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When it is received, the wave is demodulated and the original audio- frequency signal is recovered.. Thehigh frequency radio acts only to carry the audio - frequency signal and is called

TABLE OF CONTENT

Unit 1 1

Unit 2 7

Unit 3 11

Unit 4 17

Unit 5 22

Unit 6 27

Grammar review 43

Figure 1 illustrates what happens when a voltage is applied across a silicon pn junctiondiode The first quadrant of the graph shows the characteristics of the diode when the source isconnected with the positive to the p-side of the junction and the negative to the n-side In theother words, the diode is forward biased With forward bias, the current at first increased slowly.When the applied voltage reaches about 600 mV, the current rises rapidly The diode is then agood conductor The current will continue to rise with increased voltage but eventually a pointwill be reached where the diode is destroyed by heat.

The third quadrant shows the characteristics when the source is connected with thepositive the n- side and the negative to the p-side When the diode is reverse biased, there isalmost no current flow The junction is therefore a good rectifier it conducts well in one direction

Reverse voltage

6.0 4.0 2.0

1.0 2.0 Forward voltage

and not all in the other However, there is a small reverse leakage current, this leakage currentremains substantially constant until what is known as breakdown voltage (Vb) is reached At thispoint, there is a sharp increase in the reverse current This sudden increase in current is called theZener effect.

Normal diodes are never operated in the breakdown region but Zener diode is designed tomake use of the breakdown phenomenon Because any slight increase in voltage beyond thebreakdown point causes a large increase in current Zener diodes are often used as a kind ofoverspill to protect sensitive circuit from fluctuations in the power supply

1 The positive of the source is connected to the p-side of the diode and the negative to the n-side.

2 When a forward voltage is applied across the diode, there is, at first, only a slow rise in current.

3 The diode allows current to flow freely

4 If a reverse voltage is applied to the diode, it conducts badly

5 There is almost no change in leakage current until the reverse voltage reaches breakdown point.

Task 2: Meaning from context

Select a word from three alternatives given which is most similar in meaning to the word initalics as it is used in the reading passage:

5 fluctuations (line 22)

a rises and falls

b increases

c failures

Task 3: Work in-group of three; complete this description of the current-voltage

characteristics of silicon diode Use the reading passage and Figure 1 to help you.

At first, when a forward voltage is applied, ……… When the forward voltagehas reached about 600 mV, ……… If the forward voltage is furtherincreased,……… …….………… only a very small leakage current flows.When the breakdown voltage is reached ……… After the breakdown point, anyfurther increase in reverse voltage causes ………

Task 4: Checking facts and ideas

Decide if these statements are true or false Quote from the reading passage to support yourdecisions

1 The first quadrant of the graph shows the characteristics of the diode in forward bias

2 For forward voltages over 600mV, the diode conducts well

3 When the source is connected with the negative to the n-side and the positive to the side, the diode is reverse biased

p-4 When a reverse voltage is first applied, a diode conducts badly

5 Zener diodes are never used beyond breakdown point

Task 5: Writing description from graphs

This graph describes the characteristics of a tunnel diode Use the information it contains tocomplete the description below:

As the forward voltage is increased, the current (1) until point P is reached.

P is as known as the peak point The peak voltage for a germanium tunnel diode is about (2) After P the current (3) until V V is known as the valley point.From P to V the diode has a negative resistance The forward voltage at V is about (4) … After the volley point, the current (5) … with increased voltage and the diodebehaves like a normal diode When a reverse voltage is applied however, the reverse current(6) unlike normal diodes

Reverse voltage

0.2 0.1 0.1 0.2 0.3 0.4 0.5 0.6

0 0.

0.

2 0.

