International Telecommunication Union Telecommunication Standardization Sector (ITU-T)
The International Telecommunication Union (ITU), headquartered in Geneva, Switzerland, is the leading United Nations agency for information and communication technologies. It has three sectors - ITU-R for radiocommunication, ITU-T for standardization, and ITU-D for development.
The Telecommunication Standardization Sector (ITU-T) specifies a lot of Telecommunication and Data communication protocols.
The ITU-T (formerly the CCITT) is an international organization within the United Nations System where governments and the private sector coordinate global telecom networks and services.
ITU-T Recommendations: You can download ITU protocol specification from this page, but they cost money.
ITU-T Recommendations for free: As of 2007, PDF formats of in-force, posted ITU-T recommendations can be downloaded for free.
ASN.1 Module Database: ASN.1 Descriptions of some protocols can be downloaded for free.