Data communications are the transfer of data from one device to another via some form of transmission medium. A data communications system must transmit data to the correct destination in an accurate and timely manner. The five components that make up a communications system are the message, transmitter, receiver, medium, and protocol. Text, numbers, images, audio, and video are different forms of information. The transmitter injects a signal into the channel that delivers it to the receiver. The receiver must recover the information contained in the receiver signal despite the limitations introduced by the channel.
The channel can be a physical one, like a copper cable and an optical fiber, or simply air or even vacuum that transmits electromagnetic waves. Any channel is subject to some kind of electromagnetic “noise” and interference.
In order to transmit a digital signal at a reasonable distance it has to be processed bya modulator. The modulator can:
1) Select the frequency at which the signal will be transmitted over the
channel.
2) Allow for different signals to share the same modulation channel, in a
process known as multiplexing.
3) Adapt the signals parameters to suit the requirements of a given channel
(bandwidth, spectral properties, noise robustness, etc.).
4) Provide the flexibility to exchange spectral efficiency for robustness, as
needed.
Of course, at the receiving end, the inverse operation, called demodulation, needs to be performed. So in bidirectional systems a single device will perform both operations and therefore be called a modem.
The word modemis a combination of the words modulation and demodulation which is precisely what a modem does. A modem can also be viewed as a device that takes information, transfers it on to a medium to allow transportation of the information, and at the other end, removes the information from the medium and restores it to its original form. This brings up two distinguishing characteristics of a modem, the type of information it accepts and the media that it operates upon.
The type of medium employed by the modem dictates the type of modulation it will employ. The medium can be a copper cable, an optical fiber or an electromagnetic wave in free space.
Although the modem is a separate building block, it is often embedded in a laptop or in a wireless router.
Multiplexing is the sharing of a single communication channel among different users. The communication channel can be a copper wire, an optical fiber or the space between a transmitting and a receiving antenna. Different users can be distinguished by means of different frequencies, time slots, codes or regions of space.
3.1 In the text above, find English equivalents for the following:
Обеспечить надежность (прочность); информация, содержащаяся в сигнале; отличительная особенность (свойство); встроенный в компьютер блок; временной интервал.
3.2 Answer the questions:
1) What are the five components of a data communications system?
2) What functions do transmitters and receivers have?
3) What does the process known as multiplexing consist in?
4) What is a modem and what is it designed for?
3.3 Translate into Russian:
1) The type of medium employed by the modem dictates the type of modulation it will employ.
2) Any channel is subject to some kind of electromagnetic “noise” and interference.
3) A modem can also be viewed as a device that takes information and transfers it on to a medium.
4) Of course, at the receiving end, the inverse operation, called demodulation, needs to be performed.
Text B. Transmission modes
Communication between two devices can occur in one of three ways: simplex, half-duplex, or full-duplex.
Simplex.
In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive.
Keyboards and traditional monitors are examples of simplex devices. The keyboard
can only introduce input; the monitor can only accept output. The simplex mode
can use the entire capacity of the channel to send data in one direction.
Half-Duplex.
In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa. The half-duplex mode is like a one-lane road with traffic allowed in both directions.
When cars are traveling in one direction, cars going the other way must wait. In a
half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems.
The half-duplex mode is used in cases where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction.
Full-Duplex.
In full-duplex mode (also called duplex), both stations can transmit and receive simultaneously. The full-duplex mode is like a two-way street with traffic flowing in both directions at the same time. In full-duplex mode, signals going in one direction share the capacity of the link with signals going in the other direction. This sharing can occur in two ways: Either the link must contain two physically separate transmission paths, one for sending and the other for receiving; or the capacity of the channel is divided between signals traveling in both directions. One common example of full-duplex communication is the telephone network. When two people are communicating by a telephone line, both can talk and listen at the same time.
The full-duplex mode is used when communication in both directions is required all the time. The capacity of the channel, however, must be divided between the two directions.
3.4 Answer the following questions:
1) What is the difference between half-duplex and full-duplex transmission modes?
