●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．

(71) A．HDLC

B．Frame. Relay

C．X．25

D．X．21

(72) A．a peer to peer

B．a point-to-point

C．not a peer to peer

D．a peer to peers

(73) A．a primary station

B．one or more primary stations

C．a secondary station

D．one or more secondary stations

(74) A．a primary station

B．one or more primary stations

C．a secondary station

D．one or more secondary stations

(75) A．half duplex

B．full duplex

C．half or full duplex

D．half and full duplex