H.323 specifies mandatory support for the G.711 codec--also known as Pulse Code Modulation (PCM)--a widely available codec used in many forms of digital telephony. But G.711 requires 64 Kbps of continuous bandwidth for every network end point. On a full-duplex voice circuit, a single 64-Kbps feed suffices, but on a packet-switched network such as IP, 128 Kbps of cumulative data is required if two users are speaking simultaneously.

The H.323 standard also specifies a laundry list of more efficient codecs that may be used. The two most popular implementations are G.723.1, which can use 5.3 Kbps or 6.3 Kbps for each end of the connection, and G.729, which uses 8 Kbps at each end. To complicate matters, some first-generation products support G.723.1 while others support G.729. So, to guarantee interoperability among different vendors' products you must use G.711 everywhere--and this means you must expect every call to consume 128 Kbps of continuous network bandwidth, or else you have to implement products from only one vendor.

Security is another major consideration. In version 2 of H.323, encryption and authentication are optional, though most implementations include no security protections at all. As a result, an H.323-aware network analyzer becomes an effortless wiretap. If you're on a shared-media network, anyone can monitor any conversation without ever leaving his or her desk.

Another problem is network congestion, an inevitable result of the high-utilization levels engendered by widespread deployment of G.711. To deal effectively with the congestion, you need to implement prioritization services at the physical, data-link and network layers of your enterprise network. This means using switches instead of hubs, and incorporating 802.1Q and 802.1p within your Ethernet switching fabric (see "Bringing Prioritization Services to Ethernet," www.networkcomputing.com/914/914ws1.html). Alternatively, token ring and FDDI provide these services, so if you have those technologies at your desktop, you're one step ahead of the game. Meanwhile, IP can provide native prioritization services across your entire enterprise, regardless of the media in use, via the already-present IP TOS (type of service) byte. (See "Implementing Prioritization on IP Networks," at www.networkcomputing.com/915/ 915ws1.html, for more on IP TOS.)