2) Can you explain the word “walkie-talkie”?
3.5 Give some examples of simplex, half-duplex, full-duplex communication.
3.6 Write out key terms and expressions, use them to retell the text.
Text C. Networks
Defining a network. A network is the interconnection of a set of devices capable of communication. In this definition, a device can be a host (or an end system as it is sometimes called) such as a large computer, desktop, laptop, workstation, cellular phone, or security system. A device in this definition can also be a connecting device such as a router, which connects the network to other networks, a switch, which connects devices together, a modem (modulator-demodulator), which changes the form of data, and so on. These devices in a network are connected using wired or wireless transmission media such as cable or air.
Networks may be classified by various characteristics, such as the media used to transmit signals, the communications protocols used to organize network traffic, network scale, network topology and organizational scope. The best-known computer network is the Internet.
Communication protocols define the rules and data formats for exchanging information in a computer network. Well-known communications protocols include Ethernet, a hardware and link layer standard that is widely used for local area networks, and the Internet protocol suite (TCP/IP), which defines a set of protocols for communication between multiple networks, for host-to-host data transfer, and for application-specific data transmission formats. Protocols provide the basis for network programming.
Network Criteria. A network must be able to meet a certain number of criteria. The most important of these are performance, reliability, and security.
Performance. Performance can be measured in many ways, including transit time and response time. Transit time is the amount of time required for a message to travel from one device to another. Response time is the elapsed time between an inquiry and a response. The performance of a network depends on a number of factors, including the number of users, the type of transmission medium, the capabilities of the connected hardware, and the efficiency of the software.
Performance is often evaluated by two networking metrics: throughput and delay. We often need more throughput and less delay. However, these two criteria are often contradictory. If we try to send more data to the network, we may increase throughput but we increase the delay because of traffic congestion in the network.
Reliability. In addition to accuracy of delivery, network reliability is measured by the frequency of failure, the time it takes a link to recover from a failure, and the network's robustness in a catastrophe.
Security. Network security issues include protecting data from unauthorized access, protectingdata from damage and development, and implementing policies and procedures for recovery from breaches and data losses.
3.7 Answer the questions.
1. What devices are connected in the network?
2. By what characteristics can networks be classified?
3. Why are protocols needed?
4. What are the most important network criteria?
5. Why are throughput and delay called contradictory criteria?
6. What do you understand by the terms “security” and “reliability”?
3.8 Match the terms in column A and their definitions (descriptions) in column B:
A B
1. Router a) software used to find and look at information on
the web. Each web page has its own unique address
known as a Universal Resource Locator (URL).
2. Browser b) allows two different types of network to
communicate with each other.
3. Bridge c) hardware used to change analogue signals from the
phone line into digital signals needed by the
computer, also used to change the signals from
digital to analogue.
4. Modem d) define the rules and data formats
specifying the individual address.
5. Hub e) sends the signal to all the devices on the network
and only one individual PC will accept the signal.
6. Switch f) Hardware allowing computer to access the Internet
using a phone line.
Text D. Types of networks
There are different types and sizes of networks.
A Local Area Network (LAN) is used to connect computers spread over a relatively small area. LANs are usually placed in the same building. They can be built with two main types of architecture:peer-to-peer,where the two computers have the same capabilities, or client-server,where one computer acts as the server containing the main hard disk and controlling the other workstations ornodes,all the devices linked in the network (e.g. printers, computers, etc.).
Computers in a LAN need to use the same protocol,or standard of communication. Ethernet is one of the most common protocols for LANs.
A router,a device that forwards data packets, is needed to link a LAN to another network, e.g. to the Net.
If your personal computer is connected to a network, it is called a network workstation. If your PC is not connected to a network, it is referred to as a standalone computer.
In order to connect to a network, your computer will need a network adapter. This circuitry and port could be built into the motherboard or it could be on a network interface card (NIC) in one of the computer’s expansion slots.
Typically in a LAN, hardware is connected by a cable but new Wi-Fi, wireless fidelity technologies allow the creation of WLANs,where cables or wires are replaced by radio waves.