P

V

Further Reading

extremely common – it is the basic element in sand Another element, selenium, is used in sensitive transistors A number of chemical compounds have also been found suitable for use indifferent types of transistors.

light-The transistor was developed at Bell Laboratories in 1948 by three American scientists, JohnBareen, Walter Brattain, and William Shockley The transistors created as a result of their workcan perform most of the same tasks as vacuum tubes

To make a transistor, controlled impurities are introduced into semiconductor material which hasbeen refined to a high degree of purity Impurities such as arsenic, antimony, or phosphorous addreadily available electrons, creating an n type of treated or “doped”, semiconducting material.The elements aluminum and boron, on the other hand, create tiny vacant spots, or holes, that tend

to capture the free electrons This kind of doped semiconducting material is called the p type.When n and p types of semiconductors are placed together, current flowing through them isrectified because the electrons can flow only from n to p In this way, the transistor is similar tothe diode vacuum tube, but unlike the diode, it has a three-part sandwich like structure There arethree terminals, called the emitter, the collector, and the base The base forms the middle part ofthe sandwich The first type of transistor developed for widespread use has an npn structure, thebase being made of p-type semiconductor The other is called pnp Depending upon how they areconnected in a circuit, the can perform a wide variety of functions, including those of amplifiers,switches, and oscillators

Transistors overcame most of the disadvantages of the vacuum tube Since they were very small,they could be packed closely together – observe the difference in size between a radio with tubesand a transistor radio They could take rough handling, seldom burned out, did not overheat,required no warm-up time, and used less current There were other electronic advantages – theywere inexpensive to manufacture and did not need nearly as much as wiring to connect them.Transistors were first placed on boards or sheets of insulting materials and wired into a circuit.Since then printed circuits have replaced complicated wiring systems in many cases In printedcircuits, transistors are connected by thin lines of copper which aanre boned to the surface of aninsulator

Task 1: Decide whether the following statements are true or false (T/F) by referring to the information in the text Then make the necessary changes so that the false statements become true.

1… Transistor was developed due to the disadvantage of the vacuum tube.

2… Silicon is a better conductor than aluminum

3… Arsenic is an impurity which creates an n type of semiconducting material by adding readilyavailable electrons

4… When n and p types of semiconductors are placed together, current can only flow throughthem in on direction

5… The transistor performs a lot of functions no matter how they are connected in a circuit

Task 2: Filling in the spaces of the following sentences with the appropriate words or phrases, basing on the information in the text.

1 In the ……… of treated semiconducting material, there are holes from whichelectrons have been removed

2 A transistor has three terminals: the one connected to the middle part of the “sandwich” is the

……… and the two outer parts are the ……… and the

5 ……… is one of the impurities that can create n type semiconducting material

6 ……… is one of the impurities that can create p type semiconducting material

7 Transistors perform a variety of functions including ……… of amplifiers,switches, and oscillators

Propagation by space waves applies mainly to very high frequencies Part of thetransmitted signal travels in a direct line from transmitting antenna to receiving antenna Partlythe signal is reflected from the ground The higher the frequency, the greater the possibleground_ wave reflection The range of the space wave propagation is restricted to approximatelytwice the direct optical path.

The range covered by ground waves and space waves is limited Greater distances can beachieved using sky waves Sky wave propagation depends on the ionosphere

Figure 2a

A signal transmitted from point A would not be received at B because of the curvature ofthe earth if it were not for the ionosphere This consists of a number of layers of ionized gas inthe upper atmosphere If a transmission is directed towards these layers, it will be reflected back

to the earth as shown in Figure 2b

A

B

Reading

Figure 2b

The wave may then be reflected back from the earth to the ionosphere Indeed it may becarried right round the earth by successive reflections although it will lose power both in theearth and in the ionosphere at each bounce A receiver at point C, which is outside ground waverange yet closer than B, will not receive the transmission

The bending effect of the ionosphere depends on the frequency of the signal and the angle ofradiation The higher the frequency the less the bending At a certain frequency, signal will passstraight through the layers and be lost in space The smaller the angel of radiation, the greater thedistance, which can be covered in one reflection

At any time there is a maximum usable frequency for transmissions from a given site over aparticular path This frequency depends on the state of the ionosphere, which varies according tomany factors including the time of the day and the season of the year The lower the frequency of

a transmission the greater the number of reflections needed to cover the required distance andhence the weaker signal will be For this reason, it is best to use a frequency as high as possiblewithout exceeding the maximum usable frequency as this will cover the required distance withthe smallest number of reflections and hence the least attenuation

Task 1: Answer these questions about the passage.

