The purpose of this paper is to explain in greater detail the use of H.323 Dial Plans and Service Codes used by Gatekeepers, Gateways, dedicated MCUs and Endpoints in initiating services used in Video Conferences. It discusses the H.323 User Number (E.164 number) and explains how it is used by the Gatekeeper to identify the end-user (Endpoint) being called. The concept of Service Codes for Gateways and dedicated MCUs is then introduced and how these are used in conjunction with the Gatekeeper and H.323 User Number to initiate more complex conferences. Finally, we introduce the concept of IP Video Telephony functions and how the whole H.323 infrastructure can be managed by Emblaze-VCON's Media Xchange Manger™, MXM

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.

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 Telecommunications Sector of the International Telecommunications Union, (ITU-T) has created a recommendation that has a similar goal; recommendation E.164 - The International Public Telecommunications Numbering Plan, defines the E.164 number as the number used by the calling party to establish a call to the end user. The ITU recommend 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 the Dial Plan should be and how it is to be structured, you have to decide how this number is to uniquely vary, how it will identify the H.323 Endpoint and how it will be publicised to users who want to identify you. Probably the most common way of uniquely identifying a person, group or team within an organisation is their telephone extension number. This number is also published and updated within the organisation. Therefore, an obvious form of Dial Plan would be to simply use the endusers telephone extension number, or prefix this with numbers that identifies the equipment type, location and as being used for Video Conferencing.

In the above example, the Dial Plan identifies the equipment as an VSX™7000s located in Design.

Dedicated H.323 Multipoint Control Units, (MCU), such as the SCOPIA₁₀₀ 12/24 Series from RADVISION offer various network services such as Continuous Presence or Voice Activated Video Switching between multiple users. These services are identified by using unique pre-defined Service Codes that are registered with a Gatekeeper. To initiate a conference that uses a SCOPIA₁₀₀ 12/24 service, the caller prefixes the Service Code to the endusers H.323 User Number. If more than one user is being called, the additional H.323 User Numbers are also separated by the delimiter. When the Gatekeeper receives the call, it identifies the service, the device that offers it and translates the H.323 User Numbers into an IP addresses so that the call can be established.

Using the above Service Codes as an example, if we wanted to initiate a Continuous Presence conference with a password of 1234 to 3 other users, whose H.323 User Numbers are 6125789, 6235678 and 6325980, we would call 751234**6125789**6235678**6325980.

Alternatively, the SCOPIA₁₀₀ 12/24 could have its Service Codes registered with and managed by MXM in order to take advantage of MXM's additional IP Video Telephony features such as Ad-Hoc Conferencing.

Gateways, such as the SCOPIA₁₀₀ B40 from RADVISION offer various LAN to WAN services as well as WAN to LAN dialling. These services are identified by using unique pre-defined Service Codes that are registered with a Gatekeeper. To initiate a LAN to WAN conference that uses an gw-B40 service, the caller prefixes the Service Code to destination phone number. If more than one phone number has to be dialled, the additional phone numbers are separated by a delimiter. When the Gatekeeper receives the call, it identifies the service and passes the destination phone numbers to the Gateway that offers it so that the call can be established.

Using the above Service Codes as an example, if we wanted to initiate an ISDN Video Conference at 128 kbps to an H.320 device whose phones numbers are 597202 and 597203, we would call 82597202,597203. If the two channels had the same number, 597202, we would call 82597202,

Alternatively, the SCOPIA₁₀₀ B40 could have its Service Codes registered with and managed by MXM in order to take advantage of MXM's additional Gateway services such as Simplified Outbound Dialling.

Emblaze-VCON's Media Xchange Manager™, MXM is a software based IP Video PBX. From a remote console, an authorised user has the ability to perform centralised management, configuration and administrative functions across multiple zones. MXM supports a combination of both Emblaze and other vendors endpoints as well as a wide range of MCU's, Gateways and other networking equipment. With IP Video Telephony services such as Call Forward, Call Transfer, Call Pickup, Ad-Hoc Conferencing and Hunting Groups, MXM provides the functions that make Video Conferencing as simple as making a telephone call.

With the click of a button, registered MXM users can Invite a 3rd Party into their Video Conference using a revolutionary new feature called 'Ad-Hoc Conferencing'. Similarly, whilst in a Video Conference, they can Transfer the call to another user. Users can Forward incoming calls 'unconditionally' or if they are 'busy', or Pickup a 'Ringing' call intended for another user.

A software version of VCON Conferencing Bridge™, VCB is available as an optional module of MXM that provides a basic H.323 MCU capability with both Continuous Presence or Voice Activated Video Switching modes. Alternatively, VCB v5 is a hardware version of VCB that includes MXM in a 2U chassis.

The Dial Plan and Service Codes must be unique between the Multipoint Control Units, Gateways, Video Telephony Services and H.323 User Numbers to ensure that they do not clash!