在TCP中，使用了(26)来保证网络中不出现重复请求报文，而流控则使用了(27)。

(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).