1 List three types of propagation

2 How do ground wave travel?

3 What is attenuation?

4 Name two factors, which affect the attenuation of ground waves?

5 What frequencies are propagated by space wave?

6 Name two ways in which space waves travel

7 What range have space wave?

8 What controls sky wave propagation?

9 What is the ionosphere?

10 How do sky waves cover great distances?

11 Name two factors, which determine the bending effect of the ionosphere

12 What is the maximum usable frequency?

Task 3: Meaning from context

Select a word from the three alternatives given which is most similar in meaning to the word initalics as it is used in the reading passage

1 varies with ( line 4 )

Radio waves from a transmitting aerial can travel in one or more of three different ways

Surface or ground waves: This travels along the ground, following the curvature of the

earth's surface Its range is limited mainly by the extent to which energy is absorbed from it bythe ground Poor conductors, such as sand, absorb more strongly than water, and the higher thefrequency, the greater the absorption The range is about 1500 km at low frequencies (longwaves)

Sky waves: It travels skywards and, if it is below a certain critical frequency (typically 30

MHz), is returned to earth by the ionosphere This consists of layers of air molecules stretchingfrom about 80 km above the earth to 500 km On striking the earth, the sky wave bounces back

to the ionosphere where it is again gradually refracted and returned earthwards as if by "reflection " This continues until it is completely attenuated

Further reading

The critical frequency varies with the time of day and the seasons Sky waves of high frequencies can travel thousands of kilometers but at VHF and above they usually pass through the ionosphere into outer space

Space waves: For VHF, UHF, and microwave signals, only the space wave, giving line

of sight transmission, is effective A range up to 150 km is possible on earth is the transmitting aerial is on high ground and there are no intervening obstacles such as hills, buildings, or trees Space waves are also used for satellite communications

Fill in the table, which follow

Task 1: Read the following passage and fill in the blank with appropriate information

Frequencies

Travels

Range

Difficulties

UNIT 3

TRANSMISSION

We can only communicate information by radio waves by changing the wave in some ways.This change is known as modulation The simplest form of modulation is to turn the wave on oroff This method was used in the early days of radio for telegraphic signals The wave wasstopped and started to present the dots and dashes of the Morse code by means of a telegraphkey

Speech and music produce audio frequencies, which cannot be transmitted directly Butthey can be used to modulate radio waves The modulated radio wave is then transmitted When it

is received, the wave is demodulated and the original audio- frequency signal is recovered Thehigh frequency radio acts only to carry the audio - frequency signal and is called the carrier wave.The audio - frequency signal is termed the modulating signal

The wave has three quantities: amplitude, frequency and phase Any of these quantities can

be modulated The two commonest methods of modulation are amplitude modulation (AM) andfrequency modulation, (FM)

In amplitude modulation, the amplitude of the carrier wave is changed according to theamplitude of the modulating signal The frequency of the carrier is kept constant Figure 3apresents part of an audio - frequency signal, which might be generated by a microphone Figure3b represents a radio wave of much higher frequency Figure 3c shows the same radio frequencywave after it has been modulated by the audio - frequency signal in Figure 3a

Figure 3a

Reading

Figure 3b

Figure 3c

In frequency modulation, the amplitude of the carrier wave is kept constant, but thefrequency is varied in proportion to the amplitude of the modulating signal Frequencymodulation has several advantages over amplitude modulation The most notable is thatreception is less likely to be disturbed This is because atmospheric disturbances and “noise "generated in the receiver itself result in a change in the amplitude of the received signal.However, changes in only the frequency would distort the modulated information

In FM sound broadcasting, the limit of modulation is usually 75 kHz above and below thefrequency of the unmodulated carrier wave In other words, FM broadcasts spread 75kHz eitherside of the carrier frequency This is one reason why FM stations broadcasts in the VHF band(30- 300 MHz) where stations can be spaced more than several hundred kilohertz apart unlikethe medium frequency bands where spacing of only 9 to 10 kHz are common

Task 1: Find the answers to these questions in the passage, which follows.

