Computers and Computer Networks

The computer is a tool which caused a revolution among scientific, technological and commercial fields all over the world in the second half of the 20th century.  The computer age started in the early 50’swith calculators, which in the beginning were very big and only executed a few arithmetic operations. Nowadays you have personal computers of a small size, which make long and complex operations in only few seconds,

Types of computers

Computers can be devided into five main types. These are: mainfraimes, desktop PCs, notebooks, tablet PCs and personal digital assistants (PDA).

Mainfraimes are the largest and most powerful computers. The basic configuration of a mainfraime consists of a central system which processes immense amount of data very quickly. This central system provides information and computing facilities for hundreds of terminals connected together in a network. mainframes are used by large-scale computing purposes in banks, universities and large companies.

Personal computer - PC carry out their processing on a single microchip. They are usually classified by size and portability. A desktop PC is designed to be place on a dest. it is used as a home computer or as a workstation for group work. typical examples are the IBM or the Apple Macintosh.

A notebook is a portable computer that has a flat LCD screen. and uses batteries for mobile use. The latest models can run as fast as similar dosktop computers and have similar configuration. They are ideal for business or for people who travel a lot.

A tablet PC is a type of notebook computer that has an LCD screen on which you can write. You can use the Internet on it, watch films or play games. It can be also used for work instead of a notebook computer.

A PDA is a lightweight, handheld computer. The term refers to a wide variety of handheld devices, including palmtops, pocket PCs. PDAs can be used as a mobile phone, personal organizers or media player. They also let you access the Internet via Wi-Fi notworks. Some may include a GPS navigation.

Computer system

A PC system consists of two parts - software and hardware. Software is the programs that enable a computer to perform a specific task. This includes the operating system and application software such as a graphic package and web browser.

Hardware is any electronic and mechanical part. The basic structure of a computer system is made up off three main hardware sections : the central processing unit (CPU), the main memory and peripherals.

The CPU is a processor chip which executes program instructions and coordinates the activities of all the other components. I tis a chip containing millions of tiny transistors. It serves as the brain of the computer and controls how fast the computer processes data or information.  In order to improve the computer′s performance, the user can add expansion cards for video, sounds and networking = expansion slots.

The main memory holds the instructions and data which are currently being processed by the CPU. The internal memory is made of ROM and RAM chips. RAM = random access memory, is volatile, so it looses the stored data when electricity or power is turned off. ROM = read only memory is non-volatile. Thy capacity of computer memory is stated in bytes. RAM capacity is usually in megabytes (MB) and the size of hard disk is in gigabytes (GB).

The peripherals are the physical units attached to the computer. They include input, output and storage devices. Input devices, e.g. the keyboard and the mouse, enable us to present information to the computer. Output devices allow us to extract the results from the computer. For instance, we can see the output on the monitor or in printed form. Storage devices are used to store information permanently,e.g. hard disk, DVDs or flash drives. Such can store large amounts of information.

The power supply provides a supply of elektricity throughout the computer. With notebooks, some of that power charges the notebook’s battery. When unplugged, a notebook has to rely on the chargé inside the battery.

Operation system  is a set of computer programs that manage hardware and software resources of a computer. An OS performs basic tasks such as controlling and allocationg memory, prioritizing systém requests, controlling input and output devices, facilitating networking and managing file systems. An OS forms a platfors for other application software.

Input devices

A keyboard is used to enter information into the computer. As well as having normal typewriter keys for characters and numeric keypad, it may also have function keys and editing keys for special purposes.

A mouse is a device for controlling the cursor and selecting items on the screen. The ball underneath is rolled in any direction across the surface of a desk to move the cursor on the screen. It may have an optical sensor instead of a ball. It usually features two buttons and a wheel. By clicking a button, the user can activate icons or select items and text. A wireless version works without cables.

Other input devices are: a digital camera, scanner, a trackball, a graphic tablet, a joystick, a wheel, a gamepad, lightpens, touchscreens, bar code readers.

Output devices

Monitors

Most people nowadays use modern flat LCD (liquid crystal display). The screen is measured diagonally in inches (from 17´´ to 24´´ or larger) The quality of the display depends on the maximum resolution, which is the number of pixels which make up horizontal and vertical picture. Other important factors are: Contrast ratio (kontrastní poměr), brightness, viewing angle, aperture grill pitch (rozteč obrazových bodů), bit depth (bitová hloubka).

