Within each ITUT umbrella standard, minimum mandatory standards are defined that will guarantee compatibility, albeit at a basic level, e.g. within H.320 provision must be made for H.261 video coding, G.711 audio coding and H.221, H.230, H.242 communications protocols. Similarly for H.323, the corresponding mandatory standards are H.261, G.711 and H.225/H.245 communication protocols. These mandatory requirements will allow all compliant products to communicate easily and effectively.

Some sub-standards are common throughout the range of umbrella standards, e.g. H.261 video coding and G.711 audio coding are mandatory in H.320, H.321, H.323, H.324 and H.310.