Choosing a VoIP PBX

All In the Details

How many trunks to the PSTN will you need? This will dictate the number of simultaneous incoming and outgoing calls you will be able to support. If you have an existing PBX, you should be able to do a "busy study" on trunk utilization to obtain the number of concurrent, simultaneous calls supported. You can also ask the phone company to do this for you, but don't expect it to give you results overnight.

Next you need to determine how you want to handle incoming calls. Most vendors support DID (direct inward dialing), which means that every phone can be reached from the outside. But if you want inbound calls fielded by an operator, make sure the system can be set up with an attendant console. Additionally, be sure you can monitor how quickly calls are answered. Another option is an automated attendant to direct calls to specific extensions through a menu system. Automation is fine, but be sure that the system can supply you with reports showing the call-completion rate. If a lot of callers are bailing out before they are satisfied, you'll want to make adjustments.

A more specialized method for handling incoming calls is an ACD (automatic call distributor). This is often used in a customer-service environment, where a pool of individuals are available to answer the next call. Make sure the system lets you monitor calls in progress for quality and can provide statistics on the performance of those answering calls, hold times and call completion rates.

Level Playing FieldStandards for phones give manufacturers incentive to mass-produce products that support those standards. That drives down the price and ups the feature set. In our survey, vendors indicated that the most common standards their phones support are H.323, MEGACO (Media Gateway Controller) and SIP (Session Initiation Protocol). H.323 has been around the longest, while SIP is the newcomer. One of the vendors listed in our Buyer's Guide charts, Zultys Technologies, supports only SIP. But the company supports five phone vendors, more than any other survey participant.

Alcatel supports SIP, MEGACO and H.323, and three phone vendors. It's important to clarify which features are supported on each corresponding protocol, regardless of the vendor. Siemens and Shoreline support only their own VoIP phones, though Shoreline pointed out that it supports analog phones from other vendors.

If you've ever gone to Wal-Mart to buy a phone for your home, you took it for granted that an analog phone will work with any vendor's phone system. Unfortunately that's not true with VoIP. SIP has emerged as the standard that will make cross-vendor support possible in the future (for more information about the SIP protocol, see "It's Time To Take a Look At SIP").

Before purchasing a VoIP phone system, you have to verify that your data network is up to the task. At minimum you will want to have switching on your LAN. If you're not moving around large amounts of data, 10 Mbps to the desktop may be sufficient, but 100 Mbps will give you a little more headroom. Most VoIP phones have a built-in, two-port switch that lets you share your Ethernet connection with your PC. Most also support QoS (quality of service), which will ensure that your voice packets get priority if you blast a lot of data from your PC.

You'll also want to have both bandwidth and QoS on your backbone connections, which is a concern when bandwidth gets expensive, as with WAN connections. Consider putting your switches and your PBX on UPSs, so you won't lose your phones during a quick (or prolonged) power outage. This works especially well if you can power the phones from the switches. The IEEE 802.3af (Power over Ethernet) standard is about to receive final ratification, and many vendors are releasing switches and phones with support for this standard.All the vendors that responded to our Buyer's Guide survey say they support analog phones. You will still need some analog phone connections for fax machines or for modems and to support systems that automatically call analog phones or beepers. You will likely be using legacy connections for access to the PSTN as well.

Peter Morrissey is a full-time faculty member of Syracuse University's School of Information Studies, and a contributing editor and columnist for Network Computing. Write to him at [email protected].

Post a comment or question on this story.We wouldn't recommend ordering a PBX based solely on our Buyer's Guide charts. You should get a detailed quote and make sure the PBX has all the features you need to support your business and end users. Of course, pricing will vary based on your needs.

Security is also an important consideration. Using a VPN is a common way to ensure that communications going across the Internet are encrypted. When you combine a VPN with IP codecs, it starts to add milliseconds of latency. When this gets into the triple digits, it can seriously impact your call quality. Before you sign on the dotted line, verify that you can put your IP phones and the IP PBX behind a firewall (see "Hold the IP Phone").

Peruse our online survey results. Choose the vendor whose offering most closely matches your price range and requirements. Then issue an RFI (Request for Information) or an RFP (Request for Proposal) and send it to your local resellers. These companies usually sell and install the IP PBXs.Be sure to account for the cost of maintenance and configuration changes such as adding new external trunks. And beware of long-term relationships and costly after-sales support. Technology is becoming more standards-based. If your requirements change, you will want some flexibility. If you find this to be a bit overwhelming and your budget permits, a consultant can save you time and money by keeping the resellers honest and helping to negotiate the best deal for you.Interactive Buyer's Guide Charts on IP PBXs
Digital convergence white papers & research reports
Digital convergence books
IEEE802 Ethernet standards
The Society of Telecommunication Consultants
Telephony and the standards and protocols related to VoIP PBXs