Computer Networking

Computer Network
A computer network is a group of computers that are connected to each other for the purpose of communication. Computer networking is the engineering discipline concerned with communication between computer systems or devices. Networking, routers, routing protocols, and networking over the public Internet have their specifications defined in documents called RFCs (Request for Comment).
A computer network allows sharing of resources and information among devices connected to the network. The Advanced Research Projects Agency (ARPA) funded the design of the “Advanced Research Projects Agency Network” (ARPANET) for the United States Department of Defense. It was the first operational computer network in the world. Development of the network began in 1969, based on designs developed during the 1960s. For a history see ARPANET, the first network. Computer networking is sometimes considered a sub-discipline of telecommunications, computer science, information technology and/or computer engineering. Computer networks rely heavily upon the theoretical and practical application of these scientific and engineering disciplines.

Types of Computer Networks
The most common types of Computer Networks are:
• LAN
• MAN
• WAN


Local Area Network is usually a small network constrained to a small geographic area. An example of a LAN would be a computer network within a building.

LAN (Local Area Network)

LAN Design
Ethernet
When we talk about a LAN, Ethernet is the most popular physical layer LAN technology today. Its standard is

Local Area Network

defined by the Institute for Electrical and Electronic Engineers as IEEE Standard 802.3, but was originally created by Digital Intel Xerox (DIX). According to IEEE, information for configuring an Ethernet as well as specifying how elements in an Ethernet network interact with one another is clearly defined in 802.3.

For half-duplex Ethernet 10BaseT topologies, data transmissions occur in one direction at a time, leading to frequent collisions and data retransmission. In contrast, full-duplex devices use separate circuits for transmitting and receiving data and as a result, collisions are largely avoided. A collision is when two nodes are trying to send data at the same time. On an Ethernet network, the node will stop sending when it detects a collision, and will wait for a random amount of time before attempting to resend, known as a jam signal. Also, with full-duplex transmissions the available bandwidth is effectively doubled, as we are using both directions simultaneously. You MUST remember: to enjoy full-duplex transmission, we need a switch port, not a hub, and NICs that are capable of handling full duplex. Ethernets media access control method is called Carrier sense multiple access/ collision dectect (CSMA/CD). Because of Ethernets collision habits it is also known as the best effort delivery system. Ethernet cannot carry data over 1518 bytes, anything over that is broken down into. Travel size packets.
Fast Ethernet
For networks that need higher transmission speeds, there is the Fast Ethernet standard called IEEE 802.3u that raises the Ethernet speed limit to 100 Mbps! Of course, we need new cabling to support this high speed. In 10BaseT network we use Cat3 cable, but in 100BaseT network we need Cat 5 cables. The three types of Fast Ethernet standards are 100BASE-TX for use with level 5 UTP cable, 100BASE-FX for use with fiber-optic cable, and 100BASE-T4 which utilizes an extra two wires for use with level 3 UTP cable.
Gigabit Ethernet
Gigabit Ethernet is an emerging technology that will provide transmission speeds of 1000mbps. It is defined by the IEEE standard The 1000BASE-X (IEEE 802.3z). Just like all other 802.3 transmission types, it uses Ethernet frame format, full-duplex and media access control technology.

Token Ring
Token Ring is an older standard that isn’t very widely used anymore as most have migrated to some form of Ethernet or other advanced technology. Ring topologies can have transmission rates of either 4 or 16mbps. Token passing is the access method used by token ring networks, whereby, a 3bit packet called a token is passed around the network. A computer that wishes to transmit must wait until it can take control of the token, allowing only one computer to transmit at a time. This method of communication aims to prevent collisions. Token Ring networks use multistation access units (MSAUs) instead of hubs on an Ethernet network.

