The purpose of this paper is to explain in greater detail the Global Dialling Scheme (GDS) recommended for use by Schools when connecting VideoConferencing endpoints to the National Research and Educational Networks. It discuss the structure of the H.323 User Number, how it is associated to the E.164 number and how it is integrated into the Global Dialling Scheme. The concept of Neighbouring and Directory Gatekeepers is then introduced and how these are used in conjunction with the Dialling Scheme to identify the end-user (Endpoint) being called.

It is assumed that the reader has a general knowledge of Video Conferencing systems and the standards involved. However, the following technical papers are available to provide more information on these topics:

When initiating an H.323 Video Conference, we need some means of identifying the User or H.323 Endpoint that we wish to conference with. The thought of having to remember IP addresses is daunting enough; but the use of DHCP to dynamically allocate the IP address of an Endpoint means that this method is impractical. Hence the concept of a Dial Plan and the use of an H.323 User Number registered to a Gatekeeper.

A Dial Plan is simply a method of allocating a unique number to an H.323 Endpoint. This number is referred to as the H.323 User Number and when registered with a Gatekeeper, such as that embedded in RADVISION's Enhanced Communications Server, (ECS) or Emblaze-VCON's Media Xchange Manager, (MXM)™, we have a means of translating this User Number into an IP address. Basically, when an Endpoint registers, the Gatekeeper places an entry into a table that maps the H.323 User Number to the current IP address of the corresponding Endpoint. Hence, it does not matter if the H.323 Endpoint uses DHCP to dynamically allocate an IP address as if it changes, it would automatically get updated when it re-registered with the Gatekeeper.

The H.323 User Number is also loosely referred to as the E.164 Number. This is because the International Telecommunications Union - Technical Section, (ITU-T) has created a recommendation (standard) that has a similar goal; recommendation E.164 - The International Public Telecommunications Numbering Plan used in the PSTN (Public Subscriber Telephone Network) defines the E.164 Number as the number used by the calling party to establish a call to the end user. Put simply, it defines globally how each telephone is allocated with a unique number. Hence the similarity and why the H.323 User Number is loosely referred to as the E.164 Number. The ITU-T recommended length of the E.164 Number is 17 digits.

As stated earlier, the Dial Plan is simply a method of allocating a unique number to an H.323 Endpoint. In determining what your Dial Plan should be and how it is to be structured, you have to decide upon how this number will vary to uniquely identify the H.323 Endpoint as well as how it will be publicised to users who may want to call you. This will most likely be determined by your organisations size, network structure and just how global you want to be seen.

Companies and Institutions who operate globally need to develop a Dial Plan that addresses every potential endpoint, no matter where it may be. An example of such a Dial Plan that adheres to the E.164 scheme has been implemented in the UK by the JANET network and other NRENs (National Research and Educational Networks) around the world to support international H.323 conferencing. Implementation of this Dial Plan is mandatory for participation over these networks.

JANET is the UK's education and research network that is managed by UKERNA. In May 2001, the JANET H.323 Architecture Group was formed to make recommendations on how to provide a central H.323 Videoconferencing service.

Acting behalf of UKERNA, the Architecture Group, other NRENs in Europe and ViDeNet in the USA agreed on an H.323 numbering scheme. This was the GDS, (Global Dialling Scheme) and is used throughout the academic, research and school communities around the world.

The above diagram shows the structure and components of the GDS number. UKERNA assigns and manages the GDS Zone Prefixes in the UK, with the day-to-day mamagement of the JANET Directory Gatekeeper, and hence the list of assigned GDS Zone Prefixes, delegated to the JANET Videoconferencing Service Management Centre at the University of Edinburgh.

The UKERNA assigned part consists of the 2 digits 00 that represents the International Prefix followed by the 2 digits 44 that represents the Country Code for the UK followed by 5 digit beginning with 0 that represents the Zone Prefix.

Currently, blocks of Zone Prefixes are assigned to different groups of users.

• 01YYY - HEIs, FECs and other organisations with a JANET primary connection
• 02YYY - Regional Broadband Consortia and Local Authorities
• 03YYY - Commercial organisations

The Endpoint Extension Number is assigned by the Local Management Authority (LMA) such as a Univerisity, Regional Broadband Corsortia, Local Authority and Charity.

For a more detailed explanation, see: http://www.jvcs.video.ja.net/docs/E164policy.pdf

There is a UK wide project funded by the DfES (Department for Education & Skills) to pilot video conferencing between schools using the JANET network. This project is also managed by UKERNA in collaboration with the Regional Broadband Consortia (RBCs).

Whilst the marority of organisations using the GDS allocate just 3 digits to specify the Endpoint ID, schools in England and Scotland expand the scheme to use 8 digits; making the absloute E.164 number 17 digits long and matching the maximum recommended by the ITU-T.

The above diagram shows the structure of the GDS number used by schools.

The UKERNA assigned part consists of the 4 digits 0044 that represents the International Prefix and Country Code for the UK followed by a unique 5 digit Local/Regional Prefix beginning with 02 that represents either a Local Education Authority or Regional Broadband Consortia.

