ODQDB同时支持(33)两种服务。DQDB子网的双总线结构由(34)总线以及接在这两条总线上的大量的节点组成。DQDB网络为双总线提供了(35)访问控制方式，其中能够提供非等时服务是(36)，它用于(37)业务。

(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).