For references for each gatekeeper, see the end of each gatekeeper configuration chapter. The following are for generic information:
An Introduction to H.323 Videoconferencing, D. Price and A. J. Spence.
JANET Videoconferencing Service – IP, R. Bolam. https://community.ja.net/library/janet-services-documentation/ip-videoconferencing
Video Technology Advisory Service https://community.ja.net/library/advisory-services/video-technology-advisory-service