ITU Recommendations are the standards that define the way telecom networks interoperate. While compliance is not required, generally operators conform because it provides assurance of interconnectivity of networks on a worldwide scale.
There is a general belief that the strategic importance of making on-line access to ITU-T Recommendations free outweighs the costs (in terms of lost revenue) to ITU. This is seen as a way to increase the transparency of ITU-T work and encourage wider participation in ITU-T activities. It is also believed that this policy will help increase developing countries’ awareness of pertinent issues and help to promote the participation of academia in ITU-T work.
There are more than 3000 Recommendations, on topics such as service definition, network architecture, security, optical transmission systems, next-generation networks and more.
You can access the ITU recommendations database here.