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描述OSI模型中网络层的主要功能。

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正确答案:

网络层提供了尽力的端到端分组传送,网络层基于 IP 路由选择将分组从源网络发送到目的网络。

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The Importance of Good Communications Effective communication is essential for all organizations. It links the activities of the various parts of the organization and ensures that everyone is working towards a common goal. It is also extremely important for motivating employees. Staff need to know how they are getting on, what they are doing right and in which areas they could improve. Working alone can be extremely difficult and it is much easier if someone takes an interest and provides support. Employees need to understand why their job is important and how it contributes to the overall success of the firm. Personal communication should also include target setting. People usually respond well to goals, provided these are agreed between the manager and subordinate and not imposed. However, firms often have communication problems that can undermine their performance. In many cases,these problems occur because messages are passed on in an inappropriate way. There are, of course, several ways of conveying information to others in the organization which include speaking to them directly, e-mailing, telephoning or sending a memo. The most appropriate method depends on what exactly it is you are communicating. For example, anything that is particularly sensitive or confidential, such as an employee's appraisal, should be done face-to-face. One of the main problems for senior executives is that they do not have the time or resources needed to communicate effectively. In large companies, for example, it is impossible for senior managers to meet and discuss progress with each employee individually. Obviously this task can be delegated but at the cost of creating a gap between senior management and staff. As a result, managers are often forced to use other methods of communication, like memos or notes, even if they know these are not necessarily the most suitable means of passing on messages. The use of technology, such as e-mail, mobile phones and network systems, is speeding up communication immensely. However, this does not mean that more investment in technology automatically proves beneficial: systems can become outdated or employees may lack appropriate training. There are many communications tools now available but a firm cannot afford all of them. Even if it could, it does not actually need them all.The potential gains must be weighed up against the costs, and firms should realize that more communicationdoes not necessarily mean better communication." As the number of people involved in an organization increase, the use of written communication rises even faster. Instead of a quick conversation to sort something out numerous messages can be passed backwards and forwards. This can lead to a tremendous amount of paperwork and is often less effective than face-to-face communication. When you are actually talking to someone you can discuss things until you are happy that they have understood and feedback is immediate. With written messages, however, you are never quite sure how it will be received what you think you have said and what the other person thinks you have said can be very different. The amount of written information generated in large organizations today can lead to communication overload. So much information is gathered that it gets in the way of making decisions. Take a look at the average manager's desk and you will see the problem -- it is often covered with letters, reports and memos. This overload can lead to inefficiencies. For example, managers may not be able to find the information they want when they need it. Communication is also becoming more difficult with the changes occurring in employment patterns. With more people working part-time and working at home, managing communication is becoming increasingly complex. 1.In the first paragraph the writer recommends that communication with staff should includeA. some feedback on their job performance. B. an explanation of how company targets have been set. C. information on promotion prospects within the company. D. an indication of which duties they can expect assistance with.areas they could improve 2.According to the writer, the best way of achieving effective communication is toA. adapt the message to suit a particular audience. B. make the content of messages brief and direct. C. select the most suitable means of conveying a particular message. D. ensure that information is targeted at the appropriate group of people. 3.What does the writer say about the communication options available to senior managers?A. Sending memos to staff is one of the most efficient methods. B. It is important to find the time to discuss certain matters with staff. C. They should increase the range of options that they use. D. Getting junior managers to talk to staff can create different problems. 4.What advice is given about the communication tools made available by technology?A. Aim to limit staff use of certain communication tools. B. Evaluate them in terms of the expenditure involved. C. Select them on the basis of the facilities they offer. D. Encourage more staff to attend training courses in their use. 5.According to the writer, a problem with written communication is thatA. the message can be interpreted differently to what was intended. B. it can be easy for people to ignore the contents of a written message. C. most people are more comfortable with face-to-face communication. D. it is possible for correspondence to get lost within a large organization. 6.According to the article, what is the effect of receiving large amounts of written information?A. It is counter-productive. " B. It causes conflict in a company. C. It leads to changes in work patterns. D. It makes the main points more difficult to identify.
