问题详情

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.
A．HDLC
B．Frame. Relay
C．X.25
D．X.21请帮忙给出正确答案和分析，谢谢！