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Problem Statement Three-phase AC systems are the norm for most industrial applications. AC power in the form of voltage and current it delivered from the power company using three-phase distribution systems and many larger loads are three-phase loads in the form of motors, compressors, or similar. Sources and loads can be configured in either wye (where sources or loads are connected from line to neutral/ground) or delta (where sources or loads are connected from line to line) configurations and mixing between the types is common. Figure 1 shows the general wiring of a wye-wye three-phase system modeling all of the impedances typically found in such a system. During the typical analysis undertaken in most circuits textbooks, it is assumed that the system is entirely balanced. This means that all the source, line, and load impedances are equivalent, that is,

Under this assumption, the circuit is then typically reduced to a single-phase equivalent circuit model and the resultant circuit is solved with a single loop equation. What happens, however, when the system is unbalanced? Typically because the three load impedances and are not equal, which results in different currents through each load, is often measured in terms of the percentage difference between the load currents.

Chapter No: 06 COMMUNICATIONS, NETWORKS, & SAFEGUARDS The Wired & Wireless World.

Chapter Topics 6.1 From the Analog to the Digital Age 6.2 Networks 6.3 Wired Communications Media 6.4 Wireless Communications Media 6.5 Cyberthreats, Hackers, & Safeguards

   Digital convergence is the gradual merger of computing and communications into a new information environment, in which the same information is exchanged among many kinds of equipment, using the language of computers.  At the same time, there has been a convergence of several important industries—computers, telecommunications, consumer electronics, entertainment, mass media— producing new electronic products that perform multiple functions.

6.1 From the Analog to the Digital Age

	Digital

 Computers use digital signals—0s and 1s, off and on.  All the data that a computer processes is a series

of 0s and 1s.

 Each signal is a bit.

	Analog

 But most phenomena in life are analog.  Analog signals use wave variations.  Sound, light, and temperature are analog forms.  Traditional TV and radio use analog signals.  Humans’ vision operates in analog mode.

But analog data can be converted into digital form. Even though digital data is not as exact as analog data, it is easier to manipulate.

 
	For data transmission over telephone lines and cables, modems are needed to convert analog data into digital data that computers can use.
	Modem is short for modulate/demodulate. Modems modulate (convert) a computer’s digital data to analog data, transmit it, then demodulate (reconvert) it back to digital data for the receiving computer.
	Modems can convert data by modulating either a analog wave’s amplitude or its frequency.

 

	Converting Reality to Digital Form: Sampling

 Tape recorders, voices, and musical instruments are analog; CDs are digital  To burn a CD, the digital recording equipment must convert from analog to digital  The analog-to-digital converter samples the sound and converts the height of the wave to a number  Samples of the sound wave are taken at regular intervals – about 44,100 times each second  Because the digital samples are played back faster than our ears can react, it sounds to us like a single continuous sound wave

6.2 Networks

	A network is a system of interconnected computers, telephones, and/or other communications devices that can communicate with one another and share applications and data
	Benefits of Networks

 Share peripheral devices, such as printers, scanners, disk drives  Share programs and data  Better communications, including email  Centralized communications  Security of information, because of improved backup systems  Access to shared databases

	Some disadvantages of networks:

 Expense: The initial set up cost of a computer network can be high depending on the number of computers to be connected and the number of connecting devices and NICs .  Security Issues: If a computer is on a network, a computer hacker can get unauthorized access by using different tools.  Rapid Spread of Computer Viruses: If any computer system in a network gets affected by computer virus, there is a possible threat of other systems getting affected, too. Viruses get spread on a network easily because of the interconnectivity of workstations.  Dependency on the Main File Server: If the main file server of a computer network breaks down, the entire system can become useless. In case of big networks, the file server is often a powerful computer, which often makes a failure expensive—not to mention causing a service outage for many customers or system users.

	Main types of networks

 A wide area network (WAN) is a communications network that covers a wide geographic area, such as a country or the world. Most long-distance and regional telephone companies are WANs. WANs are used to connect local area networks, so that users and computers in one location can communicate with users and computers in other locations. The best example of a WAN is the Internet.  A metropolitan area network (MAN) is a communications network covering a city or a suburb. Many cellphone systems are MANs, and some cities set up wireless MANs to connect local area networks to the Internet.  A local area network (LAN) connects computers and devices in a limited geographic area, such as one office, one building, or a group of buildings close together. LANs are the basis for most office networks, and the organization that runs the LAN owns it. WANs and MANs generally use a common carrier—a telecommunications company that hires itself out to the public to provide communications transmission services—for at least part of its connections.

