Switching to VoIP

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Chapter 7: Replacing Call Signaling with VoIP

signaling protocol, then H.225 can establish the media parameters immediately, fore-

going the H.245 session after the called party has picked up the phone. In this event,

the RTP session will be open by the time the called party answers. This is useful in

minimizing the perception of lag in call startup when gateways are involved.

SoftPBX H.323 implementations

Because H.323 is such a distributed approach (gatekeepers, gateways, MCUs can all

be on different hardware), it’s tempting not to think of the standard as a way of

achieving software-based PBX functionality. But in reality, most softPBX makers

who’ve standardized on H.323 are putting gatekeepers, gateways, and MCUs all in

one box and calling it a softPBX. This is an accurate description of what a fully func-

tional H.323 network does—uses software to switch calls across multiple data links

using a variety of physical interfaces, the same purpose as a softPBX.

Avaya, Nortel, and others that support H.323 offer most of these functions on a sin-

gle Ethernet host—often a server blade that mounts inside their PBX chassis.

Cisco offers a consolidated approach in their media gateways. While they don’t offer

an MCU, Cisco’s media gateways do provide H.323 gatekeeper and gateway func-

tions and therefore are softPBXs in their own right. H.323 terminals can register with

a Cisco media gateway if it’s running a gatekeeper and place calls through it to the

PSTN if it has a POTS or PRI interface with a phone company dial-tone plugged in.

Telephony vendors’ use of the term media gateway doesn’t necessarily

mean an H.323 gateway, a device for connecting VoIP networks to the

PSTN. Cisco’s media gateways, for example, are modular router/

server hybrids that offer some softPBX functionality, often including

an H.323 gateway, but not always.

Project 7.1. Build an H.323 Gatekeeper Using Open H.323

What you need for this project:

• Open H.323 software

• GnuGK (Gnu gatekeeper) software

• H.323 softphone such as MS NetMeeting

• LAN

Open H.323 is an open source implementation of the H.323 signaling protocol suite,

managed by QuickNet Technologies, the same company that makes the Internet

Phone Jack line of analog interface cards. Open H.323 is distributed in binary and

source code forms for both Linux and Windows, though a crafty hacker should be

able to get it running on a BSD-ish OS, too.

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H.323

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This project will allow a Microsoft NetMeeting H.323 softphone and an Open H.323

OhPhone softphone to place calls through an H.323 gatekeeper running on a Linux

computer. In this example, we’ll use Microsoft NetMeeting on Windows XP and

OhPhone on Mac OS X.

Although Open H.323 provides a framework of tools for developing H.323 servers

and endpoints, it also natively implements a complete H.323 gateway, MCU, and

endpoint. Here’s a partial list of software packages that accompany Open H.323:

OpenGK

A simple H.323 gatekeeper server example

OhPhone

An H.323 softphone for Linux and Windows (OhPhoneX is the Macintosh ver-

sion)

OpenMCU

An H.323 conference bridge server

PSTNgw

An H.323 gateway server

Each of these requires the base distributions of Open H.323 and its prerequisite,

PWLib, a project-specific class library.

Open H.323’s requirements

A machine with specs like those required by Asterisk should be sufficient to run a

small-scale H.323 gatekeeper. A Pentium III clocked at 600 mHz will be able to han-

dle this project. The PC should be running Linux (though H.323 is also Windows-

compatible). It can be the same PC that runs Asterisk, if you like.

Download and compile Open H.323

The best place to get Open H.323 is from its maintainer’s web site, http://www.

openh323.org/code.html. Compiling all of these elements is pretty straightforward on

Linux. If you want to run Open H.323 on Windows, find the precompile executa-

bles. That said, these instructions are for Linux.

First, download and install PWLib. Save pwlib_1.5.2.tar.gz (or the filename appro-

priate for the version you download) to /root as the root user. Then, unzip and untar

it:

# tar xvzf pwlib_1.5.2.tar.gz

Now, you’ll need to set some environment variables so Open H.323 software knows

where to find the PWLib libraries:

# PWLIBDIR=$HOME/pwlib

# export PWLIBDIR

# OPENH323DIR=$HOME/openh323