多路复用技术能够提高传输系统利用率。常用的多路复用技术有(36)。将一条物理信道分成若干时间片，轮换地给多个信号使用，实现一条物理信道传输多个数字信号，这是(37)。将物理信道的总频带宽分割成若干个子信道，每个信道传输一路信号，这是(38)。在光纤中采用的多路复用技术是(39)，多路复用技术一般不用于(40)中。

(143)

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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.

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Frame. Relay is simplified form. of(66), similar in principle to(67), in which synchronous, frames of data are routed to different destinations depending on header information.

 Packets are routed throught one or more Virtual Circuits known as(68). Most Virtual Circuits are(69), which means that the network provider sets up all DLCI connections at subscription time.(70)are also part of the Frame. Relay specification. They privide a link that only lasts only as long as the session.