 

 
	A home area network uses wired, cable, or wireless connections to link a household’s digital devices—not only multiple computers, printers, and storage devices but also VCRs, DVDs, televisions, fax machines, videogame machines, and home security systems. 
	A personal area network uses short-range wireless technology to connect an individual’s personal electronics, such as cellphone, MP3 player, notebook PC, and printer. These networks are made possible with such inexpensive, short-range wireless technologies as Bluetooth, ultra wideband, and wireless USB.
	A home automation network relies on inexpensive, very short-range, low-power wireless technology to link switches and sensors around the house. Such networks run on inexpensive AA batteries and use wireless remotes, in-wall touch screens, and smartphones, along with special software, to control lights and switches, thermostats and furnaces, smoke alarms and outdoor floodlights, etc.

	How Networks Are Structured: Two Principal Structures
	Client/Server
	Consists of clients, which are computers that request data, and servers, which are computers that supply data

 File servers act like a network-based shared disk drive  Database servers store data but don’t store programs  Print servers connect one or more printers and schedule and control print jobs  Mail servers manage email

	 How Networks Are Structured: Two Principal Structures (continued)
	2. Peer-to-Peer (P2P)

 All computers on the network are “equal” and communicate directly with one another, without relying on servers

   

	Network Components — all networks have several things in common: 

 wired = twisted-pair, coaxial cable, fiber-optic cable  wireless = infrared, microwave, radio, Wi-Fi, satellite  Hosts and Nodes: Client/server network has a host computer, which controls the network; a node is any device attached to the network.  Packets – fixed-length blocks of data for transmission, reassembled after transmission  Protocols—set of conventions, or rules, governing the exchange of data between hardware and/or software components in the network; built into the hardware or software you are using

	Network linking devices:

 Switch—Full-duplex device that connects computers to a network; sends only to intended recipients  Bridge—Interface device that connects same type of networks  Gateway—Interface device that connects dissimilar networks  Router—Device that directs messages among several networks, wired or and/or wireless  Backbone—Main Internet highway that connects all networks in an organization; includes switches, gateways, routers, etc.  NIC—Network interface card; inserted in a slot on the motherboard, enables computer to operate as part of a network  NOS—network operating system; the system software that manages network activity

 

	Network topologies: Bus, Ring, Star, Mesh 

 Bus – all nodes are connected to a single wire or cable  Ring – all nodes are connected in a continuous loop  Star – all nodes are connected through a central host  Mesh – messages sent to the destination can take any possible shortest, easiest route to reach its destination. There must be at least two paths to any individual computer to create a mesh network. (Wireless networks are often implemented as a mesh, and the Internet is a mesh.)     6.3 Wired Communications Media

	Communications media carry signals over a communications path between devices
	Twisted-Pair Wire (dial-up connections)

1. 2 strands of insulated copper wire twisted around each other 2. Twisting reduces interference (crosstalk) from electrical signals 3. Data rates are 1 – 128 megabits per second (slow)

	Coaxial Cable

1. Insulated copper wire wrapped in a metal shield and then in an external plastic cover 2. Used for cable TV and cable Internet electric signals 3. Carries voice and data up to 200 megabits per second (fast)

	Communications media (continued)
	Fiber-optic cable

 Dozens or hundreds of thin strands of glass or plastic that transmit pulses of light, not electricity  Can transmit up to 2 gigabits per second (very fast)  Have lower error rate than twisted-pair or coax  More expensive than twisted-pair or coax  Lighter and more durable than twisted-pair or coax  More difficult to tap into than twisted-pair or coax

	Wired connections for the home: 
	Ethernet

 Connect to PC’s Ethernet network interface card (NIC)  For several PCs, get a switch to connect them all  10 or 100 megabits per second

	HomePNA

 Uses existing telephone wiring and jacks  Requires HomePNA NIC in your PC  Speeds of about 320 megabits per second

	Homeplug

 Uses existing home electrical lines  Speeds of 200 megabits per second

  6.4 Wireless Communications Media

	Electromagnetic spectrum of radiation is the basis of all telecommunications signals, wired and wireless
	Radio-frequency (RF) spectrum is the part of the electromagnetic spectrum that carries most communications signals

           

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