In order to ensure standardised H.323 videoconferences can take place both within the UK and world wide, an H.323 Global Dialing Scheme (GDS) has been developed. SURFnet in the Netherlands, HEAnet in Ireland and ViDeNet in the US have also implemented this scheme. The GDS has been developed because a standardised H.323 addressing scheme is yet to be created. The GDS uses standards based E.164 addressing.

The GDS is based on numeric addressing which allows the greatest flexibility in interfacing with other communication devices e.g. ISDN based videoconferencing systems and conventional or mobile phones. The GDS comprises four components:

• an international dialling prefix, which is specified as 00;
• the ITU-T country telephone code, that is between one and three digits, in the case of the UK 44;
• a zone prefix, which is five digits starting with a zero e.g. 01100 that is the zone prefix for an organisation;
• a three to five digit extension number.

Here is an example number that might be used to dial a system in a university:

Zone prefixes are assigned by JANET(UK) in the UK and by other National Education and Research Networks in other countries. Once an organisation has installed a gatekeeper it can apply to the JVCS to register it. After successfully completing the registration a zone prefix will be assigned. The organisation can then assign its own three to five digit extension number. The only restriction to the allocation of extension numbers is that they cannot start with a zero.

The diagram below shows how a UK call would take place between Swansea University and Edinburgh University (grey arrows), and how an international call would take place between Swansea University and SURFnet in the Netherlands.