http://wikiless.i2p/wiki/Public_switched_telephone_network?lang=en
Later, more sophisticated address signaling, including multi-frequency signaling methods, enabled direct-dialed long-distance calls by subscribers, culminating in the Signalling System 7 (SS7) network that controlled calls between most exchanges by the end of the 20th century. The growth of the PSTN was enabled by teletraffic engineering techniques to deliver quality of service (QoS) in the network.