Printers

Printing is the final stage in creating a document. There exist many types of printers. They can vary in cost, speed, print quality, noise or printing method.

There are: dot matrix printers, inkjet printers, laser printers, thermal transfer printers, image setters, plotters etc.

Storage devices

A storage drive reads or writes informationato magnetic or optivcal storage media. The drive can be used to store data permanently or to retrieve information from a media disk. Storage drivers can be installed inside the computer case, such as hard drive. For portability some storage drivers can be connected to the computer using a USB port, a FireWire port or an SCSI port. These portable storage drivers are sometimes reffered to as removable drivers and can be used on multiply computers.

Common types of storage drivers are:

A hard drive - magnetic storage device that is installed inside the computer.  I tis a storage device that stores digitally encoded data on rapidly rotating platters with magnetic surfaces. The surface consists of tracks divided into sectors. They spin at a high speed. When you save a file, the operating systém moves the read/write heads towards empty sectors, records the data and writes an entry for the direktory. When you open that file later, the read/write heads look for the required tracks and sectors and transfer the information to the main memory. It is used as a permanent storage for data. In the computer it is usually configured as the C: drive. It contains the operating system and applications. The storage capacity of a hard drive is measured in billions of bytes, or gigabytes (GB). The speed is measured in revolutions per minute (RPM). There are internal and external hard drives, or removable drives, which allow to record the information cartridges and store off-line.  Multiple hard drives can be added to increase the storage capacity.

An optical drive - uses lasers to read data on the optical media. there are three types: compact discs or CDs (CD-ROMs, CD-R, CD-RW - capacity of 700MB), digital versatile discs od DVDs (DVD-ROMs, DVD-R, DVD+R, DVD-RV, DVD+RW - capacity of 4,7GB), blue-ray discs (capacity of 25GB to 100GB), flash drive or USB flash disk  (USB storage device,small enaugh to fit on a key ring, used to store and transport data)

Computer Network

A network is a group of two or more  interconnected computers. The network allows its users to communicate with each other and share data, software applications and hardware resources, such as printers.

Basic hardware components are : network adapters, bridges, hubs, switches and routers.

The software consists of the protocols, i.e. the rules which define the formats by which information may be exchanged between different systems.

The network can have a physical or logical topology. The physical topology describes the layout of computers, the logical topology describes how the information flows through the network. The most common topologies are star, bus, ring and mesh.

                        A star topology has a server computer at the centre and a separate cable connecting the server to each of the other computers in the network. The central server controls the flow of data in the network. If the central server fails, the whole network will fail.

In a ring topology , each computer is connected to its neighrour in a circle. The data flows in one direction round the ring,. If a cable breaks or one of the computer fails, the whole network will be affected.

A bus topology  has all the computers connected to a common cable. The data travels in both directions along the cable. If a computer fails, o rif we remove one from the network, it won’t affect the other computers. But when two devices send messages at the same time, there is a problém and the message needs to be sent again.

 A mesh topology, where computers are connected with each other, is used in wireless networks. A mesh network is a local area network (LAN), wireless local area network (WLAN) or virtual LAN (VLAN) that employs one of two decentralized connection arrangements: full mesh topology or partial mesh topology. In a full mesh topology, each network node (workstation or other device) is connected directly to each of the others. In a partial mesh topology, some nodes are connected to all the others, but others are only connected to those nodes with which they exchange the most data. 

Types of networks

PAN = a  personal area network is used for communication among computer devices (including telephones, PDAs, printers, scanners) with one person’s reach and the Internet. The reach of a PAN is typically a few meters.

LAN = local area network = a network that is connected over a small area (e.g. one building) There is usually a file server that acts as a controller which regulates communication among computers and peripherals in the network. Other computers that use the services are called clients.

MAN = a metropolitan area network  usually covers a city area, typically uses wireless infrastructure or optical fiber connections.

WAN = a wide area network is a network connected over a vest area (regions/counties). Computers in WAN are linked over long distances by DSL lines, fibre-optic cables or satellites. The best known and largest example of WAN is the Internet.