Resource Availability Indication

586 Chapter 8: Call Admission Control and QoS Signaling

When the number of accessible channels is known, the utilization can be calculated from the following formula:

Utilization = Channels being used

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Accessible channels

Suppose, for instance, that you have four T1 CAS circuits. Two of the T1 CAS circuits are used for incoming calls, and the remaining two T1 CAS circuits are used for outgoing calls. You have busied out 46 time slots of the outgoing time slots, and you have one call on one of the outgoing time slots. You will have the following:

The outgoing accessible channels in this situation are as follows:

1 (voice channels being used) + 1 (free voice channels)= 2

The DS0 utilization for this device is as follows:

Channels being used

Utilization = -------------------------------------------------

Accessible channels

or

1

50% = ----------------

( 1 + 1)

The utilization for the outgoing channels is equal to 50 percent. If the configured high threshold is 90 percent, the gateway will still accept calls. Only DS0s reachable through a VoIP dial peer are included in the calculation.

The preceding calculation took the DS0 resources into consideration. Remember that the DSP resources are monitored and calculated in the same manner. The terminating gateway sends an RAI message in the event that either the DS0 or DSP resources reach the low or high threshold.

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.

Resource-Based CAC 587

Figure 8-17 Service Provider VoIP Network Topology

Service Provider VoIP

Backbone

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.

588 Chapter 8: Call Admission Control and QoS Signaling

RAI in Enterprise Networks

RAI is generally less applicable in enterprise networks than in SP networks because there is often only one gateway at each site, as shown in Figure 8-18. This is almost always true for the typical hub-and-spoke enterprise network. Even at the large sites, there may be multiple T1/E1 trunks to the attached PBX, but there are seldom multiple gateways.

Figure 8-18 Enterprise VoIP Network Topology

If a single gateway is used to terminate a call, where the called user resides on a specific PBX and is reachable only through a specific gateway in the network, RAI does not provide additional network intelligence. With no alternate gateway to handle excess calls, a call will always