(Work as quickly as you can Try to ignore information, which will not help you to answer thesequestions)

1 What is modulation?

2 Which three quantities of a wave can be modulated?

3 Why is frequency modulation better than amplitude modulation?

Task 2: Meaning from context

Select a word from the three alternatives given which is most similar in meaning to theword in italics as it is used in the reading passage

Task 3: Checking facts and ideas

Decide if the statements are true or false Quote from the reading passage to support yourdecisions

1 The first application of radio wave modulation was for telegraphic signals

2 Phase modulation is commonly used

3 In amplitude modulation, the frequency of the carrier wave is proportional to the amplitude

of the modulating signal

4 FM reception is less likely to be disturbed than AM

5 FM stations broadcast in the VHF band

Task 4: Now read the passage carefully Each of these sentences summarizes part of the passage Identify the lines summarized.

1 In fm the frequency of the carrier wave is modulated according to the amplitude of themodulating signal (lines )

2 Audio frequencies cannot be transmitted (line )

3 The amplitude, frequency and phase of a wave can be modulated (line )

4 Fm broadcasts are in the VHF band partly because FM stations require greater spacing.(line )

5 The carrier wave is demodulated by the receiver and the audio - frequency signalrecovered (line )

6 No information can be communicated by radio waves without modulating them (line )

7 In AM, the amplitude of the carrier wave is modulated according to the amplitude of themodulating signal (line )

8 Stopping and starting the wave is the simplest method of modulation (line )

9 Fm is better than AM because there is less interference (line )

10 Audio frequencies can be used to modulate high frequency radio waves, which can thenact as carriers of the audio - frequency signal

Radio frequency (AF) waves are used to carry audio frequency (AF) waves over longdistances through the air The audio signals can be combined with the RF carrier wave in such away that it varies the amplitude of the carrier This gives an amplitude - modulated (AM) carrierwave (see Figure 3d)

Figure 3eThe block diagram of a radio is shown in Figure 3f below The turner selects the required

RF wave from those picked up by the aerial The selected RF wave is amplified and passed to thedetector, which separates the audio modulation from the RM carrier wave The audio frequencyamplifier amplifies the audio signal to make it strong enough to drive the loudspeaker

Figure 3f

Turner

A typical radio turner circuit consists of an inductor and capacitor connected in parallel(see Figure 3g) The size of the aerial inductance coil can be kept small by winding it on a ferriterod core

Detector or Demodulato r

AF power amplifier

The RF waves fed to the turner cause the circuit to oscillate The impedance of the circuit

is smallest and the oscillation is greatest at a particular frequency known as the resonantfrequency This frequency is determined by the values of the inductance and the capacitance Byusing a variable capacitor, the circuit can be tuned to the required radio frequency, and selected

RF wave passed on to the RF amplifier

UNIT 4

SWITCHING

Task 1 Read the following TEXT As you read, complete Figure 4a.

On the march 10th 1876, in Boston USA, Alexander Graham Bell spoke the firstrecognizable words over that was certainly his most famous invention, the telephone “MrWatson, come here I want you” he called out to an astonished assistant

It quickly became necessary to link up people using the new invention This was donethrough the telephone exchange In the first simple exchanges, all calls were handled manually

by the operator Using the switchboard in front of her, she plugged the line of the subscribercalling her into the line of the subscriber being asked for

The idea of an automatic exchange was soon suggested In 1892, three years afterpatenting his ideas, Almon B.Strowger saw his system installed at La Porte, Indiana Strowger,

an undertaker from Kansas City, found his business was becoming less and less profitablebecause the operator always connected calls intended for him to other undertakers

Strowger’s automatic exchange underwent several modifications over the next fifty years,but the principle has remained the same ever since It is known as the “step-by-step” system andthere are still thousands of Strowger exchanges in service throughout the world today

The next generation of exchanges was first developed in Sweden Crossbar exchanges asthey are called, consist of a series of vertically and horizontally crossed bars For the first time acommon control system was used This made crossbar exchanges cheaper than Strowger, foreach selector could now carry up to ten calls They were not as noisy as Strowger, either Theseexchanges are electromechanically operated using electromagnets

