A better way to support H.323 across firewalls is to employ a circuit gateway that can disassemble the
packets on the control streams and dynamically open the firewall as indicated. This strategy also requires
implementation of an application proxy device that includes a partial H.323 stack to manage the
connections on both sides. Intel provides online support documentation at its Web site. One commendable
document, "The Problems and Pitfalls of Getting H.323 Safely Through Firewalls" is available at
http://www.intel.com/support/videophone/trial21/h323_wpr.htm.
Now, let's look at calling possibilities over a LAN/WAN. An H.323 gatekeeper's functional software can be
placed in a device that has other roles, such as a router or gateway. But usually, a gatekeeper runs in its
own box and can be thought of as a video PBX. Telephony concepts apply.
End-user devices (telephones) are hard wired into a telephone exchange. In a network that supports H.323,
terminals automatically search for a gatekeeper that they can "see" in their virtual LAN or LAN segment,
and then attempt to register with that gatekeeper. In the registry, the terminal's IP address can be associated
with a recognizable station identifier such as an alias string that looks like a telephone number or an e-mail
address.
As part of the gatekeeper's original configuration, lists of allowed station identifiers are maintained (with
passwords, if desired). These can be partitioned into zones that are similar to telephone area codes. These
not only provide for logical network segmentation, but also can be used to enforce permissions of who can
call whom. Telephony functions, such as Automatic Call Forwarding (to an operator, for instance) or Call
Forward When Busy, can be implemented in a gatekeeper.
The H.323 gateway is an optional device that provides a window to switched circuits and other networks.
The most common application is interconnection with ISDN for H.323>H.320 conferences. However, the
H.323 architecture provides other useful possibilities, including H.324 over POTS lines (useful in
telemedicine home visit applications) and speech-only over POTS lines.
You could use an H.323 gateway to accept dial-in voice calls, carry them over the enterprise backbone
using G.728 codecs and switch them back out to a telephone network in a distant city. This application, IP
telephony, is beyond the scope of this chapter.
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