在系统转换的过程中，旧系统和新系统一起工作一段时间，再由新系统代替旧系统的转换策略称为(15)。

(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.