The RBC Technical Group recommended that the structure for the End User assigned part should consist of the following 3 components; Category, Organisational ID and Endpoint Number.

Category - A single digit is used to define the following current categories.

• 1 - School
• 2 - Museums, Library and Archives
• 3 - Local Authority
• 4 - Adult or Community Learning Centre

Organisational ID - 4 digits is used to define the particular organisation within the category. For schools, this will be the last 4 digits of their 7 digit DfES national school number and will be unique within the LEA.

Endpoint Number - 3 digits defined by the organisation to uniquely identify an endpoint.

For a more detailed explanation, see: http://www.ja.net/community/schools/vc/E164.pdf

Although the H.323 standard describes the Gatekeeper, as an optional component, it is in practice an essential tool for defining and controlling how voice and video communications are managed over the IP network. Gatekeepers are responsible for providing address translation between an endpoints current IP address and its various H.323 aliases, call control and routing services to H.323 endpoints, system management and security policies.

Gatekeepers provide the intelligence for delivering new IP services and applications. They allow network administrators to configure, monitor and manage the activities of registered endpoints, set policies and control network resources such as bandwidth usage within their H.323 zone. Registered endpoints can be H.323 Terminals, Gateways or MCU's, (Multipoint Control Units).

Only one Gatekeeper can manage an H.323 zone, but this zone could include several Gateways and MCU's.

As stated earlier, the Gatekeeper defines the zone and manages the registered endpoints within. To call an endpoint within the same zone, we simply dial that endpoints H.323 User Number. But what happens when we want to call an endpoint that is located in another zone? Well, we then also need to know the zone where that endpoint is registered. Each Gatekeeper on the same network is identified by a unique number, its Zone Number. To call an endpoint in a different zone, we prefix that endpoints H.323 User Number with its Zone Number and dial this extended number.

The telephone analogy to the Gatekeeper Zone Number is the STD code for the local exchange. If we want to telephone a person locally, we just dial their local number, but if we want to telephone somebody further afield, we need to prefix their local number with their STD code.

Behind the scenes, all the Gatekeepers on the network must know how they are related to eachother. The diagram below shows the two different relationships in which Gatekeepers can be networked and interoperate together.

When Gatekeepers are arranged in a single tier 'Peer-to-Peer' manner with no particular hierarchical structure, they are termed as being Neighbour Gatekeepers. This would typically be on a corporate network within a multi-site company who has a Gatekeeper at each site. Each Gatekeeper manages its own site (Zone), with inter-zone communications routed directly between zones and controlled on an individual basis specifically defined by the direct relationship between each Gatekeeper.

When the Gatekeepers are arranged in a multi-tier manner with a hierarchical structure, they are termed as being Directory Gatekeepers (DGK). This would typically be within a large scale deployment such as the national schools network. Whilst each Gatekeeper still manages its own zone, inter-zone communications are routed indirectly on a Parent-Child basis between zones.

A Directory Gatekeeper only knows its Parent and Child Gatekeepers as well as any directly registered endpoints. If the Gatekeeper does not know the Zone of the dialled number, it routes the call to its Parent DGK, which then searches its database to see if the Zone is known. If not known, this Parent routes the call to its Parent and so on until it eventually reaches a Parent DGK that has a Child DGK that matches the Zone. The call is then routed down through each Child DGK tier until it reaches the specific endpoint.

In the above scenario School A only has one endpoint, 124161, which is registered with its RBC Gatekeeper 02706. This RBC Gatekeeper is registered to its Parent DGK, the JANET UK National Gatekeeper 44, which is registered to the World Gatekeeper 00. Hence the full E.164 number of the endpoint 124161 is 00 44 02706 124161

School B has several endpoints 100, 101, 102 that are registered with its own Gatekeeper 13040. This Gatekeeper is registered as a Child to its Parent DGK, the RBC Gatekeeper 02750. Like other RBC Gatekeepers, it is also registered to the JANET UK National Gatekeeper 44. Hence the full E.164 number of the endpoint 100 is 00 44 02750 13040 100

School C is in the Netherlands and has one endpoint, 67, registered to the Regional Gatekeeper 0323053. This Gatekeeper is registered to the Netherlands Gatekeeper 31, which like the JANET UK Gatekeeper is registered to the World Gatekeeper 00. Hence the full E.164 number of the endpoint 67 is 00 31 3023053 67

Taking this example further, for endpoint 124161 at School A to call endpoint 67 at School C, they would dial 00 31 3023053 67. This call would be forwarded to Gatekeeper 02706, which as it does not know Zone 00 31, it routes the call up a tier to its Parent DGK, the JANET UK National Gatekeeper 44. As this does not know Zone 00 31, it routes the call up a tier to its Parent DGK, the World Gatekeeper 00. Now the World Gatekeeper knows Zone 31 as it's Child Gatekeeper, the Netherlands Gatekeeper, so it routes the call down a tier. And the Netherlands Gatekeeper knows Zone 0323053 as one of it's Child Gatekeepers, so it routes the call down a tier to it. Finally, Gatekeeper 0323053 completes the call by forwarding it to it's registered endpoint 67.