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Rogue theory of smell gets a boost 1.A controversial theory of how we smell, which claims that our fine sense of odour depends on quantum mechanics, has been given the thumbs up by a team of physicists. 2.Calculations by researchers at University College London (UCL) show that the idea that we smell odour molecules by sensing their molecular vibrations makes sense in terms of the physics involved. 3.That's still some way from proving that the theory, proposed in the mid-1990s by biophysicist Luca Turin, is correct.But it should make other scientists take the idea more seriously. 4."This is a big step forward," says Turin, who has now set up his own perfume company Flexitral in Virginia.He says that since he published his theory, "it has been ignored rather than criticized." 5.Most scientists have assumed that our sense of smell depends on receptors in the nose detecting the shape of incoming molecules, which triggers a signal to the brain.This molecular 'lock and key' process is thought to lie behind a wide range of the body's detection systems: it is how some parts of the immune system recognise invaders, for example, and how the tongue recognizes some tastes. 6.But Turin argued that smell doesn't seem to fit this picture very well.Molecules that look almost identical can smell very different — such as alcohols, which smell like spirits, and thiols, which smell like rotten eggs.And molecules with very different structures can smell similar.Most strikingly, some molecules can smell different — to animals, if not necessarily to humans — simply because they contain different isotopes (atoms that are chemically identical but have a different mass). 7.Turin's explanation for these smelly facts invokes the idea that the smell signal in olfactory receptor proteins is triggered not by an odour molecule's shape, but by its vibrations, which can enourage an electron to jump between two parts of the receptor in a quantum-mechanical process called tunnelling.This electron movement could initiate the smell signal being sent to the brain. 8.This would explain why isotopes can smell different: their vibration frequencies are changed if the atoms are heavier.Turin's mechanism, says Marshall Stoneham of the UCL team, is more like swipe-card identification than a key fitting a lock. 9.Vibration-assisted electron tunnelling can undoubtedly occur — it is used in an experimental technique for measuring molecular vibrations."The question is whether this is possible in the nose," says Stoneham's colleague, Andrew Horsfield. 10.Stoneham says that when he first heard about Turin's idea, while Turin was himself based at UCL, "I didn't believe it".But, he adds, "because it was an interesting idea, I thought I should prove it couldn't work.I did some simple calculations, and only then began to feel Luca could be right." Now Stoneham and his co-workers have done the job more thoroughly, in a paper soon to be published in Physical Review Letters. 11.The UCL team calculated the rates of electron hopping in a nose receptor that has an odorant molecule bound to it.This rate depends on various properties of the biomolecular system that are not known, but the researchers could estimate these parameters based on typical values for molecules of this sort. 12.The key issue is whether the hopping rate with the odorant in place is significantly greater than that without it.The calculations show that it is — which means that odour identification in this way seems theoretically possible. 13.But Horsfield stresses that that's different from a proof of Turin's idea."So far things look plausible, but we need proper experimental verification.We're beginning to think about what experiments could be performed." 14.Meanwhile, Turin is pressing ahead with his hypothesis."At Flexitral we have been designing odorants exclusively on the basis of their computed vibrations," he says."Our success rate at odorant discovery is two orders of magnitude better than the competition." At the very least, he is putting his money where his nose is. Questions 5-9 Complete the sentences below with words from the passage.Use NO MORE THAN THREE WORDS for each answer. 5.The hypothesis that we smell by sensing the molecular vibration was made by ______. 6.Turin's company is based in ______. 7.Most scientists believed that our nose works in the same way as our ______. 8.Different isotopes can smell different when ______ weigh differently. 9.According to Audrew Horsfield, it is still to be proved that ______ could really occur in human nose.
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1.The failure of a high-profile cholesterol drug has thrown a spotlight on the complicated machinery that regulates cholesterol levels.But many researchers remain confident that drugs to boost levels of 'good' cholesterol are still one of the most promising means to combat spiralling heart disease. 2.Drug company Pfizer announced on 2 December that it was cancelling all clinical trials of torcetrapib,a drug designed to raise heart-protective high-density lipoproteins (HDLs).In a trial of 15000 patients,a safety board found that more people died or suffered cardiovascular problems after taking the drug plus a cholesterol-lowering statin than those in a control group who took the statin alone. 3.The news came as a kick in the teeth to many cardiologists because earlier tests in animals and people suggested it would lower rates of cardiovascular disease."There have been no red flags to my knowledge," says John Chapman,a specialist in lipoproteins and atherosclerosis at the National Institute for Health and Medical Research (INSERM) in Paris who has also studied torcetrapib."This cancellation came as a complete shock." 