PDF of this tutorialThe gatekeeper is the focal point of the H.323 multimedia network. The H.323 standard is implemented in production networks through zones. Zones are the set of endpoints over which one and only one gatekeeper has jurisdiction. They include any number of terminals, gateways, and multipoint conferencing units (MCUs); any combination of these entities may register with the gatekeeper. Regardless of the physical location of the gatekeeper program code, there must be only one active runtime gatekeeper per zone. Zones can be defined according to geographic locations (such as different branch locations) or in accordance with overlap of a physical network connection (such as a subnet on the floor of a building or a range of IP addresses) or by a functional (organizational) paradigm (see Figure 1).
Figure 1. The Gatekeeper Environment
A gatekeeper manages all zone activities. Whenever an H.323 entity goes live, it will either send out a query to the network, asking which (if any) gatekeepers are present and willing to accept a registration request from this entity or send a registration request to a predefined gatekeeper. This endpoint discovery and registration process is a prerequisite for gatekeeper zone management.
The choice of gatekeeper is critical to the optimal operation of a total H.323 solution. The gatekeeper allows developers the ability to scale up the system to a large number of users while taking care of interzone call routing.
Identifying endpoints in a zone is done using IP addresses, alias names (such as H.323 identifiers, e-mail addresses, and universal resource locators [URLs]) or phone numbers. The gatekeeper is the focal point for insertion of logic into the H.323 network. It can be configured and controlled remotely by third-party applications using hypertext transfer protocol (HTTP) or signaling network management protocol (SNMP). For example, a network planner can configure a gatekeeper to allow a specific set of endpoints into the zone and provide users with unique policies and procedures, as shown in Table 1.
| Required Zone Management Procedures | Optional Policies and Procedures |
|---|---|
| address translation | call authorization |
| admissions control | bandwidth management |
| bandwidth control | supplementary services |
| directory services | |
| call-management services | |
| call control signaling |
Table 1. Policies and Procedures