MAN (Metropolitan Area Network)

A metropolitan area network (MAN) is a network that interconnects users with computer resources in a geographic area or region larger than that covered by even a large local area network (LAN) but smaller than the area covered by a wide area network (WAN). It might cover a

Metropolitan Area Network

group of nearby corporate offices or a city and might be either private or public. A MAN can support both data and voice, and might even be related to the local cable television network. A MAN just has one or two cables and does not contain switching elements, which shunt packets over one of several potential output lines. Not having to switch simplifies the design.
In simple Language we can define MAN as
A metropolitan area network (MAN) is a network that connects two or more local area networks or campus area networks together but does not extend beyond the boundaries of the immediate town/city. Routers, switches and hubs are connected to create a metropolitan area network. Such networks are being implemented by innovative techniques, such as running optical fibre through subway tunnels. A popular example of a MAN is SMDS. The term is applied to the interconnection of networks in a city into a single larger network (which may then also offer efficient connection to a wide area network). It is also used to mean the interconnection of several local area networks by bridging them with backbone lines. The latter usage is also sometimes referred to as a campus network.
The main reason for even distinguishing MANs as a special category is that a standard has been adopted for them, and this standard is now being implemented. It is called DQDB (Distributed Queue Dual Bus) for people who prefer numbers to letters, 802.6. DQDB consists of two unidirection buses (cables) to which all the computers are connected. Each bus has a head-end, a device that initiates transmission activity. Traffic that is destined for a computer to the “right” of the sender uses the “upper” bus. Traffic to the “left” uses the “lower” one.
Examples of metropolitan area networks of various sizes can be found in the metropolitan areas of London, England; Lodz, Poland; and Geneva, Switzerland. Large universities also sometimes use the term to describe their networks. A recent trend is the installation of wireless MANs.

WAN (Wide Area Network)

The term Wide Area Network (WAN) usually refers to a network which covers a large geographical area, and use communications circuits to

Wide Area Network

connect the intermediate nodes. WANs often connect multiple smaller networks, such as local area networks (LANs) or metro area networks (MANs). The world’s most popular WAN is the Internet. Some segments of the Internet, like VPN-based extranets, are also WANs in themselves. Finally, many WANs are corporate or research networks that utilize leased lines.
Numerous WANs have been constructed, including public packet networks, large corporate networks, military networks, banking networks, stock brokerage networks, and airline reservation networks. Some WANs are very extensive, spanning the globe, but most do not provide true global coverage. Organisations supporting WANs using the Internet Protocol are known as Network Service Providers (NSPs). These form the core of the Internet.

WAN Protocols
In general, there are three broad types of WAN access technology. With Leased Lines, we have point-to-point dedicated connection that uses pre-established WAN path provided by the ISP. With Circuit Switching such as ISDN, a dedicated circuit path exist only for the duration of the call. Compare to traditional phone service, ISDN is more reliable and is faster. With Packet Switching, all network devices share a single point-to-point link to transport packets across the carrier network – this is known as virtual circuits.

When we talk about Customer premises equipment(CPE), we are referring to devices physically located at the subscriber?s location. Demarcation is the place where the CPE ends and the local loop begins. A Central Office(CO) has switching facility that provides point of presence for its service. Data Terminal Equipment(DTE) are devices where the switching application resides, and Date Circuit-terminating Equipment(DCE) are devices that convert user data from the DTE into the appropriate WAN protocol. A router is a DTE, while a DSU/CSU device or modem are often being referred to as DCEs.

• Peer to Peer - A peer to peer network is one in which lacks a dedicated server and every computer acts as both a client and a server. This is a good networking solution when there are 10 or less users that are in close proximity to each other. A peer to peer network can be a security nightmare, because the people setting permissions for shared resources will be users rather than administrators and the right people may not have access to the right resources. More importantly the wrong people may have access to the wrong resources, thus, this is only recommended in situations where security is not an issue.

Some more types of Networks

• Client/Server - This type of network is designed to support a large number of users and uses dedicated server/s to accomplish this. Clients log in to the server/s in order to run applications or obtain files. Security and permissions can be managed by 1 or more administrators which cuts down on network users medling with things that they shouldn’t be. This type of network also allows for convenient backup services, reduces network traffic and provides a host of other services that comes with the network operating system(NOS).
• Centralized - This is also a client/server based model that is most often seen in UNIX environments, but the clients are “dumb terminals”. This means that the client may not have a floppy drive, hard disk or CDROM and all applications and processing occur on the server/s. As you can imagine, this requires fast and expensive server/s. Security is very high on this type of network.

