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 connections 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 applications are strictly half duplex．
请选择（1）处最佳答案：
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