Understanding the Avaya H.323 ALG

The H.323 standard is a legacy voice-over-IP (VoIP) protocol defined by the International Telecommunication Union (ITU-T). H.323 consists of a suite of protocols (such as H.225.0 and H.245) that are used for call signaling and call control for VoIP. The processes for configuring the H.323 standard Application Layer Gateway (ALG) and the proprietary Avaya H.323 ALG are the same.

However, Avaya H.323 ALG has some special features. To understand and configure the Avaya H.323-specific features listed here, see the Administrator Guide for Avaya Communication Manager, Avaya IP Telephony Implementation Guide, and Avaya Application Solutions IP Telephony Deployment Guide at http://support.avaya.com.

This topic contains the following sections:

• Avaya H.323 ALG-Specific Features
• Call Flow Details in the Avaya H.323 ALG

Avaya H.323 ALG-Specific Features

Avaya H.323-specific features are as follows:

• H.323 Fast Connect
• H.323 asymmetric media
• Call waiting
• Call forwarding
• Voice mail
• Call identification
• Conference calling

Call Flow Details in the Avaya H.323 ALG

• Connecting the Phone into the Network—Avaya performs the Q.931 Setup/Connect negotiation when the phone is wired into the network rather than when a call is being initiated.
• Making a call—When a call is made, because the PBX has already stored the capabilities for each phone when the phone is connected to the network, no further Q.931 and PBX negotiations are required to set up the call. It no longer exchanges Q.931 Setup and Connect messages with the PBX. The phone and the PBX exchange H.323 Facility messages to set up the call.
• Registering with a CM—When a call has been made, Avaya H.323 registers with the Avaya Communication Manager (CM). The registration process is similar to a generic H.323 standard registration process.

  Note: The direct mode and tunnel mode are not defined by Avaya H.323 ALG.

  For a call to work, the CM must be deployed with Avaya Endpoints. During the call, RAS and Q.931 messages are exchanged between the CM and the Avaya Endpoints.

  Note: For Avaya H.323 with a source Network Address Translation (NAT) pool, the registration process allows only one IP address in the pool.

• Setting up Real-Time Transport Protocol (RTP)/Real-Time Control Protocol (RTCP) ports—The Q.931 Setup, Facility and Information messages are used to set up RTP/RTCP ports. The hierarchy for an Avaya H.323 session is Q.931, RTP/RTCP, Parent, and then Child.

  Note: H.245 ports are not used in an Avaya call flow process.

• Using Avaya H.323 counters—The counters for calls and active calls are not applicable to the Avaya H.323 ALG. The call creation and tearing down is done by Facility messages afterward. When resources are allocated for a call, all counters for calls and active calls increment. If resources are allocated for a call multiple times, messages belonging to the same call that pass the firewall multiple times will trigger multiple increments of the counters. In other words, messages that belong to the same call and pass the firewall multiple times might trigger multiple increments of the counters if the resource for a call needs to be allocated multiple times.

  For example, in the two-zone case, the setup and connect message pair allocates one call resource. The active call counter is increased once. Each time the setup and connect message pair passes the firewall, a different call resource with unique interfaces and NAT is allocated. Therefore, the counter increments twice in a three-zone scenario.

Related Topics

• JUNOS Software Feature Support Reference for SRX Series and J Series Devices
• ALG Overview
• Understanding H.323 ALGs
• H.323 ALG Configuration Overview