Peer to Peer Network

Client/Server Network

Electrical machine

Losses in a D.C. Motor

Saturday, September 19th, 2009

The losses occurring in a d.c. motor are the same as in a d.c. generator  (i) copper losses (ii) Iron losses or magnetic losses (iii) mechanical losses As in a generator, these losses cause (a) an increase of machine temperature and (b) reduction in the efficiency of the d.c. motor. The following points may be noted: (i) Apart from armature Cu loss, field Cu loss and brush contact loss, Cu losses also occur in interpoles (commutating poles) and compensating windings. Since these windings carry armature current (Ia), Loss in interpole winding = Ia 2× Resistance of interpole winding Loss in compensating winding = Ia 2× Resistance of compensating winding (ii) Since d.c. machines (generators or motors) are generally operated at constant flux density and constant speed, the iron losses read more
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Commutation in D.C. Motors

Friday, September 18th, 2009

Since the armature of a motor is the same as that of a generator, the current from the supply line must divide and pass through the paths of the armature windings. In order to produce unidirectional force (or torque) on the armature conductors of a motor, the conductors under any pole must carry the current in the same direction at all times. This is illustrated in Fig. (4.10). In this case, the current flows away from the observer in the conductors under the N-pole and towards the observer in the conductors under the S-pole. Therefore, when a conductor moves from the influence of N-pole to that of S-pole, the direction of current in the conductor must be reversed. This is termed as commutation. The function of the commutator and the brush gear in a d.c. motor is to cause the reversal read more
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Armature Reaction in D.C. Motors

Wednesday, September 16th, 2009

As in a d.c. generator, armature reaction also occurs in a d.c. motor. This is expected because when current flows through the armature conductors of a d.c. motor, it produces flux (armature flux) which lets on the flux produced by the main poles. For a motor with the same polarity and direction of rotation as is for generator, the direction of armature reaction field is reversed. (i) In a generator, the armature current flows in the direction of the induced e.m.f. (i.e. generated e.m.f. Eg) whereas in a motor, the armature current flows against the induced e.m.f. (i.e. back e.m.f. Eg). Therefore, it should be expected that for the same direction of rotation and field polarity, the armature flux of the motor will be in the opposite direction to that of the generator. Hence instead of read more
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Torque and Speed of a D.C. Motor

Wednesday, September 16th, 2009

For any motor, the torque and speed are very important factors. When the torque increases, the speed of a motor increases and vice-versa. We have seen that for a d.c. motor; N = K (V- IaRa)/ Ф = K Eb/ Ф…………………………………………….(i) Ta α ФIa…………………………………………………………………………(ii) If the flux decreases, from Eq.(i), the motor speed increases but from Eq.(ii) the motor torque decreases. This is not possible because the increase in motor speed must be the result of increased torque. Indeed, it is so in this case. When the flux decreases read more
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Speed of a D.C. Motor

Monday, September 14th, 2009

Eb = V-IaRa But Eb=PФZN/60A PФZN/60A  = V- IaRa Or  N = (V- IaRa)/ Ф ×  60A/ PZ Or N = K (V- IaRa)/ Ф But         V- IaRa = Ea Therefore N= K Eb/ Ф Or N α Eb/ Ф Therefore, in a d.c. motor, speed is directly proportional to back e.m.f. Eb and inversely proportional to flux per pole Ф. Speed Relations If a d.c. motor has initial values of speed, flux per pole and back e.m.f. as N1 ,Ф1 and Eb1 respectively and the corresponding final values are N2 ,Ф2 and Eb2 then, N1 α Eb1/ Ф1 and N2 α Eb2/ Ф2 Therefore N2/ N1 = (Eb2/ Eb1) ×( Ф1 / Ф2) (i) For a shunt motor, flux practically remains constant so that Ф1 = Ф2. therefore  N2/ N1 = Eb2/ Eb1 (ii) For a series motor, Ф α Ia prior to saturation. therefore N2/ N1 = (Eb2/ Eb1) × (Ia1/Ia2) where Ia1 = initial read more
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types of computer users

