RAI in Service Provider Networks

RAI is an indispensable feature in SP networks that provide VoIP calling services such as debit and credit card calling and VoIP long-distance phone service. Figure 8-17 shows the general structure of these networks.

Figure 8-17 Service Provider VoIP Network Topology

Directory-GK HSRP

Directory-GK

Service Provider VoIP Backbone

Alternate

West GK West GK

HSRP

Alternate East GK East GK

Alternate

West GK West GK

HSRP

Alternate East GK East GK

Around the world there are points of presence (POPs) where racks of gateways, such as Cisco AS5300 access servers, connect to the PSTN with T1/E1 trunks. The call routing is managed through several levels of gatekeepers as shown in Figure 8-17. Call volume is high, and these gateways handle voice traffic only (no data traffic other than minimal IP routing and network management traffic).

When a customer on the West Coast dials a number residing on the East Coast PSTN, the East Coast gatekeeper must select an East Coast gateway that has an available PSTN trunk to terminate the call. If an East Cost gateway cannot be found, the customer's call fails. In the event of a failed call, the originating gateway must retry the call or the customer must redial the call. In either case, there is no guarantee that the same out-of-capacity terminating gateway will not be selected again.

This scenario is inefficient and provides poor customer service. It is important that calls are not routed by the gatekeeper to a terminating gatekeeper that cannot terminate the call due to the lack of PSTN trunk capacity.

In general, calls are load balanced by the gatekeeper across the terminating gateways in its zone. But the gateways could have different levels of T1/E1 capacity and by load balancing across the gateways one gateway could become shorter on resources than another. It is in this situation that RAI is imperative. The overloaded terminating gateway has the capability to initiate an indication to the gatekeeper that it is too busy to take more calls.

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