RAM, can consolidate multiple functions into single keys, accepts digital signatures, and has a built-in acoustical modem. Its laser scanner reads package labels quickly and accurately, smart software knows the driver s next street, and the device interacts with UPS cellular service. WIRELESS APPLICATION PROTOCOL. Wireless Application Protocol (WAP) is a technology that enable wireless trasmissions. For example, one popular application that utilizes WAP is i-mode, a wireless portal that enables users to connect to the Internet. Developed by NTT DoCoMo, i-mode provides an always-on connection to the Internet and content sites from popular media outlets, all accessible via color-screen handsets with polyphonic sound. It is charged at actual usage instead of on a pre-paid basis. WAP is criticized for browsing with small screens, little compelling content, and bad connections at great cost through a browser. Despite these drawbacks, it offers users the ability to make wireless connections to the Internet, which has enormous commercial appeal. NEWER WIRELESS TECHNOLOGIES. Because of the requirements of faster speed and strict security requirements that existing WAP cannot fulfill, newer wireless technologies are being created for future purposes. Listed below are some of the major new wireless technologies. Bluetooth. A relatively new technology for wireless connectivity is called Bluetooth. It is the term used to describe the protocol of a short-range (10- meter), frequency-hopping radio link between devices. Bluetooth allows wireless communication between mobile phones, laptops, and other portable devices. Bluetooth technology is currently being built into mobile PCs, mobile telephones, and PDAs. Bluetooth is the code name for a technology designed to provide an open specification for wireless communication of data and voice. It is based on a low-cost, short-range radio link built into a 9 9 mm microchip, providing protected ad hoc connections for stationary and mobile communication environments. It allows for the replacement of the many existing proprietary cables that connect one device to another with one universal short-range radio link. Designed to operate in a noisy radio-frequency environment, the Bluetooth radio uses a fast acknowledgement and frequency-hopping system scheme to
T4.16 TECHNOLOGY GUIDES TELECOMMUNICATIONS make the link robust. Its modules avoid interference from other signals by hopping to a new frequency after transmitting or receiving a packet. It operates in ISM band at 2.4 GHz. Bluetooth is also used in WPAN (described below) in a new standard 802.15. Fiber optics without the fiber. Another new technology is fiber optics without the fiber. With this technology, laser beams are transmitted through the air between two buildings or other points. Terabeam Corporation recently introduced such a service. Like other wireless media, the chief advantage of this technology is that there is no need to gain rights of way to lay cabling. However, weather can have a negative impact on transmission quality. Ultrawideband. Ultrawideband (UWB) is a superfast, short distance wireless technology that will have data speeds 10 times faster than Wi-Fi, which is actually a nickname for the 802.11b protocol. (See Chapter 6 for more on Wi- Fi.) It works by transmitting its signal over a wide swath of frequencies at a low power that does not interfere with the other occupants of the spectrum. It can give rise to a new generation of portable and home entertainment products with quality equal to a hardwired system that is perfect for home networking. Software-Defined Radio. Software-defined radio is a concept of a reconfigurable device that can automatically recognize and communicate with other devices. This concept could impose wireless standards that compete with the existing ones like CDMA, GSM, TDMA to transform today s rigid networks into an open system. The benefits are: improved system performance, cheaper service cost, seamless roaming (i.e., you could carry a single device for multiple purposes). Intel is now developing a new CPU that will include a type of software-defined radio that can adapt to different wireless LAN standards. Mesh Networks. A mesh network is created by a device that can turn nearly any wireless device into a router, creating an ad hoc network. Members of a network no longer rely on a central routing hub to distribute data; rather, the information hops from one user s device to another until it gets where it s going. Benefits are cheaper service, wider coverage areas, and speed (the mesh network can send data at speeds above 6 Mbps). Drawbacks are a security problem because of numerous pass-throughs and billing problems due to changes in connectivity. Wireless Personal Area Networks. Defined by IEEE as wireless networks that cover an area of at least 10 meters around a person, wireless personal area networks (WPANs) could eliminate cable and wire networks. Computing devices within a WPAN create a flow of machine-to-machine communication that personalizes services spontaneously. Possible problems are managing device interoperability, maintaining always-on connectivity between devices, and leakage of privacy information. (See Chapter 6 for more.) Adaptive Radio. Adaptive radio is a technology that lets wireless devices scout out the spectrum wherever they are, avoiding interference by tuning their transmissions to the available gaps. The primary benefit of this technology is it enables wireless devices to modify their power, frequencies, or timing to suit the environment they find themselves in, making such adjustments at occasional intervals or constantly checking and changing as airwave traffic shifts around them. HomePlug. A product called HomePlug makes it easy to use existing inwall electrical wiring for fast home networks. It lets you network devices by plugging an external adapter into a standard wall outlet, and delivers performance
T4.2 COMMUNICATIONS MEDIA (CHANNELS) T4.17 TABLE T4.4 802.11 Wireless Networking Standards 802.11 standard Functions 802.11a 54-mbps top speed; incompatible with 802.11b 802.11b 11-mbps top speed; popular in home and small-business networks 802.11e Enhances audio and video transmission on 802.11a, b, or g 802.11g New standard with 54-mbps top speed; compatible with 802.11b 802.11i Adds enhanced 128-bit encryption to 802.11a, b, or g superior to that of 802.11b wireless networks at only a small price premium. Its security protection is more robust than 802.11b because it uses DES encryption while 802.11b uses RC4 algorithm. It has a maximum speed of 14 mbps which is slightly faster than 802.11b s 11 mbps. Moreover, it is not subjected to other wireless traffic or to interference from walls and doors like that of 802.11b. Table T4.4 shows 802.11 wireless networking standard. Characteristics of Communications Media Communications media have several characteristics that determine their efficiency and capabilities. These characteristics include the speed, direction, mode, and accuracy of transmission. TRANSMISSION SPEED. Bandwidth refers to the range of frequencies that can be sent over a communications channel. Frequencies are measured in the number of cycles per second (or Hertz, abbreviated Hz). Bandwidth is an important concept in communications because the transmission capacity of a channel is largely dependent on its bandwidth. Capacity is stated in bits per second (bps), thousands of bps (Kbps), millions of bps (Mbps), and billions of bps (Gbps). In general, the greater the bandwidth of a channel, the greater the channel capacity. A baud is a detectable change in a signal (i.e., a change from a positive to a negative voltage in a wire). The amount of data that can be transmitted through a channel is known as its baud rate, measured in bits per second (bps). A baud represents a signal change from positive to negative, or vice versa. The baud rate is not always the same as the bit rate. At higher transmission speeds, a single signal change can transmit more than one bit at a time, so the bit rate can be greater than the baud rate. For many data communications applications (i.e, those that involve textual data), a low bandwidth (2400 to 14,400 bps) is adequate. On the other hand, acceptable performance for transmission of graphical information requires bandwidth in the Mbps range. Channel capacity is usually divided into three bandwidths: narrowband, voiceband, and broadband channels. Slow, low-capacity transmissions, such as those transmitted over telegraph lines, make use of narrowband channels, while telephone lines utilize voiceband channels. The channel bandwidth with the highest capacity is broadband, used by microwave, cable, and fiber-optic media. Communications channels have a wide range of speeds based on the technology used. Transfer rates measure the speed with which a line carries data. The faster the transfer rate, the faster you can send and receive data and information.