A LAN which uses radio waves rather than cables to transmit the data from machine to machine is called a wireless local area network (WLAN).
To build a WLAN you need access points,radio-based receiver-transmitters that are connected to the wired LAN, and wireless adapters installed in your computer to link it to the network.
Hotspotsare WLANs available for public use in places like airports and hotels, but sometimes the service is also available outdoors (e.g. university campuses, squares, etc.).
Awide area network (WAN) is a network which is spread over a large geographical area. WANs have no geographical limit and may connect computers or LANs on opposite sides of the world. They are usually linked through telephone lines, fiber-optic cables or satellites. The main transmission paths within a WAN are high-speed links called backbones.
Wireless WANsuse mobile telephone networks. The largest WAN in existence is the Internet.
The ideas associated with LANs and WLANs of sharing data and making communications easier are still true with WANs. The difference is that because of the greater distances involved it is not possible to link the computers with cables or by radio.
A WAN normally uses communication media provided by a third party. An
example would be a telephone company providing links via telephone cables or via broadband facilities.
Often, several LANs are linked up by a WAN. For example, a firm that has offices in London, Lahore and Colombo would probably use LANs in each of the offices. The company would then use telecommunication media to link the individual LANs together as a WAN.
1) Define the terms LAN, WLAN and WAN.
2) What is the difference between a LAN and a WAN?
3) Is the Internet a LAN or a WAN?
3.9 Now read and memorize these definitions:
A bridge is a hardware and software combination used to connect the same type of networks. Bridges can also partition a large network into two smaller ones and connect two LANs that are nearby each other.
A router is a special computer that directs communicating messages when several networks are connected together. High-speed routers can serve as part of the Internet backbone.
A gateway is an interface that enables dissimilar networks to communicate, such as two LANs based on different topologies or network operating systems.
A backbone is the main transmission path, handling the major data traffic, connecting different LANs together.
A LAN is a network contained within a small area, for example a company department.
A modem is a device for converting digital signals to analogue signals and vice versa to enable a computer to transmit and receive data using an ordinary telephone line.
3.10 Complete these definitions with the correct participle given in brackets.
1. A gateway is an interface (enable) dissimilar networks to communicate.
2. A bridge is a hardware and software combination (use) to connect the same type of networks.
3. A backbone is a network transmission path (handle) major data traffic.
4. A router is a special computer (direct) messages when several networks are linked.
5. A network is a number of computers and peripherals (link) together.
6. A LAN is a network (connect) computers over a small distance such as within a company.
7. A server is a powerful computer (store) many programs (share) by all the clients in the network.
8. A client is a network computer (use) for accessing a service on a server.
9. A thin client is a simple computer (comprise) a processor and memory, display, keyboard, mouse and hard drives only.
10. A hub is an electronic device (connect) all the data cabling in a network.
Text E. Network topology
Topologyrefers to the shape of a network. There are three basic physical topologies:
Bus: In abus network, the computers are all connected to a main cable which is known as the bus. All the data is sent along this one cable. It needs special devices at the ends of the bus calledterminators which absorb signals so that they don't get reflected back into the network and cause data corruption. Bus networks are usually easy and cheap to install. There is little to go wrong, but if a fault occurs the whole network is likely to be affected. They have generally been superseded by star networks.
Star:In astar network, each computer and peripheral is connected to a central hub or switch by its own cable. Star networks are more difficult to install than bus networks because there is more cabling. However, if one of the cables fails, the rest of the network will continue to operate. But if a hub or switch fails, none of the devices connected to it will be able to communicate.
Ring: the workstations are connected to one another in a closed loop configuration.
There are also mixed topologies like the tree,a group of stars connected to a central bus. Two common patterns are a bus network and a star network.
3.11 Read the descriptions of different physical topologies of communication
networks and match them with the terms in the text “Network topology”:
1) All the devices are connected to a central station.
2) In this type of network there is a cable to which all the computers and
peripherals are connected.
3) Two or more star networks connected together; the central computers
are connected to a main bus.
