题目内容：
根据下列题干，回答题：SDLC was invented by IBM to replace the older Bisynchronous protocol for wide area connec- tions between IBM equipment. A varietion of the （ 1 ）protocol called NRM is essentially the same as SDLC. SDLC is （ 2 ）protocol like HDLC , Frame Relay or X. 25. An SDLC network is made up of （ 3 ） that controls all communications , and （ 4 ）. It is capable of （ 5 ）operation, but almost all practical ap- plications are strictly half duplex.
请选择（ 1 ）处最佳答案
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