sentences:  The British developed System X (telephony), and other smaller systems also emerged in the early 1980s.Some digital switches, notably the 5ESS and very early versions of Ericsson AXE 10, continued to use analog concentrator stages, using SPC-like technologies, rather than direct connections to the digital line cards containing the CODEC.Early in the 21st century the industry began using a 5th generation of telephony switching, as TDM and specialist hardware based digital circuit switching is being replaced by softswitches and voice over IP VoIP technologies.