"History of EEsof and Touchstone at Microwaves 101". http://www.microwaves101.com/encyclopedia/historyCAD.cfm#touchstone
"W2200 Advanced Design System (ADS) Core". http://www.keysight.com/en/pd-1385381-pn-W2200BP/advanced-design-system-ads-core
.S1P for one-port (e.g. termination), .S2P for two-port (e.g. transmission line), .S3P for three-port (e.g. a balun), .S4P for four-port (e.g. a differential transmission line) and so on.
"IBIS (I/O Buffer Information Specification)". http://www.ibis.org/
"Touchstone File Format Specification Version 2.0" (PDF). http://www.ibis.org/touchstone_ver2.0/touchstone_ver2_0.pdf
"Touchstone File Format Specification Rev 1.1" (PDF). http://www.ibis.org/connector/touchstone_spec11.pdf
"Touchstone File Format Specification Version 2.1" (PDF). https://ibis.org/touchstone_ver2.1/touchstone_ver2_1.pdf
"E8854A RF System, now part of W2200 Advanced Design System (ADS) Core". http://www.keysight.com/en/pd-1385381-pn-W2200BP/advanced-design-system-ads-core