Telecom Tips - Building Your Own Conference Bridge

by Brian McConnell

Conference calling is a poorly implemented feature on most telephone systems (do you even know how to set up a conference call on your PBX? if not, join the club). "Meet Me" conferencing systems and services make this much easier to do.

In "Meet Me" conferences, you give participants a telephone number and extension to dial. This rings into a virtual conference room that allows up to several dozen people to participate in a two-way conversation. This is the easiest way to deal with conference calls because it is analogous to walking in or out of a real conference room. People simply call 800-nnn-xxxx extension xyz at the appointed time, and can come and go as they please.

The problem is that many telephone systems make setting a conference call up difficult at best. The conference host, or secretary, must often call out to people and manually add them to a conference. The results can be frustrating to say the least, especially if participants are on cellular phones.

If your telephone system doesn't have built-in meet me conferencing, there are a couple of ways you can do this. One is to subscribe to outsourced conferencing services such as Webex, FreeConference.Com, and others (most long distance companies offer some type of teleconferencing service. There's no equipment to buy. You just tell participants what 800 number and extension to dial at an appointed time.

However, if you conduct a lot of conference calls, the toll charges can add up, in which case you might want to buy your own conference bridge. Off the shelf conference bridges are expensive beasts, costing several hundred dollars to a thousand dollars per port (phone line). A system big enough to handle 100 callers can cost a pretty penny.

If you don't mind getting your hands dirty, you can build an El Cheapo conference bridge using the Asterisk open source PBX package combined with inexpensive telephone network interface cards from Digium (Asterisk's corporate sponsor). Asterisk supports meet me conferencing out of the box.

To configure this feature in Asterisk, edit the meetme.conf file to map one or more extension numbers to conference rooms (as shown in the example below, which creates three conference rooms numbered 9000, 9001 and 9002).

; Configuration file for MeetMe simple conference rooms
; for Asterisk of course.
; Usage is conf => confno,pincode,adminpin
conf => 9000
conf => 9002,123456
conf => 9003,123456,654321

The conference bridge can be accessed in one of several ways depending on the type of telephone system you have, whether it supports voice over IP, and how easily it can be expanded:

  • Internal T1/PRI - here you connect an internal T1 or ISDN Primary Rate circuit from the PBX directly to the conference bridge. This is a good option if your PBX is expandable, but does not support VoIP. This way you don't have to order a local loop T1 from the local phone company, which will cost you several hundred dollars per month.

  • Connect direct to public telephone network - order one or more local loop T1/PRI circuits from your phone company, connect them directly to the conference bridge. This is a good option if your PBX is not expandable. The downside is you have to pay for more local telephone service.

  • Connect to PBX via Voice over IP. If your PBX or local telephone provider supports SIP, H.323 or IAX voice over IP service, you can route calls to Asterisk via your LAN/WAN. This is the best option if it's available as you can eliminate the need for T1 interface cards in the Asterisk box, as well as expansion cards for your PBX.

Diagram - Conference Bridge Connected to PBX


Diagram - Conference Bridge Attached to Public Telephone Network


NOTE: installing and configuring an Asterisk PBX is not trivial, and involves telephone network interface configuration, extension numbering and dial plans, which is beyond the scope of this short recipe.

How much does an Asterisk conferencing bridge cost. If you'll be using digital telephone service (T1 or ISDN Primary Rate), you'll need:

  • A fast Linux box with plenty of memory (~$700)

  • A Digium T1 interface card (~$700 for a single T1, ~$1500 for a quad T1 card)

  • Asterisk PBX software (FREE)

  • One or more T1 or ISDN PRI circuits (23 or 24 simultaneous callers per circuit), connected from the Asterisk box to your PBX or directly to the public telephone network

Compare this to a conventional conference bridge, which will typically set you back $10,000 to $20,000 just for an entry level model. Granted, Asterisk is freeware, and experimental compared to telephone systems that have been in use for well over a decade. However it is so much less expensive, it's worth playing around with just to learn what you can do with it.

Do you know of other teleconferencing services that support meet me conferencing, post your comments and references here...


2006-03-14 14:54:17
Admin pin code in conference room
Please let me know what the admin pin code in a conference room allows the admin person to do and how.
2006-04-04 09:51:47
Conference Bridge
Does the conference bridge from the Digium Asterisk solution have any of the following features?

1) Sercurity passwords to control access to who can call into a conference bridge.
2) Conference Leader passcodes that restrict the callers from talking to one another until the conference leader has logged in.
2) Name Announce which asks the caller to announce themselves and then plays their name when they call prior to placing the caller into the bridge