In 1948, the invention of the transistor at Bell Telephone Laboratories led to a revolution

in electronics and to the creation of semi-electronic telephone exchanges

The old mechanical and electromechanical exchanges have now begun to disappear, andsince the 1970s they have started to be replaced by exchanges built around electroniccomponents The latest digital switching centers have several advantages over other types ofexchange, for they provide: a substantial reduction in equipment cost; a larger reduction in

Reading

equipment size; shorter procurement times; shorter installation and commissioning times; greaterreliability and reduced running costs; new services for the customer; and new facilities for theadministration

Main characteristics of the systems

Task 2 Answer the following questions:

a Who invented the telephone? When and where?

b What did the first operators do?

c Why did Strowger invent the automatic telephone exchange?

d What is a “crossbar” system?

e What advantages does crossbar offer over Strowger?

f What important development took place in 1948?

g What are mechanical and electromechanical exchanges being replaced by?

Task 1 Read the following passage about electronic exchange As you read complete Figure 4b

Classification Electromechanical exchange Digital exchange

Transmission type Analogue transmission Digital transmission and

switching

Further Reading

Figure 4bMost of the world’s telephone traffic is still handled by exchanges of electromechanicaldesign, each with thousands of wear-prone exposed moving parts Analogue exchanges of thistype are costly to install and maintain, and are subject to familiar faults such as crossed lines,noise and wrong numbers

Many of the world’s telecommunications administrations have explored other, digital,solutions They are designing networks that offer: a much higher quality of service than before,using the latest microchip technology with few or no moving parts; less interference thanpreviously; lower installation and maintenance costs; and much faster connection speed for callsand fewer wrong numbers than in the past

The main characteristics of a digital exchange are:

1 Integrated Digital Transmission and Switching

Speech and other signals are digitally encoded and a common method of time divisionmultiplexing is used in both transmission and switching equipment The main advantage of thissystem is that the transmission loss encountered by speech becomes more or less independent ofboth distance and the number of exchanges through which a call is routed

2 Stored Program Control (SPC) is the application of data processing and computer

techniques to an exchange, thus providing a powerful, flexible method of controlling theoperation of the exchange

3 Common Channel Signaling (CCS) uses just one (go and return) pair of signaling channels,

not directly associated with the traffic circuits, for performing all the signaling functions of anentire route which may contain several hundred traffic circuits

4 Microelectronics technology is the application of solid- state semiconductor technology to

provide components, which range in function from a signal active element (e.g transistor) tolarge scale integrated circuits Use of this technology offers small physical size and reliabilitytogether with automated design, manufacture and testing The components are fixed onto printedcircuit boards, and if there is a fault the defective printed circuit board can be taken out andreplaced by another in a matter of seconds

Task 2.

a Match each of the headings with one set of functions.

1 stored program control

2 common channel signaling

3 integrated digital transmission and

switching

4 microelectronics technology

a the application of solid -state semiconductors toprovide greater reliability small size and automateddesign

b the application of data-programming techniques

to an exchange, providing greater control andflexibility in the operative of an exchange

c the use of one pair of channels not directlyassociated with the traffic circuits, to perform allsignaling functions on a complete route

d speech and other signals are digitally encoded and

a common method of time division multiplexing isused

b Match the following terms with a suitable explanation

a a call is incorrectly routed

b a system in which a varying electrical currenttransmits the caller’s voice pattern

c an exchange which uses microchip technology

d the cost of a telephone call

e during a telephone conversation, you hear anotherconversation at the same time

f a switching centre, the design of which is based onthousands of moving parts

Another important use of this means of communicating is shipping A ship that is indifficulty can call the nearest coast station, giving details of its situation and, if necessary, ask forhelp We call this “ship –to- shore” radio Radio can ensure greater safety in navigation (forexample, to want of bad weather or of hazards in the shipping lanes) and it enables large amounts

of information to be sent over land or water without the support of several hundred kilometres ofwires and cables Radio networks can, therefore, be cheaper to install but often have fewer

circuits than cable links Radar systems also enable air traffic controllers to follow and guide thelight paths of planes from take-off to landing.

