Understanding IP private branch exchange (PBX)

Private branch exchanges (PBXs) handle call switching and routing within an organization, and IP PBX systems are the next generation of Internet-based PBX communications. Deb Shinder gives an overview of IP PBX systems, and explains how businesses can easily make the switch.

To provide the cost savings of IP-based phone calls for small companies, a couple of VoIP lines with a business plan from a low-cost provider such as Vonage or Lingo will suffice. But as the organization grows, users will want to look at more sophisticated options, the same as if using a public switched telephone network (PSTN).

Medium- to large-sized businesses have long used private branch exchange (PBX) switchboards to route incoming calls from a number of external lines among a larger number of internal lines. This saves money because there's no need to have a separate line for every employee, and makes line sharing more efficient. Most systems in use today are technically PABX, which means the call routing is automated, rather than dependent on a human switchboard operator.

According to an article by Robert Poe in VoIP News, major research firms reported that a majority of PBX systems sold in the first quarter of 2006 were IP-based. Slowly but surely, companies are moving away from traditional PBXs to IP PBX solutions.

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How IP PBX works

To the end user, the IP version of PBX is essentially the same. Under the hood, though, there's a big difference. The heart of the system is the IP PBX server, which works somewhat like a proxy server. With a Session Initiation Protocol (SIP)-based system, the VoIP clients register their SIP addresses with the server, which maintains a database of all the clients and their addresses. (The VoIP clients can be soft phone software installed on a computer or self-contained hard phone devices.)

When a user places a call, the PBX server recognizes whether it's an internal or external call. Internal calls route to the SIP address of the phone or user receiving the call. External calls route to the VoIP gateway.

The company may have its own VoIP gateway incorporated into the system, or use the gateway of a VoIP service provider. The purpose of the gateway is to link an IP system with the PSTN. VoIP gateways can connect either to analog phone lines or to digital lines (ISDN or standard multiplex T-carrier lines).

The gateway device can take the form of a dedicated appliance, or as an expansion card in the server. Like other gateway devices, it has two or more ports to allow it to span multiple networks. In this case, one connection is to the IP network and one is to the PSTN. A virtual PBX refers to a card and PBX software installed on an existing PC.

Traditional PSTN PBX lines connect to the VoIP network (or to the Internet) using an FSX gateway. Or, going in the other direction, PSTN phone lines may connect to an IP PBX with a foreign exchange office (FXO) gateway.

IP Centrex is a service model whereby the VoIP provider owns and hosts the PBX equipment. Hosted IP PBX solutions are turnkey solutions that don't require organizations to have personnel on staff capable of managing the system.

Advantages of IP PBX

Along with the cost savings of VoIP, users can get some extra features with IP PBX that don't typically come with a traditional PBX system. PBX systems support caller ID, three-way calling, call forwarding, voice mail, and similar features that business phone users take for granted. The nice thing about IP-based systems is that users' voice mail messages can be forwarded to their e-mail inboxes, so they not only get instant notification that a call has come in, but can also play the messages from their computers or handheld devices.

Other sophisticated automated features are available, too. "Smart" systems can be set up to route calls differently depending on the caller ID information. Thus, a user could have calls from his or her boss forwarded directly to a cell phone, while calls from that pesky sales rep automatically go to voice mail.

Businesses also find the monitoring and reporting capabilities of IP systems useful in maintaining an audit trail and keeping up with costs. The system can track usage by phone number/SIP address, monitor performance information and Quality of Service (QoS), and detect security breaches.

Why not IP PBX?

What are the disadvantages of transitioning to an IP-based PBX system? There can be a large upfront cost. In addition to the IP PBX hardware and/or software, IP phones may need to replace traditional handsets—or adapters used to allow them on the VoIP network.

Even worse, if the IP network is an outdated one, an upgrade to the entire network may be necessary to support high quality voice transmissions, which require more bandwidth than many data transmissions.

Finally, implementing an IP PBX system on-site (as opposed to using a hosted service) requires personnel with knowledge and experience in VoIP. It's an IT specialty area with which many systems administrators are unfamiliar.

IP PBX vendors

There are hundreds of vendors who make IP PBX equipment. This list just provides a sampling and is by no means conclusive:

  • Axon Virtual PBX software from NCH Swift Sound can be installed on almost any Windows computer (server operating system is not required) to provide switching between multiple phone lines and extensions. It uses a SIP protocol to connect to VoIP hard phones or soft phone clients, and is especially good for small- to medium-sized businesses on a budget. Axon is a free product that can be downloaded at http://www.nch.com.au/pbx/index.html.
  • SwyxWare is a Windows server-based PBX software that comes in two flavors: Compact for small businesses (10 or fewer users) and Essential for larger businesses. It's scalable in that users can add features and applications such as conferencing by buying a new license key. Learn more at http://www.swyx.com/uk/products/product.html?product=18.
  • Cisco Unified CallManager is an enterprise-level VoIP call processing system that supports sophisticated features such as multimedia conferencing, and can handle up to 30,000 lines per server cluster. Users can implement this system in both Windows and Linux environments. See http://www.cisco.com/en/US/products/sw/voicesw/ps556/index.html for an overview.
  • Sphericall from Sphere Communications is a software-based IP PBX for Service Oriented Architecture (SOA) and can scale to 30,000 ports. It uses a distributed software architecture rather than clustering to increase reliability/fault tolerance. Find out more at http://www.spherecom.com/sphericall_ip_pbx.php.
  • Summary

    IP PBX technology is growing in sophistication and popularity, and can provide businesses with substantial cost savings due to ease of installation (resulting in much lower rollout costs), calling features that are not standard on traditional PBX equipment, better monitoring and usage tracking, and lower overall calling costs.

    By Deb Shinder

    Debra Littlejohn Shinder, MCSE, MVP is a technology consultant, trainer, and writer who has authored a number of books on computer operating systems, networking, and security. Deb is a tech editor, developmental editor, and contributor to over 20 add...