PDF of this tutorialIn an effort to differentiate their products from their competitors, network designers can customize their gatekeeper by implementing the following optional features:
- call control routing
- call authentication
- call authorization/access
- call accounting
- bandwidth management
- call management services
- supplementary services
- directory services
Call Control Routing
There are two models for call routing: direct mode and routed mode. The routed mode is the more widely preferred model. When the gatekeeper performs address translation, the gatekeeper provides endpoints with the transport address for the call signaling channel destination.
In the direct mode, the gatekeeper provides the endpoints with the address of the destination endpoint and directs them to the call-signaling channel so that all messages can be exchanged directly between the two endpoints without gatekeeper involvement (see Figure 2).
Figure 2. Direct Call Signaling
With the routed mode, the gatekeeper provides its own address as the destination address so that it receives all call-signaling messages and handles routing the call signals between itself and all endpoints during a session. In this case, the gatekeeper keeps a signaling channel open while routing the call for the duration of the call. The routed mode is fundamental for call management, as it performs line-hunting functions, provides separate control over each leg of the call by disconnecting and reconnecting each leg separately, and provides supplementary and proprietary services. This gatekeeper-routed model is the preferred method for ensuring the efficient delivery of supplementary services as well as a more robust management of the network (see Figure 3).
Figure 3. Routed Call Signaling
Call Authentication
The gatekeeper has the capability to perform call-authentication functions by identifying the user or proving a given token or certificate.
Call Authorization/Access
The gatekeeper authorizes a call based on the user’s access rights. It may reject calls from a terminal as a result of authorization failure. The reasons for rejection may include but are not limited to restricted access to or from particular terminals or gateways and restricted access during certain periods of time. Corporate management information system (MIS) and information technology (IT) management have the flexibility to determine the criteria as to whether or not authorization passes or fails, based on security reasons such as restricted access to services or out-of-zone calls.
The network manager may choose to admit all requests under certain low-use circumstances. It is important to note that admissions control is a way to preserve the integrity of the calls and sessions that are in progress when a user requests access. Policies can also be implemented to terminate an ongoing call to process a higher-priority call request.
Call Accounting
Once the call is terminated, the gatekeeper notifies the accounting entity of the call details. The gatekeeper can also function in cooperation with back-office systems to generate billing. The information generated can include many details such as call duration, origin, destination, and QoS.
Bandwidth Management
The gatekeeper can control and limit the number of H.323 terminals allowed to use the network simultaneously. Through H.225.0 signaling, the gatekeeper is able to limit the bandwidth of the call to less than what was requested as well as reject calls from a terminal if it determines that there is not sufficient bandwidth available on the network to support the call. The gatekeeper can work in conjunction with a QoS server to achieve better QoS for calls. This function also operates during an active call when a terminal user requests additional bandwidth.
Call Management Services
The gatekeeper is able to maintain a list of ongoing H.323 calls that is similar to PBX logs. This information is necessary to indicate that a called terminal is busy and to provide information for the bandwidth management function.
Supplementary Services
Supplementary services per the H.450 standard, such as call FORWARD and TRANSFER, are critical telephony functions enterprise users will expect their network to provide. Both the gatekeeper and terminal can provide support for supplementary services; however, the gatekeeper performs it with less computational complexity and load on the client.
Directory Services
The gatekeeper’s database contains user profiles that provide the necessary information for implementing directory services to help users find one another. It can access other directory services (such as Internet locator service [ILS]) that are updated or configured with the necessary information for connecting calls.