Radio-paging systems enable us to be contacted even though we do not have a telephonewithin easy reach It is a way of letting us know that something important has just happened andthat we must act quickly The system works through a pager or ‘bleeper’, a small receiver aboutthe size of a packet of cigarettes, which you can keep in your pocket When people want tocontact you, they simply pick up the nearest telephone, dial your personal paging number free ofcharge, and wherever you are in the coverage area, your pager will bleep (You must, however,

be love, not below ground lever, for the signal will not get through if you are underground) Thebleeping informs you that someone is calling you You can identify up to four callers on sometypes of equipment The most modern type of radio-pager displays the number of the telephonethe caller is ringing from The main advantage of radio-paging is that you can be contactedwherever you are, even though you may be a long way from a telephone set

Many companies which have personnel on business trips have found that radio-pagingimproves their competitive position and allows them to be more responsive to a lot of theircustomers’ needs It also increases company efficiency by cutting out many unnecessary andexpensive journeys that are sometimes made by salesman This saves time and fuel costs

Type of communication : e……

Doctors can be contacted immediately and return to their surgery or hospital, thus possiblysaving the life of a very sick patient.

Task 2 Answer the following questions:

a How does the text define radio communications?

b What examples does the text give of radio communications?

i ship-to-shore radio ii … etc

c Why can radio networks be cheaper to install?

d What equipment does the radio-paging user need?

e Where does the user keep this equipment?

f Where must you do not go if you wish to be contacted by radio-paging?

g How can companies benefit from radio-paging? (give three ways)

h Why do many doctors carry a bleeper?

Task 1 Read the description of the Nordic Mobile Telefone System (NMT) As you read, complete Figure 6d

Description of System

The NMT system is made up of the following component parts: MTX (Mobile

Telephone Exchange) is the brain of the system and is technically the most complicated part.

The MTXs form the interface between the NMT system and fixed telephone network Whereassignalling and other system components of NMT are the same in all Nordic countries, theirtelephone networks differ in some respects One of the functions of the exchanges is therefore tocompensate for these differences

The Base Stations are intermediary links without switching function between the wire and radio

transmission There will be about 1,000 base stations when the system is developed to fullcapacity

The Mobile Stations i.e the subscriber equipment, will be available in various forms.e.g…

vehicle-borne, portable or of coin-box type A mobile station is owned or leased by thesubscriber and must be type-approved by the Administrations

Further Reading

The adjoining figure shows the structure of the NMT system Each country is dividedinto a number of traffic areas In a traffic area there are a number of base stations spaced betweenabout 8 and 80 km apart Each traffic area belongs to a single exchange This means that all basestations in the area are connected to this exchange and that all traffic in the area is channeledthrough it The exchange communicates with the telephone network via the trunk exchanges.Every mobile subscriber is registered in a so-called home MTX, usually the MTX controlling thetraffic area in which the subscriber normally resides.

Setting up of calls

A call from an ordinary telephone subscriber is connected on the basis of the first digits

of the mobile subscriber’s number to his home MTX The latter stores data of his presentlocation and transmits a call signal via all base stations in that traffic area The mobile systemanswers automatically with a call acknowledgement and the MTX the assigns the mobile station

a traffic channel The call is set up

If the mobile subscriber is in a traffic area not belong to his home MTX, the home MTXhands over the call to the MTX controlling the visited traffic area, and the later MTX takes overthe subsequent setting up of the call

Task 2 Answer the following questions:

a Where does the NMT system compensate for the differences in the four countries’telephone networks?

b What does the distance 8-80 km refer to?

c How many exchanges does each traffic area have?

d What happens after the mobile station transmits a call acknowledgement?

e When does the home MTX hand over calls to another MTX?

Task 3: Writing

Now read the letter sent by Tom Davies’s secretary to Chris Bradley (see 1.2.3) confirming the subject matter of their telephone conversation.

On Monday, April 30th, Ms Whitfield received the following telex from Mr Tom

Davies who was on a business trip to Spain Using this telex, write Ms Whitfield’s letter to ChrisBradley, including all the information in the telex, and basing your letter on the exampleprovided

Swanford Tools Ltd., Slingdon Road, Thurby TH2SY 3

Following your telephone conversation yesterday with Mr Tom Davies, I should

like to confirm that Mr Davies will visit you at Bradfield Electronics on Thursday 26 th

April at 2 p.m.

