EIARS-232C定义了DTE和DCE之间的接口，其机械特性规定RS-232C的D型连接器有(11)个插脚，其电气特性与CCITT的(12)兼容。DTE和DCE之间的接口信号线按功能一般可分为(13)4类，使用EIARS-232C接口进行数据通信时，至少需用的信号线有(14)。V.11建议中规定的电气特性逻辑“1”的电子电压是(15)。

(143)

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(142)

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.

(141)

(139)

(137)

(138)

(136)

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.