H.323 and Alternatives
Of the dozens of VoIP products available now, only a handful support any standards-based implementation. The only standard with any notable presence is the ITU-T's Recommendation H.323, a version of the H.320 Multimedia-over-ISDN standard optimized for packet-based networks such as TCP/IP. Although H.323 is not specific to IP (it could also be used with IPX or AppleTalk), it relies on some IETF technologies--most notably RTP (Real Time Protocol) and RTCP (Real Time Control Protocol), which were developed within the IETF's Multimedia Working Group.

This ambivalence toward IP has created some sticky points of operation. For example, the call-setup negotiation routines in H.323 dictate that the end-point systems allocate random port numbers for the RTCP control channel and RTP data channel. While this allows for considerable portability across different kinds of packet-based networks, it makes implementing H.323 across an IP firewall very difficult. Rather than using a well-known port for all voice traffic, every H.323 node on the network must be able to listen for and send on any port number above 1,024. This is an impossible request for most corporate networks; no organization will open its entire corporate network to all UDP and TCP traffic.

You can work around this, of course. The easiest solution is to contain all H.323 traffic within a specific region of your network. If you filter traffic between your corporate backbone and your branch-office networks, you will need to keep the H.323 traffic contained within those sites and rely on voice trunks for any interconnection services. Another option is to use a firewall that is H.323-aware; such products are becoming widely available.

Another VoIP standard in the works is SIP (Session Initialization Protocol). It is currently under development within the IETF's Multimedia Working Group, with a particular focus on IP implementation. SIP offers many of the same architectural features of H.323, but relies on IP-specific technologies, such as DNS, as well. It also incorporates the concept of fixed port numbers for all devices and allows the use of proxy servers, both of which ease firewall implementation concerns.

Another standard making the rounds at the IETF is SGCP (Simple Gateway Control Protocol), which was developed by Bellcore. SGCP introduces a new call-management tier known as the Call Agent, which off-loads much of the signaling intelligence from the end node. This makes it a good fit for traditional telephone handsets. SGCP also promises to reduce the delays associated with H.323's use of signaling translators and TCP/IP.

Finally, at press time, IPDC (Internet Protocol Device Control) was announced. Developed by Level 3 and friends, IPDC is intended for use between centralized switches and IP-based gateways, providing integration and management on a very large scale.

SIP, SGCP and IPDC are in draft form and a long way from implementation. Until then, limit your purchases to products that support H.323. Although only a small percentage of vendors support H.323, it's your only guarantee of interoperability. No matter what else a vendor tells you, if a product doesn't support H.323, tell the rep to come back when it does.