Finally, you can bring VoIP to the desktop via high-end dedicated telephony equipment that off-loads all telephony services from the PC, such as Selsius Systems' H.323 telephones. Selsius' telephone units look and feel like regular multifunction handsets, but they have Ethernet jacks instead of RJ-11 ports. Using dedicated processors, firmware-based codecs and a local TCP/IP stack, these phones offer the highest level of quality and reliability of any H.323 terminal on the market.

Back-End Integration
We believe VoIP today is best-suited for use at the back end, where it can be used as a toll-bypass service. Most high-end vendors are working this angle, with first-generation products focusing on the H.323 gateway space.

H.323 gateways come in many flavors, as you can see in the "Mixed Voice and Data Network" and "Total VoIP Network" diagrams (on pages 42 and 46).

Toll-bypass gateways, for example, work as a VoIP bridge between voice networks, conceptually similar to the voice-over-frame-relay products we tested earlier this year (see "FRADs Make Sound Sacrifices To Get the Data Through," at www.networkcomputing. com/902/902r1.html). This kind of gateway lets you take voice traffic from one PBX and route it to another PBX (local or remote), using H.323 and IP as the interconnect technology instead of voice trunks. Unlike voice over frame relay, voice over IP works with any underlying network technology.

This type of implementation lets you use the Internet--or a private data network--for interoffice calls, greatly reducing long-distance toll charges, particularly for international calls. Let's say your company spends 9 cents a minute on calls between offices, paying $10,000 on such calls every month. If introducing VoIP trunks can trim those net charges to 5 cents, you'll save 45 percent on your monthly bill. That's a savings of $54,000 in annual usage costs alone.

Another class of H.323 gateways consists of those that flip the coin, bridging H.323-based desktop systems with an existing voice network, as shown in the "Mixed Voice and Data Network" diagram (on page 42).

These gateways essentially act as PBX systems in their own right, routing calls between H.323 clients on one side of the gateway and trunk lines on the other. Assuming you have sufficient bandwidth, you can deploy islands of H.323 that you join using analog or digital circuits.

Finally, we come to the H.323 gatekeepers. These are similar to the H.323 gateways described above except they use H.323 on both the desktop and back-end segments of the network (as shown in the "Total VoIP Network" diagram on page 46), eliminating any need for voice trunks. H.323 gatekeepers provide lookup and routing services for the downstream devices under their control, allowing different gateways to be deployed across a network while preserving local extension management and access services.

Most products available today fall into one of the first two categories, either supplying VoIP bridging services for the toll-bypass market or providing PBX/PSTN integration services to H.323 desktop users; there has been little product development to date in the H.323 gatekeeper category. Vendors in the first camp include big-name data networking companies such as Ascend Communications and 3Com Corp., as well as newcomers such as RADVision and VocalTec.

As might be expected, traditional PBX vendors, such as Northern Telecom and Lucent Technologies, are working on H.323-based gateways that plug directly into their existing product lines. Given their familiarity with voice networks, expect their solutions to carry more voice-specific features, though these features may have to be used with these vendors' PBXs as well.

Typically, these VoIP systems include an Ethernet/H.323 interface, as well as POTS, T1 or ISDN PRI interfaces to the voice network. Dialing patterns route calls to specific destinations according to predefined masks (dialing "8-XXX" might route a call to an H.323 gateway at a New York facility, for example, while "9" might route the call to a PSTN gateway).