Centralized Call Control
Centralized call control applies when the voice-capable device does not support call control on its own but only through the use of a call agent. In this example, both voice gateways have the MGCP protocol enabled. With MGCP call control enabled, the gateways use the call agent to perform these functions:
■ Process dialed digits and route the call: The figure shows an example of this sequence.
1. After detecting a service request (the phone goes off hook), the first gateway (R1) informs its call agent.
2. The call agent tells R1 to play a dial tone and receive the digits dialed by the user.
3. R1 passes each received digit to the call agent (one by one).
4. The call agent looks up the called number in its call routing table. According to the call routing table, the called number can be reached via the second gateway (R2). R2 is also controlled by this call agent and the call agent, therefore, knows about the phone numbers that can be reached by R2. As a consequence, the call agent knows which port at R2 the call has to be routed to.
5. The call agent now sends a message to R2 requesting that the call be passed to a certain port (the port that connects to the destination phone number).
Both call routing decisions—which gateway to use after the call has been received at R1 and how to pass the call on at that next gateway (R2)—are made by the call agent. This is an example of the centralized call control model, where all call routing intelligence (required for the first stage of a call, the call setup) is at the call agent. The call agent then instructs the gateways how to handle the call. Therefore, only the call agent has a call routing table.
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© 2006 Cisco Systems, Inc.
■ Supervise the call: During the call, R1 and R2 both monitor the quality of the call. In the centralized call control model, if one of the gateways detects that the quality is no longer adequate, it will pass that information to the call agent. The call agent then terminates the call.
Supervision of the call occurs during the second stage of a call, call maintenance.
■ Terminate the call: If the caller that is connected to R1 finishes the call, R1 informs the call agent. The call agent notifies both gateways that the VoIP call should be terminated and resources should be released.
In the centralized call control model, the call agent initiates the third stage of a call, call teardown.
As illustrated in the example, with centralized call control, the gateways do not make any local decisions. Instead, they inform the call agent about events (such as incoming or dropped calls). Only the call agent makes call routing decisions, and the gateways depend on the availability of their call agent. Availability of the call agent is critical, because the call agent is a single point of failure. However, only the call agent needs to have a call routing table. Therefore, administration of the centralized call control model is more scalable.
Centralized call control allows an external device (call agent) to handle signaling and call processing, leaving the gateway to translate audio signals into voice packets after call setup. After the call is set up, the voice path runs directly between the two gateways and does not involve the call agent. The difference between distributed and centralized call control applies only to signaling, never to the media exchange, which always goes on directly between the two gateways.
© 2006 Cisco Systems, Inc. Describe Cisco VoIP Implementations 2-15
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