I remember my first computer teacher relaying this very important piece of wisdom. “There are two types of computer users – those who have lost their data and those about to.”
Disaster can strike at any minute – a web server getting fried or even a power surge in your own system. In one second, much or even all you may have worked on may disappear; unless of course you regularly back up. Backing up is something we all know we should be doing, but more often than not we don’t do it enough – or at all.
I was speaking to someone tonight who is paying the price for not backing up. A few hours downtime already while he struggles to breathe life into what appears to be a dodgy hard drive. If the drive is cactus, the only option left is data recovery. This will mean a couple more days downtime and the cost for the recovery service will be anywhere from $500 – $2000. That sounds pretty steep but as one technician said to me a while ago, “Well, how much is your data worth?”
In regards to your web sites, don’t trust your web host to maintain backups. It doesn’t matter what type of great backup scheme they have in place or what whizz-bang equipment they use to help ensure reliability and recovery in a disaster; stuff happens – and their terms of service will usually have that disclaimer as well.
Backing up your online and offline data can be a pain; it’s certainly one of the more mundane tasks of running an online business; but ask yourself – where would you be if your hard drive died this minute and you had no backup

working of thyrister

types of moniters

Different types of computer monitor
As the technology has improved and the prices have come down, LCD (Liquid Crystal Display) monitors have rapidly been replacing CRT (Cathode Ray Tube) monitors on desktops around the world. ComputerWorld first reported that LCD sales would surpass CRT sales for the first time in 2003, a lead that it didn't hold for good.
But according to DisplaySearch, a flat panel display market research and consulting company, the sales of LCD monitors regained the lead over CRT sales in the third quarter of 2004, a lead that it should eventually hold for good.
 
The question is why choose LCD over CRT?

There are several pros and cons to consider, and the few items listed below will be considered in this Geek Tip.
• Price
• Size
• Image Quality
• Energy Consumption
• Personal Comfort
• Response Time

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Price
The price of LCD monitors is much lower than a few years (or even months) ago, but still far exceeds the price of a comparable CRT monitor. For example, I spent about $600 (US) on a Viewsonic VA-720 17" LCD monitor in early 2003, and see that the same model now sells for less than $300. A significant price drop, but in comparison a 17" Viewsonic CRT monitor can currently be purchased for less than $100. The ratio of prices may have narrowed from about 5:1 to 3:1, but the aging technology behind CRTs still allows it to hold the lead.
You can't even compare prices of CRTs to LCDs in CompGeeks monitor section as they are right in step with the sales information provided above, and now only carry LCD monitors. Prices vary, even among LCD monitors of the same screen size, so there has to be something more to it than price.

Size
One reason that LCDs have gained in popularity is because of their small foot print. The overall size and weight of CRT monitors far exceeds that of LCD monitors. CRTs share the same image processing technology with tube televisions, and therefore share the same bulky style of housing.
Desktop real estate is precious, and an LCD will require only a small fraction of the depth that a CRT would require. And if there isn't even enough room on your desk for a slim LCD monitor, the low weight makes them perfectly adaptable to be hung on the wall, or off of a radial arm mount, such as this one from

Image Quality
Image quality is generally considered to be better on an LCD, as each pixel is generated by a specific set of transistors in the screen, which produces a crisp image. But some features that fall under the general heading of image quality might not favor an LCD, including viewing angle, brightness, and contrast.
Early LCD monitors had a fairly narrow viewing angle that made clearly seeing the screen from anywhere but directly in front of it difficult. This has improved greatly, but still doesn't quite rival the viewing angle of CRTs which provide the same picture quality regardless of the angle. A monitor with a maximum vertical viewing angle of 120 degrees should not be hard to find at this point, with many monitors now being able to provide an even greater angle.
Brightness is an area that LCD monitors may have the edge over CRTs, but it varies widely from unit to unit. The standard measure for brightness is referred to as "nits", which have units of cd/m2 (candelas per square meter), where a higher number is better.
Contrast is similar to brightness in the fact that it varies widely from unit to unit, and is a specification where a higher number is desired. The contrast is represented as a ratio, where higher numbers imply that bright colors can be displayed next to dark colors without them appearing washed out. Monitors with lower numbers in the ratio may also result in dark shades being displayed as just black, and any detail in these areas may be lost. As a point of reference, CRT monitors may have contrast ratios around 700:1.
Energy Consumption
LCD monitors definitely hold the edge over CRT monitors when it comes to being energy efficient. The huge tube in a CRT monitor is the source of most of its energy consumption, and a comparably sized LCD may use just a fraction of the electricity.
For example a 19" LCD monitor consumes 48 Watts during normal operation, which is less than your typical light bulb. In contrast, a 19" CRT may draw up to 160 Watts.
Therefore the fraction of electricity used in this case is 3/10, and could translate to noticeable savings on your electricity bill.
Personal Health and Comfort
The main benefit that LCDs have when it comes to comfort is the reduced strain on your eyes. The reduced glare on the screen's surface, and the elimination of a typical CRT's "refresh", can prevent your eyes from getting tired from extended use.
A CRT monitor redraws the image on the entire screen as it refreshes, whereas an LCD monitor only changes the necessary pixels during a refresh.
There may also be the unquantifiable effect of reduced electromagnetic emissions on LCD monitors. The exact impact of electromagnetic emissions may not be fully understood, but in general less is considered to better, as addressed in this article.
And, your back may also appreciate an LCD when it comes time to move, as the example above shows a 19" LCD monitor weighs about ¼ as much as its CRT counterpart.
check out different types of computer monitor

