Call Flows with a Gatekeeper
Figure 8-14 illustrates how an endpoint locates and registers with a gatekeeper.
464 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
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Figure 8-14 Finding and Registering with a Gatekeeper
A attendant adds scalability to H.323. Without a gatekeeper, an endpoint charge recognize
or acquire the adeptness to boldness the IP abode of a destination endpoint.
Before an endpoint can use a gatekeeper, it charge annals with the gatekeeper. To register,
an endpoint charge admit the IP abode of the gatekeeper.
As ahead mentioned, one of the afterward two methods is acclimated to actuate the
address of the gatekeeper:
■ An endpoint can be preconfigured to admit the area name or IP abode of its
gatekeeper. If configured to admit the name, an endpoint charge acquire a agency to
resolve the name to an IP address. A accepted abode resolution abode is to use
DNS.
■ An endpoint can affair a multicast GRQ to the attendant analysis address
(224.0.1.41) to ascertain the IP abode of its gatekeeper. If the endpoint receives a
GCF to the request, it uses the IP abode to advance with registration.
To admit registration, an endpoint sends an RRQ to the gatekeeper. In the RRQ, the
endpoint identifies itself with its ID and provides its IP address. Optionally, the endpoint
lists the prefixes (for example, blast numbers) that it supports. These prefixes are
gleaned from the POTS dial-peer destination patterns associated with any Foreign
Exchange Station (FXS) port.
With this procedure, a attendant determines the area and character of endpoints and
the identities of SCN endpoints from aperture registrations.
In Figure 8-15, both endpoints acquire registered with the aforementioned gatekeeper.
Figure 8-15 Alarm Breeze with a Gatekeeper
Call breeze with a attendant accretion as follows:
1. The aperture sends an ARQ to the attendant to admit the procedure. The gateway
is configured with the area or abode of the gatekeeper.
2. The attendant responds to the ARQ with an ACF. In the confirmation, the gatekeeper
provides the IP abode of the alien endpoint.
3. When the basic endpoint identifies the absolute endpoint, it initiates a basic
call setup.
4. Before the absolute endpoint accepts the acceptance call, it sends an ARQ to the
gatekeeper to accretion permission.
5. The attendant responds affirmatively, and the absolute endpoint accretion with
the alarm bureaucracy procedure.
During this procedure, if the attendant responds to either endpoint with an ARJ to the
ARQ, the endpoint that receives the bounce terminates the procedure.
Figure 8-16 shows an archetype of gatekeeper-routed alarm signaling.
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Figure 8-16 Gatekeeper-Routed Alarm Signaling
In the antecedent examples, the call-signaling approach is created from endpoint to endpoint.
In some cases, it is adorable to acquire the attendant represent the added endpoint for signaling
purposes. This adjustment is alleged gatekeeper-routed alarm signaling. The action for
gatekeeper-routed alarm signaling is as follows:
1. The attendant responds to an ARQ and advises the endpoint to accomplish the call
setup action with the gatekeeper, not with the absolute endpoint.
2. The endpoint initiates the bureaucracy appeal with the gatekeeper.
3. The attendant sends its own appeal to the absolute endpoint and incorporates
some of the capacity acquired from the basic request.
4. When a affix bulletin is accustomed from the absolute endpoint, the gatekeeper
sends a affix bulletin to the basic endpoint.
5. The two endpoints authorize an H.245 ascendancy approach amid them. The alarm procedure
continues commonly from this point.
Call Flows with Multiple Gatekeepers
By simplifying agreement of the endpoints, gatekeepers aid in architecture large-scale
VoIP networks, as apparent in Figure 8-17. As a VoIP arrangement grows, accumulation additional
gatekeepers enhances the network’s scalability.
Without a gatekeeper, endpoints charge acquisition anniversary added by any agency available. This limits
the advance abeyant of a VoIP network. Through the allotment and abode resolution
services of a gatekeeper, advance abeyant improves significantly.
A distinct attendant architecture ability not be adapted for several reasons. A distinct gatekeeper
can become overloaded, or it can acquire a suboptimal arrangement location. Deploying
multiple gatekeepers offers a added scalable and able-bodied environment.
Figure 8-18 illustrates a alarm bureaucracy involving two gatekeepers.
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H.225.0 RAS H.225.0 RAS
H.225.0/H.245
Call Signaling
RTP Voice Bearer Traffic
Figure 8-17 Scalability with Multiple Gatekeepers
Chapter 8: Configuring H.323 Gatekeepers 467
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Figure 8-18 Alarm Breeze with Multiple Gatekeepers
In this scenario, anniversary endpoint is registered with a altered gatekeeper. Notice the
changes in the afterward alarm bureaucracy procedure:
1. The basic endpoint sends an ARQ to its attendant requesting permission to
proceed and allurement for the affair ambit for the absolute endpoint.
2. The attendant for the basic endpoint (Gatekeeper 1) determines from its configuration
or from a agenda ability that the absolute endpoint is potentially
associated with Attendant 2. Attendant 1 sends an LRQ to Attendant 2.
3. Attendant 2 recognizes the abode and sends aback an LCF. In the confirmation,
Gatekeeper 2 provides the IP abode of the absolute endpoint.
4. If Attendant 1 considers the alarm adequate for aegis and bandwidth reasons, it
maps the LCF to an ARQ and sends the acceptance aback to the originating
endpoint.
5. The endpoint initiates a alarm bureaucracy to the alien endpoint.
6. Before accepting the acceptance call, the alien endpoint sends an ARQ to
Gatekeeper 2 requesting permission to acquire the acceptance call.
7. Attendant 2 performs acceptance ascendancy on the appeal and responds with a
confirmation.
8. The endpoint responds to the alarm bureaucracy request.
9. The alarm bureaucracy progresses through the H.225.0 alarm action and H.245 ascendancy function
procedures until the RTP sessions are initiated.
10. At the cessation of the call, anniversary endpoint sends a abstract appeal to its gatekeeper
to admonish the attendant that the alarm is complete.
11. The attendant responds with a confirmation.