单项选择题 Pharming is a scamming practice in which malicious code is installed on a personal computer or server, misdirecting users to 1 fraudulent Web sites without their knowledge or consent. Pharming has been called "phishing without a lure". In phishing, the perpetrator sends out legitimate- 2 looking e-malls, appearing to come from some of the Web's most popular sites, in an effort to obtain personal and financial information from individual recipients. But in pharming, larger numbers of computer users can be 3 victimized because it is not necessary to target individuals one by one and no conscious action is required on the part of the victim. In one form of pharming attack, code sent in an e-mail modifies local host files on a personal computer. The host files convert URLs into the number strings that the computer uses to access Web sites. A computer with a compromised host file will go to the fake Web site even if a user types in the correct Internet address or clicks on an affected 4 bookmark entry. Some spyware removal programs can correct the corruption, but it frequently recurs unless the user changes browsing. 5 habits,
Serialization delay and 37 delay are the two components of network delay that are improved by increasing bandwidth. Serialization delay, i.e. the amount of time it takes to put the 38 on the wire, and queuing delay (depth of the queue) are improved by increasing the 39 from a 128Kbps circuit to a T1. However, three other components of delay, routing/switching deiay, distance delay, and protocol delay are components that can not be positively affected by an 40 in bandwidth. If the circuits are not over-utilized, then increasing the bandwidth to improve the 41 of the application will only result in an increased bandwidth with no positive effects on performance.
[解析] 在ATM网络中，有两个接口十分重要。即用户—网络接口(UNI)和网络—网络接口(NNI)，前者定义了主机和ATM网络之间的边界，后者定义了两台。ATM交换机之间。ATM信元头有40位，GFC字段只出现在UNI中，一般流量控制字段GPC(Generic Flow Control)，又称接入控制字段。当多个信元等待传输时，用以确定发送顺序的优先级。信元从用户端进入网络的第一个交换机是UNI口，改变的是 GFC字段。 ATM信元传输采用异步．时分复用(Asynchronous Time Division Multiolex)，又称统计复用(Statistic Multiptx)。信息源随机地产生信息，因为信元到达队列也是随机的。高速的业务信元来得十分频繁、集中，低速的业务信元来得很稀疏。这些信元都按先来后到在队列中排队，然后按输出次序复用到传输线上。具有同样标志的信元在传输线上并不对应某个固定的时间间隙，也不是按周期出现的池即信息和它在时域的位置之间没有关系，信息只是按信头中的标志来区分的。而在同步时分复用方式(如PCM复用方式)中，信息以它在一帧中的时间位置 (时隙)来区分，一个时隙对应着一条信道，不需要另外的信息头来标识信息的身份。 VPI字段用于选择一条特定的虚通路，VCI字段在一条选定的虚通路上选择一条特定的虚电路。当进行VP交换时，是选择一条特定的虚通路。 未定比特速率(UBR：Unspecified Bit Rate)：对传输速率没有指定，但可靠性要求很高；即所谓“尽力传输”(Best Effort)，用于局域网仿真 (LAN Emulation)。 不变比特速率(CBR：Constant Bit Rate)：有固定的带宽(速率)要求，适用实时的话音和视频信号传输。 可用比特速率(ABR：Available Bit Rate)：只需指定峰值(Peak)和谷值(Minimum)信元速率，应用不多。 可变比特速率(VBR：Variable Bit Rate)：允许随时可变的带宽，但必须指定峰值带宽、最大突发数据长度和必须维持的最低速率。 实时可变比特速率(rt-VBR：Real Time Variable Bit Rate)：主要用于速率可变的实时业务(如视频监控和压缩话音通信等业务)。 ATM适配层共有6种类型：AAL0,AAL1,AAL2,AAL3,AAL4, AAL5。但是只有采用AAL5时，信元净荷Payload才能达到48Bytes。即有效的数据传输率是48/53＝90.5%。
localhost ～]# ls -lh install.log -rw-r--r-- 1 root root 53K 03-16 08:54 install.log
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