4.Torcetrapib is one of the most advanced of a new breed of drugs designed to raise levels of HDLs,which ferry cholesterol out of artery-clogging plaques to the liver for removal from the body.Specifically,torcetrapib blocks a protein called cholesterol ester transfer protein (CETP),which normally transfers the cholesterol from high-density lipoproteins to low density,plaque-promoting ones.Statins,in contrast,mainly work by lowering the 'bad' low-density lipoproteins. Under pressure 5.Researchers are now trying to work out why and how the drug backfired,something that will not become clear until the clinical details are released by Pfizer.One hint lies in evidence from earlier trials that it slightly raises blood pressure in some patients.It was thought that this mild problem would be offset by the heart benefits of the drug.But it is possible that it actually proved fatal in some patients who already suffered high blood pressure.If blood pressure is the explanation,it would actually be good news for drug developers because it suggests that the problems are specific to this compound.Other prototype drugs that are being developed to block CETP work in a slightly different way and might not suffer the same downfall. 6.But it is also possible that the whole idea of blocking CETP is flawed,says Moti Kashyap,who directs atherosclerosis research at the VA Medical Center in Long Beach,California.When HDLs excrete cholesterol in the liver,they actually rely on LDLs for part of this process.So inhibiting CETP,which prevents the transfer of cholesterol from HDL to LDL,might actually cause an abnormal and irreversible accumulation of cholesterol in the body."You're blocking a physiologic mechanism to eliminate cholesterol and effectively constipating the pathway," says Kashyap.Going up 7.Most researchers remain confident that elevating high density lipoproteins levels by one means or another is one of the best routes for helping heart disease patients.But HDLs are complex and not entirely understood.One approved drug,called niacin,is known to both raise HDL and reduce cardiovascular risk but also causes an unpleasant sensation of heat and tingling.Researchers are exploring whether they can bypass this side effect and whether niacin can lower disease risk more than statins alone.Scientists are also working on several other means to bump up high-density lipoproteins by,for example,introducing synthetic HDLs."The only thing we know is dead in the water is torcetrapib,not the whole idea of raising HDL," says Michael Miller,director of preventive cardiology at the University of Maryland Medical Center,Baltimore. Questions 7-13 Match torcetrapib,HDLs,statin and CETP with their functions (Questions 8-13)..Write the correct letter A,B,C or D in boxes 8-13 on your answer sheet.NB You may use any letter more than once. 7.It has been administered to over 10,000 subjects in a clinical trial. 8.It could help rid human body of cholesterol. 9.Researchers are yet to find more about it. 10.It was used to reduce the level of cholesterol. 11.According to Kashyap,it might lead to unwanted result if it's blocked. 12.It produced contradictory results in different trials. 13.It could inhibit LDLs.List of choices A.TorcetrapicB.HDLSC.StatinD.CETP
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1 There's a dimmer switch inside the sun that causes its brightness to rise and fall on timescales of around 100,000 years - exactly the same period as between ice ages on Earth. So says a physicist who has created a computer model of our star's core. 2 Robert Ehrlich of George Mason University in Fairfax, Virginia, modelled the effect of temperature fluctuations in the sun's interior. According to the standard view, the temperature of the sun's core is held constant by the opposing pressures of gravity and nuclear fusion. However, Ehrlich believed that slight variations should be possible. 3 He took as his starting point the work of Attila Grandpierre of the Konkoly Observatory of the Hungarian Academy of Sciences. In 2005, Grandpierre and a collaborator, Gábor ágoston, calculated that magnetic fields in the sun's core could produce small instabilities in the solar plasma. These instabilities would induce localised oscillations in temperature. 4 Ehrlich's model shows that whilst most of these oscillations cancel each other out, some reinforce one another and become long-lived temperature variations. The favoured frequencies allow the sun's core temperature to oscillate around its average temperature of 13.6 million kelvin in cycles lasting either 100,000 or 41,000 years. Ehrlich says that random interactions within the sun's magnetic field could flip the fluctuations from one cycle length to the other. 5 These two timescales are instantly recognisable to anyone familiar with Earth's ice ages: for the past million years, ice ages have occurred roughly every 100,000 years. Before that, they occurred roughly every 41,000 years. 6 Most scientists believe that the ice ages are the result of subtle changes in Earth's orbit, known as the Milankovitch cycles. One such cycle describes the way Earth's orbit gradually changes shape from a circle to a slight ellipse and back again roughly every 100,000 years. The theory says this alters the amount of solar radiation that Earth receives, triggering the ice ages. However, a persistent problem with this theory has been its inability to explain why the ice ages changed frequency a million years ago. 7 "In Milankovitch, there is certainly no good idea why the frequency should change from one to another," says Neil Edwards, a climatologist at the Open University in Milton Keynes, UK. Nor is the transition problem the only one the Milankovitch theory faces. Ehrlich and other critics claim that the temperature variations caused by Milankovitch cycles are simply not big enough to drive ice ages. 8 However, Edwards believes the small changes in solar heating produced by Milankovitch cycles are then amplified by feedback mechanisms on Earth. For example, if sea ice begins to form because of a slight cooling, carbon dioxide that would otherwise have found its way into the atmosphere as part of the carbon cycle is locked into the ice. That weakens the greenhouse effect and Earth grows even colder. 9 According to Edwards, there is no lack of such mechanisms. "If you add their effects together, there is more than enough feedback to make Milankovitch work," he says. "The problem now is identifying which mechanisms are at work." This is why scientists like Edwards are not yet ready to give up on the current theory. "Milankovitch cycles give us ice ages roughly when we observe them to happen. We can calculate where we are in the cycle and compare it with observation," he says. "I can't see any way of testing [Ehrlich's] idea to see where we are in the temperature oscillation." 