以下关于IEE802.3ae标准的描述中，错误的是（）。 A．支持802.3_爱下问搜题

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问题单项选择题

以下关于IEE802.3ae标准的描述中，错误的是（）。

A．支持802.3标准中定义的最小和最大帧长

B．支持IEE802.3ad链路汇聚协议

C．使用1310nm单模光纤作为传输介质，最大段长可达10公里

D．使用850nm多模光纤作为传输介质，最大段长可达10公里

答案

参考答案：D

解析：

[分析]： IEEE 802.3ae万兆以太网技术标准的物理层只支持光纤作为传输介质，但提供了两种物理连接。一种是与传统以太网进行连接的、速率为10Gb/s的LAN物理层设备，即 LAN PHY，另一种是与SDH/SONET进行连接的速率为9.58464Gb/s的WAN物理层设备，即WAN PHY。通过引入WAN PHY，万兆以太网的帧可以与SONETOC-192帧结构融合，从而能够通过SONET城域网提供端到端的以太网连接。

每种物理连接都可使用10GBASE-S (850nm短波)、10GBASE-L (1310nm 长波)和10GBASE-E (1550nm 长波)这三种规格的传输介质，最大传输距离分别为300m、 10000m和40000m。

在物理拓扑上，万兆位以太网既支持星状连接(或扩展星状连接)，也支持点到点连接，以及星状连接与点到点连接的组合。在万兆位以太网的MAC子层，已不再采用 CSMA/CD机制，只支持全双工传输方式。另外，万兆以太网还继承了802.3以太网的帧格式和最大/最小帧长度，从而能充分兼容已有的以太网技术，进而降低了对现有以太网进行万兆位升级的成本。

单项选择题

Questions 11-15People value money desperately because they value one another desperately; thus the cause of panic in the stock-market plunge is not that people will lose their dollars but that they will lose their sense of community. For the past couple of weeks, the nation has watched itself roll toward ruin because people were losing their money in bales. If one were tasteless enough to ask a big loser what exactly he was losing, he would sputter, "What am I losing My car! My beautiful home! My children’s educations! My clothes! My dinner! My dollars!" They are all true. People have been mourning the passing of their money for all the things that money can do, and what money can do is impressive. Money can build cities, cure diseases, and win wars. The sudden acquisition of the stuff can toss our spirits into the air like a hat.Money can do considerably more. It offers power, an almost unique form of power, not simply because it allows us to acquire and possess things but because it is we who determine its worth; we who say a ruby costs more than an apple; we who decide that a tennis court is more valuable than a book. Paradoxically, money creates a deep sense of powerlessness as well, since technically we cannot provide money for ourselves; someone or something else must do that for us-our employers or, until recently, our stocks. All that, money can do: and when such essential, familiar functions are snatched from one’s life, small wonder that people may grow wild, frantic, and even murderous.What money can do, however, is not the same as what money is. Let’s return for a moment to the theory: people value money because they value one another. In other words, the usefulness of money is directly related to and established by continuous mutual need. People work for money to buy things that other people make or do, things that they cannot or will not make or do for themselves but that they deem necessary for some definition of self-improvement.Abstractly, money is one of the ways, indeed a universally accepted way, by which we make connections. Cash is cold. So the connections may feel cold, but real blood flows through them. These connections constitute one of the central means by which societies cohere; by which they sustain and characterize themselves.When the coin begins to wobble, as it has in the past weeks, a fear seizes the mind that is disorienting. The fear is not merely that of the loss of possessions but of self-possession, which in some sense is bought and sold from person to person in infinite daily bargains. To lose money is frightening. To lose touch with others is more frightening still. Losing touch may cause the panic of the times.

According to the author, what can be a regular source of money provided for us

A．Possessions.

B．Bargains.

C．Stocks.

D．Employers.

问答题简答题

高处作业的危险区应有何防护措施？