Mr Davies hopes that you will be able to provide a demonstration of the

radio-paging system which is of great interest to Swanford Tools Ltd.

I can also confirm that three other members of our Sales department will be

present with Mr Davies.

Yours sincerely

Ms J Whitfield

83 4 30 43751 64032 TLX NO 3624

13 30 SWANTOOL SPANCOM

ATTENTION: J.WHITFIELD REF: MY VISIT TO BRADFIELD RE RPH RADIOPHONE PLS SEND LETTER BRADLEY CONFIRMING:

1 INTERESTING DEMONSTRAT BRADFIELD 26.4 CONFIRM ORDER TEN PAGERS FOR SALESMEN.

2 FINALIZE DETAILS AT MEETING 29.5 AT SWATFOED 15.30

3 HOPE ABOVE TIME AND DATE CONVENIENT THANKS.

TDAVIES

64032 SPANCOM

43751 SWANTOOL

UNIT 6

INTERNET AND THE COMPUTER

Where is Access in the Network?

Internet access has different meanings to different people In this text, the concept ofaccess can be explained as follow There are three major entities in the provision of Internetservice:

· End users who want to have Internet service as well as other services, such as telephony orcable TV

· Data service providers who want to supply Internet access, content services, or other dataservices, such as virtual private networking

· Telephony, wireless and cable service providers who want to provide connectivity between endusers and data service providers

Note that, at times, the last two entities are combined For example, several companiessuch as MCI, AT&T or Pacific Bell provide both telephony and Internet access

Now to the question: “What is access?” To the telephony, wireless or cable serviceprovider, access is the network connection from the end user’s home or business to the outsideplant termination points within the service node In traditional telephony architecture this is mostcommonly thought of as the twisted-pair, cross-connect point and is referred to as the maindistribution frame (MDF) The remainder of the telephony’s provider’s network would bereferred to as the switching and transport network

In contradistinction, an Internet service provider (ISP) views access as the connectionfrom its customer to its network Here, access is the connection from the end-user’s home orbusiness to the gateway-access node belonging to the ISP Thus, to the ISP, the telephony’saccess, switching and transport network are all parts of access

Access Evolution

Access evolution is being driven primarily by strong demands for increasing bandwidth

to support a growing variety of user services In the past, traffic sent over the Internet was largelytext-based information with file transfer and e-mail being among the most popular services The

Reading

surge in growth of the Internet during 1995 was in part due to the graphical nature of the WorldWide Web (WWW).

A significant aspect of this shift is that graphical images generally consist of a largenumber of bits To transfer large graphical image files quickly with satisfactory performancemeant that higher-speed access technologies were needed than those used to deliver relativelysmall text files The WWW also became the base for nurturing other capabilities such asanimated graphics, audio, and low-rate video Each of these capabilities has been pushing theneed for increasingly higher-speed access

An Overview of Access Alternatives

Internet-access technologies fit into four broad categories:

• Twisted pair

• Fiber/coax

• Wireless

• All fiber

Several technologies and implementations exist within each of these broad categories

Twisted-pair telephone lines are the access media used in the vast majority of individualresidential subscribers today Over time, a number of technologies have been introduced toprovide faster data speeds over this medium

Fiber/ coax systems were originally introduced for video-broadcast applications Becausethese systems are inherently broadband, techniques have been developed to use this advantage toprovide high-speed data transmission, principally for residential Internet access

Wireless Internet access has two origins: satellite systems established for broadcast videohave the ability to distribute Internet data at high speed, and cellular/ personal communicationsservice (PCS) systems are designed to serve mobile users

The predominant access systems for business users are optical-fiber synchronous opticalnetwork (SONET) and synchronous digital hierarchy (SDH) systems In the future, passiveoptical network (PON) systems are expected to become an all-fiber access medium forresidential users as well

Task 1 Match the beginnings with the endings.

c) want to provide connectivity between endusers and data-service providers

Task 2: Give the full forms of the acronyms.