4) All devices (computers, printers, etc.) are connected to one another
forming a continuous loop.
3.12 Use the words in the box to complete the sentences:
| LAN nodes hub backbones WLAN peer-to peer server |
1) All the PCs on a ______________are connected to one ___________,
which is a powerful PC with a large hard disk that can be shared by everyone.
2) The style of ___________networking permits each user to share
resources such as printers.
3) The star is a topology for a computer network in which one computer
occupies the central part and the remaining _________are linked solely to it.
4) At present Wi-Fi systems transmit data at much more than 100 times
the rate of a dial-up modem, making it an ideal technology for linking computers to one another and to the Net in a __________.
5) All of the fiber-optic ____________of the United States, Canada and
Latin America cross Panama.
6) A ____________ joins multiple computers (or other network devices)
together `to form a single network segment, where all computers can communicate directly with each other.
Text F. Wireless networks
3.13 Before you read the text try to answer these questions:
1) What is a WAN?
2) How can computers be linked up over a long distance?
3) What are the advantages of optical-fiber cables over telephone lines?
4) What is the function of communications satellites?
3.14 Now read the passage and find out if your answers were correct:
For long-distance or worldwide communications, computers and LANs are usually connected into a wide area network (WAN) to form a single, integrated network. The largest WAN in existence is the Internet.
Networks can be linked together by either telephone lines orfiber-opticcables. An optical-fiber network transmits data at great speed - 100 megabits per second. A variation called FDDT transmits data at 200 Mbps. FDDI networks are typically used as backbones for wide area networks.
Modern telecommunications use fiber-optic cables because data can be transmitted at a very high speed through the extremely wide bandwidths of glass fibers. The fiber system operates by transmitting light pulses at high frequencies along the glass fiber. This offers considerable advantages:
- the cables require little physical space;
- they are safe because they don't carry electricity;
- they avoid electromagnetic interference.
Networks on different continents can also be connected viasatellite. Computers are connected by a modem either to ordinary telephone wires or fiber-optic cables, which are linked to a dish aerial. This aerial has a large concave reflector for the reception and sending of signals. Then, when signals are received by the satellite, they are amplified and sent on to workstations in another part of the world.
Wireless (WiFi) networks are just like fixed LANs but instead of using cables, devices are linked by radio waves.
Each computer in a wireless network requires a wireless network interface card (NIC). These can be built in or you can use plug-in adapters. These allow each component in the network to communicate with a wireless access point (AP) to create a wireless local area network (WLAN). The AP operates like a router in a fixed LAN. It also provides a bridge which plugs into the hub of a fxed LAN allowing both fixed and wireless users to talk to each other. If your LAN is connected to the Internet, the WLAN can also use it. If not, you can connect the WLAN to the Internet via an ADSL or cable modem.
What are the advantages of a wireless network? You don’t need cabling. In older buildings it can be expensive to install cables and access points. With WiFi, one access point can cover an entire floor or even a building. You can work anywhere within range of the access point. On a sunny day you could work outside. You can make any room in the house your study. There are now hotspots in hotels, libraries and airports, so you can link to a network away from home or your officce.
There are disadvantages. Fixed LANs can run at 1000 Mbps. Wireless networks are much slower and the further you are from an access point, the slower the rate. Although there are savings on the cost of cabling, wireless NICs are more expensive than the wired versions. Then there is the problem of interference, if a neighbour uses the same channel, and security. Other users may be able to intercept your data. Encription programs like Wired Equivalent Privacy (WEP) can help.
3.15 Write a list of the advantages and disadvantages of using wireless networks.
3.16 Look through the texts above and correct the following statements.
1. In a client-server architecture, all the workstations have the same
capabilities.
2. LANs link computers and other devices that are placed far apart.
3. The word protocol refers to the shape of the network.
4. Routers are used to link two computers.
5. Access points don’t need to be connected to a wired LAN.
6. Wireless adapters are optional when you using a WLAN.
7. Hotspots can only be found inside a building.
8. The Internet is an example of a LAN.
9. Wireless WANs use fiber and cable as linking devices.