Response Time
<>The transistors that create the image on a TFT LCD can be a bottleneck to its performance, especially in fast paced 3D games where speed is critical. Related to the different approach taken with screen refreshes, the amount of time it takes the pixels to change in order to display the new image is referred to as the response time.
If the response time is too slow, one may experience blurred images or ghost effects where the previous image is still slightly visible with the new image.
LCD monitor response times have greatly improved over the past few years, and many LCDs are now fast enough to consider for serious 3D gaming use, but specifications still vary from unit to unit.
A few years ago a typical response time on an LCD monitor may have been anywhere from 30 to 50 milliseconds, and today these numbers can get down into the single digits, with anything 25 milliseconds or less being quite common (lower is definitely better)
Final Words
In addition to some of the positives mentioned, many LCD monitors now incorporate other features to make them more practical and even fun. LCD monitors can now be found with integrated USB hubs, stereo speakers, and TV tuners
LCD monitors will continue to replace CRTs as they become less expensive and the many benefits are realized by consumers, but CRTs won't disappear all together as many situations require the performance that LCDs currently can't provide.

types of the computer

Analog Computers: These are almost extinct today. These are different from a digital computer because an analog computer can perform several mathematical operations simultaneously. It uses continuous variables for mathematical operations and utilizes mechanical or electrical energy.

Hybrid Computers: These computers are a combination of both digital and analog computers. In this type of computers, the digital segments perform process control by conversion of analog signals to digital ones.

Following are some of the other important types of computers.

Mainframe Computers: Large organizations use mainframes for highly critical applications such as bulk data processing and ERP. Most of the mainframe computers have the capacities to host multiple operating systems and operate as a number of virtual machines and can thus substitute for several small servers.

Microcomputers: A computer with a microprocessor and its central processing unit is known as a microcomputer. They do not occupy space as much as mainframes. When supplemented with a keyboard and a mouse, microcomputers can be called as personal computers. A monitor, a keyboard and other similar input output devices, computer memory in the form of RAM and a power supply unit come packaged in a microcomputer. These computers can fit on desks or tables and serve as the best choices for single-user tasks.

Personal computers come in a variety of forms such as desktops, laptops and personal digital assistants. Let us look at each of these types of computers.

Desktops: A desktop is intended to be used on a single location. The spare parts of a desktop computer are readily available at relative lower costs. Power consumption is not as critical as that in laptops. Desktops are widely popular for daily use in workplaces and households.

Laptops: Similar in operation to desktops, laptop computers are miniaturized and optimized for mobile use. Laptops run on a single battery or an external adapter that charges the computer batteries. They are enabled with an inbuilt keyboard, touch pad acting as a mouse and a liquid crystal display. Its portability and capacity to operate on battery power have served as a boon for mobile users.

Personal Digital Assistants (PDAs): It is a handheld computer and popularly known as a palmtop. It has a touch screen and a memory card for storage of data. PDAs can also be effectively used as portable audio players, web browsers and smart phones. Most of them can access the Internet by means of Bluetooth or Wi-Fi communication.

