为了进行差错控制，必须对传送的数据帧进行校验。在局域网中广泛使用的校验方法是循环冗余校验。当接收端发现错误后采取的措施是(62)。

(143)

(144)

(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.