A case study: DX 200 Exchange
			n SSU: Subscriber Signaling Unit: controls access network n CCSU:Common Channel Signaling Unit (SS7). n CCMU: Common Channel Signaling Management Unit: (as MTP, SCCP) n PAU: Primary Rate Access Unit: controls basic (64 kbit/s) system interfaces n  LSU: Line Signaling Unit: takes care of signaling between transit exchanges and access networks n MFSU: MULti-frequency Service Unit: Takes care of signaling when multiple frequency signals are used
	MTP: Message transfer part of SS7 SCCP: Signaling connection control part
					=SS7 bearer part: network service                               part (NSP)