10 Ehrlich concedes this. "If there is a way to test this theory on the sun, I can't think of one that is practical," he says. That's because variation over 41,000 to 100,000 years is too gradual to be observed. However, there may be a way to test it in other stars: red dwarfs. Their cores are much smaller than that of the sun, and so Ehrlich believes that the oscillation periods could be short enough to be observed. He has yet to calculate the precise period or the extent of variation in brightness to be expected. 11 Nigel Weiss, a solar physicist at the University of Cambridge, is far from convinced. He describes Ehrlich's claims as "utterly implausible". Ehrlich counters that Weiss's opinion is based on the standard solar model, which fails to take into account the magnetic instabilities that cause the temperature fluctuations. Questions 1-4 Complete each of the following statements with One or Two names of the scientists from the box below. Write the appropriate letters A-E in boxes 1-4 on your answer sheet. A. Attila Grandpierre B. Gábor ágoston C. Neil Edwards D. Nigel Weiss E. Robert Ehrlich 1. ...claims there a dimmer switch inside the sun that causes its brightness to rise and fall in periods as long as those between ice ages on Earth. 2. ...calculated that the internal solar magnetic fields could produce instabilities in the solar plasma. 3. ...holds that Milankovitch cycles can induce changes in solar heating on Earth and the changes are amplified on Earth. 4. ...doesn't believe in Ehrlich's viewpoints at all. Questions 5-9 Do the following statements agree with the information given in the reading passage? In boxes 5-9 on your answer sheet write TRUE if the statement is true according to the passage FALSE if the statement is false according to the passage NOT GIVEN if the information is not given in the passage 5. The ice ages changed frequency from 100,000 to 41,000 years a million years ago. 6. The sole problem that the Milankovitch theory can not solve is to explain why the ice age frequency should shift from one to another. 7. Carbon dioxide can be locked artificially into sea ice to eliminate the greenhouse effect. 8. Some scientists are not ready to give up the Milankovitch theory though they haven't figured out which mechanisms amplify the changes in solar heating. 9. Both Edwards and Ehrlich believe that there is no practical way to test when the solar temperature oscillation begins and when ends. Questions 10-14 Complete the notes below. Choose one suitable word from the Reading Passage above for each answer. Write your answers in boxes 10-14 on your answer sheet. The standard view assumes that the opposing pressures of gravity and nuclear fusions hold the temperature ...10...in the sun's interior, but the slight changes in the earth's ...11... alter the temperature on the earth and cause ice ages every 100,000 years. A British scientist, however, challenges this view by claiming that the internal solar magnetic ...12... can induce the temperature oscillations in the sun's interior. The sun's core temperature oscillates around its average temperature in ...13... lasting either 100,000 or 41,000 years. And the ...14... interactions within the sun's magnetic field could flip the fluctuations from one cycle length to the other, which explains why the ice ages changed frequency a million years ago.
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In the earliest stages of man's development he had nomore need of money than animals have.He was content with very simple forms of shelter,made his own rough tools and weapons and could provide food and clothing for himself and his family from natural materials around him.As he became more civilized,however,he began to want better shelter,more efficient tools and weapons,and more comfortable and more lasting clothing than could be provided by his own neighborhood or by the work of his own unskilled hands.For these things he had to turn to the skilled people such as smiths,leather workers or carpenters.It was then that the question of payment arose. At first he got what he wanted by a simple process ofexchange.The smith who had not the time to look after land or cattle was glad to take meat or grain from the farmer inexchange for an axe or a plough.But as more and more goods which had no fixed exchange value came on the market,exchange became too complicated to be satisfactory.Another problem arose when those who made things wanted to get stocks of wood or leather,or iron,but had nothing to offer in exchange until their finished goods were ready. Thus the difficulties of exchange led by degrees to the invention of money.In some countries easily handled things like seeds or shells were given a certain value and the farmer,instead of paying the smith for a new axe by giving him some meat or grain,gave him so many shells.If the smith had any shells left when he had bought his food,he could get stocks of the raw materials of his trade.In some countries quite large things such as cows or camels or even big flat stones were used for trade.Later,pieces of metal,bearing values according to the rarity of the metal and the size of the pieces,or coins were used.Money as we know it had arrived. 1.Exchange of goods became difficult because _________. A man became more civilized B smiths began to look after land or cattle in their spare time C more and more goods which had no fixed exchange alues came to the marker D farmers hadn't enough grain or meat to provide for skilled workers 2.Money was not used until _______. A paper was invented B people practiced a simple process of exchange C nothing could be offered in exchangeD the exchange of one thing for another became too complicated 3.The best title for this passage is _____. A What is money B What are money's functions. C The importance of money D The beginning of money
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