This sudden expansion of the market has resulted in substantially improved quality, raised the level of audio fidelity and strengthened support for industry-standard protocols, such as the ITU-T's Recommendation H.323. Thus fortified, VoIP technology is beginning to carve a niche in corporate networks. The question is, is it really ready to make this leap?

After giving VoIP technology a tryout across Network Computing's own distributed network, we're convinced that it's a bit premature to roll it out across an entire corporatewide enterprise network. Concerns about interoperability, security and bandwidth management are creating static on the line between VoIP and widescale deployment.

For example, while we managed to coax equipment from several vendors to interoperate at a very basic level, we could do so only by using the G.711 codec. But this generated tremendous utilization across our frame relay and ISDN networks, resulting in periodic signal loss, particularly when other traffic was introduced to the network. On top of that, our attempts to use features such as "hold" or "transfer" across vendors' product lines forced calls to drop. Although H.323 specifies that these features should be implemented, vendors are not yet doing so consistently.

There's good reason to believe these hang-ups will disappear over the next year or so. Vendors in this area will incorporate support for additional low-bandwidth codecs, and feature-implementation issues also are expected to be resolved.

But that doesn't necessarily mean you should wait until next year to dip your toes in the VoIP waters. While the technology clearly is not in shape for enterprisewide deployment today, it is eminently suitable for interoffice, long-distance, toll-bypass service, and even for isolated LANs that have the right infrastructure.

Segmenting the Technology
Every enterprisewide corporate telephone network has the same basic components, including end-user equipment (telephones, premises wiring) and back-end gear (PBXs, trunk lines). VoIP devices generally fall into these same two camps, with IP-centric equipment replacing analog handsets and wiring, and IP-based equivalents filling in for PBX and/or interconnect wiring.

Although most VoIP equipment today employs proprietary protocols, many vendors are beginning to support the ITU-T's Recommendation H.323 standard. This highly modular version of the H.320 multimedia-over-ISDN specification is tailor-made for packet-based networks (see "H.323 and Alternatives" on page 55). H.323 defines a variety of node types, the most common of which are identical to those in today's typical voice networks: terminals for the desktop, gateways for bridging the packet network to a standard telephone network, and gatekeepers that set up calls and provide other administrative services to the various devices.