1. AT&T Network Averages.
https://ipnetwork.bgtmo.ip.att.net/pws/averages.html
2. Bitcoinj.
https://bitcoinj.github.io/working-with-micropayments
3. BitInfoCharts, Bitcoin avg. transaction fee.
https://bitinfocharts.com/comparison/bitcoin-transactionfees.html
4. BloXroute: A Scalable Trustless Blockchain Distribution Network.
https://bloxroute.com/wp-content/uploads/2018/03/bloXroute-whitepaper.pdf
5. Board of Governers of the Federal Reserve System, press release June 2011.
https://www.federalreserve.gov/newsevents/pressreleases/bcreg20110629a.htm