H.323 Version 2 - Overview

Approved in January of 1998, version 2 of the H.323 standard addresses many deficiencies in version 1 and introduces new functionality within existing protocols, such as H.245 and H.225, as well as new protocols. The purpose of this document is to provide a high level overview of the changes that were defined by Version 2. For a more thorough explanation, please refer to the appropriate standard. These standards can be found on the International Telecommunications Union website at


The H.235 standard addresses four general issues when dealing with security, Authentication, Integrity, Privacy, and non-Repudiation. Authentication is a mechanism to make sure that the endpoints participating in the conference are really who they say they are. Integrity provides a means to validate that the data within a packet is indeed an unchanged representation of the data. Privacy/Confidentiality is provided by encryption and decryption mechanisms that hide the data from eavesdroppers so that if it is intercepted, it cannot be viewed. Non-Repudiation is a means of protection against someone denying that they participated in a conference when you know they were there. Hooks for each of these security features are specified in H.323 Version 2. The proper usage of these hooks is specified in H.235.

Fast Connect (a.k.a. Fast Start)

Fast Connect is a new method of call setup that bypasses some usual steps in order to make it faster. In addition to the speed improvement, Fast Connect allows the media channels to be operational before the CONNECT message is sent, which is a requirement for certain billing procedures.

Supplementary Services

Supplementary Services for H.323, namely Call Transfer and Call Diversion, have been defined by the H.450 series. H.450.1 defines the signaling protocol between H.323 endpoints for the control of supplementary services. H.450.2 defines Call Transfer and H.450.3 Call Diversion. Call Transfer allows a call established between endpoint A and endpoint B to be transformed into a new call between endpoint B and a third endpoint, endpoint C. Call Diversion provides the supplementary services Call Forwarding Unconditional, Call Forwarding Busy, Call Forwarding No Reply and Call Deflection. The hooks for Supplementary Services are specified in H.323 Version 2. The proper usage of these hooks is specified in H.450.x.

T.120/H.323 Integration

Version 2 of H.323 restricts the way T.120 and H.323 calls should be made when both protocols are to be used in the same conference. Version 1 allowed establishing a T.120 call first, but Version 2 dictates that the H.323 call model is to be followed, and T.120 is to be treated as a type of Logical Channel, just like an audio or video channel. A transport address is furnished when the channel is opened, along with instructions on whether the called side should initiate the conference or wait to be called. Further, version 2 states that T.120 is an optional part of an H.323 conference and that enabling T.120 is at the discretion of each H.323 endpoint.


Tunneling is the process of sending H.245 PDUs through the Q.931 channel. The same TCP/IP socket that is already in use for the Call Signalling Channel is also used by the H.245 Control Channel.

Call Identifier

The Call Reference Value (CRV) method of identifying a call in Version 1 was not sufficiently unique when going through a gatekeeper. The Version 2 Call Identifier is a globally unique ID, so you can always correctly identify which call a packet is referencing, even when going through a gatekeeper.

Overlapped Sending

Overlapped Sending is a method of call setup where the calling party sends only a partial address to the gatekeeper in the Admission Request. If the gatekeeper does not receive enough of the address to be able to route it correctly, it asks for more information. The calling party keeps sending a more and more complete address until the gatekeeper accepts it. The advantage of this approach is that while a user is keying in an address, the process of routing the call can be taking place simultaneously, so the connection can be established quicker.

The same type of mechanism is supported in Q.931, so if you call into a PBX or Call Center, you can attempt to route the call using partial address information.

Progress Message

The Progress message may be sent be an endpoint or Gatekeeper to indicate the progress of a call in the event of interworking with the SCN. This may be used to merely indicate that a Gateway in the call is interworking with the SCN or to indicate the presence of in-band information.

Conference List

If an MCU hosts multiple conferences and wishes to provide a calling endpoint with a choice of conferences to join, it can send the calling endpoint a list of conferences in a Q.931 Facility Message. For backward compatibility with version 1 endpoints, conference lists are only provided to version 2 (or higher) endpoints.

Multiple Aliases in Call Redirection

Version 2 of H.323 allows you to specify the alias address of a called endpoint in the Q.931 Setup message when this information is needed to traverse multiple Gateways. You can also specify extra channels in the case of a 2*64 Kbps call on the WAN side, for instance.

New Alias Types

An alias address in RAS or Q.931 may represent an endpoint or it may represent conferences that an endpoint is hosting. An alias address can be used when the actual address of an endpoint (or the conference ID) is not known. The caller may call a gatekeeper and give the gatekeeper the alias of the endpoint that the caller wishes to reach. The gatekeeper resolves the alias and connects to the appropriate endpoint. Version 2 adds support for four additional alias types: Email, URL, Transport ID and Party Number.

