安全的威胁可分为两大类，即主动攻击和被动攻击。通过截取以前的合法记录稍后重新加入一个连接，叫做重放攻击。为防止这种情况，可以采用的办法是(50)。一个计算机系统被认为是可信任的，主要从其受保护的程度而言的，WindowsNT4.0以上版本目前具有的安全等级是(51)。

(65)

(64)

(63)

SDLC was invented by IBM to replace the older Bisynchronous protocol for wide area connections between IBM equipment . A varietion of the (71) protocol called NRM is essentially the same as SDLC. SDLC is (72) protocol like HDLC, Frame. Relay or X.25. An SDLC network is made up of (73) that controls all communications, and (74) . It is capable of (75) operation, but almost all practical applications are strictly half duplex.

(62)

(59)

(60)

(57)

(58)

In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.

 Spread spectrum simply means that data is sent in(66)pieces over a number of the discrete frequencies available for use at any time in the specified range. Devices using(67) spread spectrum (DSSS)communicate by splitting each(68)of data into several parts and sending them concurrently on different (69), DSSS uses a lot of the available(70), about 22 megahertz (MHz).