Communication devices and media

1. Communication devices and media By the end of this unit students should be able to: Distinguish between communication devices and communication media.

2. Efficient data communication must be: • Accurate • Timely • Cost effective Data communication allows users to transmit and receive data and information. This information is used to identify and solve problems and to make informed choices.

3. Terms Media – a collective name for materials (tape, paper, disk, etc) used to transmit data. Communication media – the material used to transmit data and includes cables, fibre-optic cables, radio waves and satellite links.

4. Communication Devices – the parts of the computer that facilitate communication by using the above media. Modem – the most commonly used computer communication device. Other communication devices include local area network cards (LAN), network hubs, and wireless cards. Telecommunications is the sending and receiving of data by means of cable or wireless devices and media.

5. Interface – a device that allows otherwise incompatible items to be connected. It is sometimes an external device with its own power supply that is plugged into the computer. Eg. Modems, network cards and wireless cards.

6. MODEMS They use the already-existing telephone network to facilitate computer communication. Telephone lines were designed to communicate the human voice, so they transmit data using sound waves or what is called an analogue data form. Before computer signals can be sent via telephone lines they must be converted from digital to an analogue format. This process is called MODULATION. For more information click here.

7. When the signal reaches the receiving computer it is converted back from analogue to digital form. A process called DEMODULATION. Modems are usually used to connect a computer to the Internet and to send and receive faxes via the computer. Modems constantly check whether the information was received properly or not. The information is broken down and sent in groups called PACKETS. Each packet is checked for errors and is re-sent if there is an error. Click on the link below to see how the modem works. http://tdi.uregina.ca/~flash/cs100/modem-general.html

8. Digital phone lines Advantages: • There is no need to use a modem when using a digital telephone line such as an ISDN (integrated services digital network) line to connect a computer (or a LAN) to the Internet because the signal is already a digital one which a computer can process. • A larger volume of data is transmitted more quickly. • ISDN line is used for voice, video, and data transmission. Users can receiving telephone calls, transmitting faxes and surfing the internet at the same time using the same line.

9. Facsimile or fax machine This scans a document with a beam of light and converts the image to a coded series of signals that can be transmitted along telephone lines. The receiving machine interprets the coded signals and prints out a copy of the document. It is an exact representation of the text, photographs and graphic images in the original document. Some computers are equipped with fax modem so that you can transmit a typed document directly from the sender’s computer to the recipient’s fax machine or computer.

10. networks • LOCAL AREA NETWORKS (LAN) • WIDE AREA NETWORKS (WAN) • WIRELESS NETWORKS (WN) • CELLULAR NETWORKS (CN)

11. LOCAL AREA NETWORK (LAN) • A network is a set of computers and peripherals that are linked together so that they can communicate with each other and share resources. • If the computers are joined by cables in a small area such as a room or building, it is called a LOCAL AREA NETWORK (LAN). • All the computers are less than a kilometer away from each other. • In a lab several computers may be linked in order to share a printer. One computer may be the server, which controls and provides services for the LAN. The printer is connected to the server but can be accessed by all the other computers.

12. Types of lans

13. A network card is a small circuit inside the computer. It allows the computer to communicate with others that are on the network. • A cable is plugged into the card and each computer then has access to any shared programs, devices and files on other computers or a main server computer. • Each network card has a unique address to make sure the correct data is delivered to it. • Some use cables or wireless. • The cables used may be twisted pair, coaxial or fibre- optics.

14. Twisted pair – cheapest but relatively slow, used where a rather small volume of data is transmitted or where speed is not important. • Coaxial cable – faster, more expensive, used where large amounts of data are being communicated eg. Voice and video transmission. • Fibre-optics – fastest and most expensive. Transmit data as digital pulses of light. They are not affected by electromagnetic interference and transmit data over large distances. Transmit large volumes of data quickly and accurately.

15. Wide area networks (WAN) • When computers which are located in different locations are linked using telephone lines or satellite, a WAN is formed. • They cover large areas such as a city, a nation or the entire world.

16. Wireless networks • Both LANS and WANs may be created using wireless technology. • As the name suggests there are no wires. • It uses infrared or microwave radio signals to transmit and receive data. Wireless networks are use where cables are inconvenient, e.g if the network is created among buildings separated by streets. • They are usually more expensive than cable-linked networks. • More readily used now as costs declines.

17. Cellular networks • Mobile telephones are now regularly used to communicate while travelling or away from home. • Some cellular phones offer facilities for browsing the internet and sending and receiving faxes and electronic mail.

18. How the cellular network works • It is created when transmitters are place in a checkerboard pattern throughout a service area. • The geographical area that is served by a transmitter is called a CELL. Unique frequencies are assigned to each cell. • Each cell phone transmits and receives a signal. When you place a call, the cell in which the caller is located detects the signal from your cell phone. • The cell then assigns a specific frequency to that call and the call is transmitted to the receiving unit.

19. A computer system monitors the strength of the signal from the cell phone. • As you move away from the transmitter, the signal weakens. • If the signal falls to a preset level, the computer recognizes that you have left the cell and scans the surrounding cells to determine the new area into which you have moved. • The transmitter in the new cell takes over the call and a new frequency is assigned to your phone. • This happens so quickly that you do not detect the transfer. If you move too far away from any network transmitter, the connection breaks.

20. The internet • The world’s largest network. It is a worldwide collection of networks that links together millions of computers by means of modems, telephone lines and other communication devices and media. • It is used by more that 100 million people. • It can be accessed either through a file server or through an Internet service provider (ISP) or online service. • A file server is a computer that is permanently connected to the Internet.

21. File servers are used by large organisations. • An ISP is an organisation that supplies connections to the Internet for a monthly fee. • An online service provides access to the Internet but also provides other services such as financial information, entertainment, hardware and software guides, news and weather, and direct messaging. • Fees for online service are usually higher than for an ISP. • The internet enables users to send and receive mail electronically. • Electronic mail (email) is so quick that traditional mail is now called ‘snail mail’.

22. Direct messaging enables users to meet and communicate with persons all around the world using either typed or voice messages. • The internet enables employees with PCs to connect to the organisation’s mainframe or supercomputer without travelling to these locations. It has also made Ecommerce possible. • Discussions boards allow participants to type messages about a given topic. Responses are usually linked to each other to allow the reader to easily follow the discussion.

23. Intranet and extranet • Intranet – private network of computers with an organisation that provides functions similar to the public internet. • These functions include: email and newsgroups • They do not allow access via the internet and has security features to prevent unauthorised access. • Extranet – an internal company network that has a connection to the public internet and allows users to gain access via the internet. It provides access to people in the organisation who are working away from the office. • Web page – a file that is saved on the computer on the internet. • Web site – a collection of web pages.

24. teleconferencing • This makes use of telecommunications technology to allow participants in two or more locations to speak with each other. • It is a group conference call in which all participants are able to speak with each other. • Online meetings allow computer users to access the Internet to create a chat room in which they can exchange typed messages instantaneously. • Video conferencing – allows users in distant locations to see each other.

25. QUESTION TIME • See how much you can remember: Click here to see the questions.

26. bibliography • Electronic Document Preparation and Management for CSEC by Ann Margaret Jacob and Agatha Augustine • http://www.webopedia.com/TERM/m/modem.html