Empty Capability Set (Third Party Re-routing)

Version 2 of H.323 gives a special meaning to receipt of an empty H.245 capability set. When a node receives an empty capability set, it must close all of its channels, enter a "paused" state, and wait for a non-empty capability set to restart the H.245 session. This will effectively allow a gatekeeper to re-route connections from an endpoint that does not support Supplementary Services.

Dynamic Replacement of Channels

This version 2 feature allows for seamless changing of modes from one codec to another without the need for two media decoders. Since opening and closing of H.245 logical channels is not synchronized with media content, media dropout could occur between the time of closing a logical channel and the opening of its replacement. This new method allows the avoidance of such media dropout.

User Input

The H.245 User Input Indication PDU has been updated to allow more complete DTMF signaling. Previously, interoperability was not guaranteed since the PDU did not contain enough information to ensure uniform interpretation of the signals. New structures have been added to provide the length of tones, and other missing information.


QoS structures have been added to the H.245 OLC packets to allow endpoints to set QoS parameters for the media streams, including RSVP parameters. H.323 only communicates QoS information between H.323 devices. Actual reservation and control of resources is outside the scope of the standard.

Remote Logical Switching

This H.245 feature gives the MC the ability to tell the receiver which media stream to render. For example, if multiple video streams are going to the receiver, the MC will tell the receiver which stream to display.

H.263 Enhancements

The H.263 video capability has been increased significantly. Many new structures have been added to the H.245 capability set to fine-tune the H.263 video stream.

Arbitration of GSM Audio Compression

GSM is a new audio capability available to the user. This new capability must be conveyed via H.245.

Arbitration of V.Chat

V.Chat is a chat protocol. H.245 now allows for the capability exchange of V.Chat.

Conference Requests

There are a number of new H.245 conference request packets that can be transmitted. They are all focused towards control (conductorship) of the conference.

Request In Progress Message

Each RAS request has a timeout value and a number of retries associated with it. If no response has been received at the end of the timeout, the request is retried. The sender performs only the given number of retries before the request is considered failed. The new Request In Progress (RIP) message allows a receiver of a request to say "I'm going to process your request, but I won't have it ready before the old timeout expires, so apply this new timeout value instead." This avoids the confusion of having to process multiple instances of the same request, or multiple responses to the same request.

Alternate Gatekeeper

In order to provide redundancy in systems that use a Gatekeeper, the Gatekeeper may indicate alternate Gatekeepers that may be used in the event of a primary Gatekeeper failure. The client should use these alternatives if a request to the gatekeeper does not respond, or if it returns a reject without redirect.

Alternate Endpoint

An endpoint may indicate a backup, redundant, or alternate Transport Address. This allows an endpoint to have a secondary network interface or a secondary H.323 endpoint as a backup. The Gatekeeper will reject ambiguous registrations.

Time To Live

The Time To Live parameter can be requested by the endpoint or specified by the gatekeeper during registration. It indicates the duration (in seconds) for which that registration is valid. This mechanism allows the gatekeeper to not worry about its "database" filling up with entries from endpoints that are no longer active.

Keep Alive

The Keep Alive parameter is sent in a lightweight Registration Request from an endpoint to its gatekeeper to indicate a desire to keep the registration active for a given amount of time. This is useful if the registration is about to expire based on the endpoint's Time to Live.

Q.931 Information in IRR

Many direct-model calls still involve a gatekeeper, for reasons of endpoint management, network management, address management, or location services. Even if the gatekeeper is not routing the call, it may be required to have knowledge of the progress of the call (e.g. for accounting purposes). The gatekeeper may request that the endpoint notify it of H.225.0 call signaling messages that the endpoint sends or receives. The Gatekeeper indicates the list of PDU types it wishes to receive copies of, and it indicates whether it wants copies when the PDUs are sent or when the PDUs are received.

Information Request ACK and NAK

An endpoint may want some of the unsolicited Information Responses to be delivered reliably. This could be especially important if the information is required for billing, as described above.

Resource Availability

In Version 2 of the RAS protocol, a gateway has the ability to notify a gatekeeper of its current call capacity (i.e. what data rate it can currently support) for each protocol, and whether the gateway is at or near capacity. This can be used to help the gatekeeper make intelligent decisions on how to route calls.

Endpoint Type Prefixes and Data Rates

Endpoint Types for gateways can now indicate the data rates supported for each protocol the device supports. They can also indicate the prefixes associated with supported protocols, and in some cases also with the data rates.


In an Admission Request, an endpoint may indicate its capability to reserve transport resources. In the Admission Confirm, the gatekeeper may indicate to the endpoint where the responsibility lies for resource reservation. (i.e. endpoint-controlled, gatekeeper-controlled, or no resource reservation needed). H.323 only communicates QoS information between H.323 devices. Actual reservation and control of resources is outside the scope of the standard.

Pre-granted ARQ

When an endpoint registers with a gatekeeper, the gatekeeper has the ability to pre-grant admission requests to that endpoint. This allows the endpoint to make and answer calls without asking the gatekeeper for permission, thus reducing call setup time.