Minicomputers: In terms of size and processing capacity, minicomputers lie in between mainframes and microcomputers. Minicomputers are also called mid-range systems or workstations. The term began to be popularly used in the 1960s to refer to relatively smaller third generation computers. They took up the space that would be needed for a refrigerator or two and used transistor and core memory technologies. The 12-bit PDP-8 minicomputer of the Digital Equipment Corporation was the first successful minicomputer.

Supercomputers: The highly calculation-intensive tasks can be effectively performed by means of supercomputers. Quantum physics, mechanics, weather forecasting, molecular theory are best studied by means of supercomputers. Their ability of parallel processing and their well-designed memory hierarchy give the supercomputers, large transaction processing powers.
Wearable Computers: A record-setting step in the evolution of computers was the creation of wearable computers. These computers can be worn on the body and are often used in the study of behavior modeling and human health. Military and health professionals have incorporated wearable computers into their daily routine, as a part of such studies. When the users’ hands and sensory organs are engaged in other activities, wearable computers are of great help in tracking human actions. Wearable computers are consistently in operation as they do not have to be turned on and off and are constantly interacting with the user.

Hydroelectric Power generation

Hydroelectric power: How it works

So just how do we get electricity from water? Actually, hydroelectric and coal-fired power plants produce electricity in a similar way. In both cases a power source is used to turn a propeller-like piece called a turbine, which then turns a metal shaft in an electric generator, which is the motor that produces electricity. A coal-fired power plant uses steam to turn the turbine blades; whereas a hydroelectric plant uses falling water to turn the turbine. The results are the same.
Take a look at this diagram (courtesy of the Tennessee Valley Authority) of a hydroelectric power plant to see the details:

The theory is to build a dam on a large river that has a large drop in elevation (there are not many hydroelectric plants in Kansas or Florida). The dam stores lots of water behind it in the reservoir. Near the bottom of the dam wall there is the water intake. Gravity causes it to fall through the penstock inside the dam. At the end of the penstock there is a turbine propeller, which is turned by the moving water. The shaft from the turbine goes up into the generator, which produces the power. Power lines are connected to the generator that carry electricity to your home and mine. The water continues past the propeller through the tailrace into the river past the dam. By the way, it is not a good idea to be playing in the water right below a dam when water is released!

This diagram of a hydroelectric generator is courtesy of U.S. Army Corps of Engineers.
As to how this generator works, the Corps of Engineers explains it this way:
"A hydraulic turbine converts the energy of flowing water into mechanical energy. A hydroelectric generator converts this mechanical energy into electricity. The operation of a generator is based on the principles discovered by Faraday. He found that when a magnet is moved past a conductor, it causes electricity to flow. In a large generator, electromagnets are made by circulating direct current through loops of wire wound around stacks of magnetic steel laminations. These are called field poles, and are mounted on the perimeter of the rotor. The rotor is attached to the turbine shaft, and rotates at a fixed speed. When the rotor turns, it causes the field poles (the electromagnets) to move past the conductors mounted in the stator. This, in turn, causes electricity to flow and a voltage to develop at the generator output terminals."

Pumped storage: Reusing water for peak electricity demand

Demand for electricity is not "flat" and constant. Demand goes up and down during the day, and overnight there is less need for electricity in homes, businesses, and other facilities. For example, here in Atlanta, Georgia at 5:00 PM on a hot August weekend day, you can bet there is a huge demand for electricity to run millions of air conditioners! But, 12 hours later at 5:00 AM .... not so much. Hydroelectric plants are more efficient at providing for peak power demands during short periods than are fossil-fuel and nuclear power plants, and one way of doing that is by using "pumped storage", which reuses the same water more than once.
Pumped storage is a method of keeping water in reserve for peak period power demands by pumping water that has already flowed through the turbines back up a storage pool above the powerplant at a time when customer demand for energy is low, such as during the middle of the night. The water is then allowed to flow back through the turbine-generators at times when demand is high and a heavy load is placed on the system.
The reservoir acts much like a battery, storing power in the form of water when demands are low and producing maximum power during daily and seasonal peak periods. An advantage of pumped storage is that hydroelectric generating units are able to start up quickly and make rapid adjustments in output. They operate efficiently when used for one hour or several hours. Because pumped storage reservoirs are relatively small, construction costs are generally low compared with conventional hydropower facilities.