- http://alumnus.caltech.edu/~dank/isdn/isdn_ai.html
- http://networking.ringofsaturn.com/Cisco/ciscoisdn.php#_troubleshooting
- http://isdn.modemhelp.net/causecodes/causecodelist.shtml
- http://www.protocols.com/pbook/h323.htm
- http://www.ak-nord.de/en/daten/manual_isdn.pdf
- http://www.cisco.com/en/US/tech/tk801/tk379/technologies_tech_note09186a0080093c1a.shtml
- http://www.cisco.com/en/US/tech/tk801/tk379/technologies_tech_note09186a0080094b78.shtml
- http://www.cisco.com/en/US/tech/tk801/tk379/technologies_tech_note09186a0080094b77.shtml
- http://www.cisco.com/en/US/docs/ios/12_1/debug/command/reference/dbdipx.html
- http://www.jet.net/isdn/isdnintro.html
- http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1917.html
- http://www.cisco.com/en/US/tech/tk801/tk379/technologies_tech_note09186a0080094bb8.shtml
- https://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1917.html
- http://www.idstech.com/troubleshoot.htm
- http://www.teach-ict.com/as_a2/topics/telephone_systems/telephone_systems/isdn.htm