A testing and switching (tester) system is disclosed for information transmission and communications networks and systems and the like. According to a preferred embodiment, a data transmission network is advantageously of the type in which remotely located users or customers are afforded time-shared access to a centrally located computer. The data transmission network includes remotely located switching stations which are linked to the central computer over data transmission lines which include selectively actuated normal and fallback local loops. The network users gain access to the centrally located computer over selectively actuated local lines to the remotely located switching stations. The tester system comprises switching and testing apparatus which includes a programmable tester computer located at the remotely located stations and preferably at the centrally located computer site also. The tester computer is programmable from the centrally located computer site and is preferably under command control of the centrally located computer site. The tester system tests, including programmed and manual tests, network data transmission parameters at sites having switching and testing apparatus, and also performs programmed and manually controlled switching at those sites. The tester system is especially suited to testing and switching in a telephone data transmission network in which the data transmission lines are typically leased data transmission long lines and in which the selectively actuated local lines to the remotely located stations are part of the telephone company DDD network. Access to the remotely located stations by the centrally located computer site is also gained over selectively actuated lines which advantageously form part of the DDD network. The tester system advantageously tests and measures voice and data transmission parameters including signal level, phase jitter, and noise. Methods and apparatus are also disclosed for performing these tests.