Resource-Based CAC 591

Table 8-13 RAI CAC Evaluation Criteria (Continued)

Cisco CallManager Resource-Based CAC

The term “locations” refers to a CAC feature in a Cisco CallManager centralized callprocessing environment. Because a centralized call-processing model is deployed in a hub- and-spoke fashion without the use of a terminating gateway at each spoke, an IP telephony CAC mechanism must be used to protect the desired number of voice conversations. Locations are designed to work in this nondistributed environment.

Figure 8-20 shows a typical CallManager centralized call-processing model.

Location-Based CAC Operation

The Cisco CallManager centralized call-processing model uses a hub-and-spoke topology. The host site, or hub, is the location of the primary Cisco CallManager controlling the network while the remote sites, containing IP endpoints registered to the primary CallManager, represent the spokes. The Cisco CallManager Administration web page can be used to create locations and assign IP endpoints, such as IP Phones, to these locations. After the locations have been created and devices have been assigned, you can allocate bandwidth for conversations between the hub and each spoke locations.

For calls between IP endpoints in the host site of Figure 8-20, the available bandwidth is assumed to be unlimited, and CAC is not considered. However, calls between the host site and remote sites travel over WAN links that have limited available bandwidth. As additional calls are placed between IP endpoints over the WAN links, the audio quality of each voice conversation can begin to degrade. To avoid this degradation in audio quality in a CallManager centralized call-processing deployment, you can use the locations feature to define the amount of available bandwidth allocated to CallManager controlled devices at each location and thereby decrease the number of allowed calls on the link.

The amount of bandwidth allocated by CallManager locations is used to track the number of voice conversations in progress across an IP WAN link on a per-CallManager basis. CallManager servers in a cluster are not aware of calls that have been set up by other members in the cluster. To effectively use location-based CAC, all devices in the same location must register with the same centralized CallManager server.

Locations and Regions

In Cisco CallManager, locations work in conjunction with regions to define how voice conversations are carried over a WAN link. Regions define the type of compression, such as G.711 or G.729, used on the link, and locations define the amount of available bandwidth allocated for that link.

Resource-Based CAC 593

Figure 8-21 illustrates a CallManager centralized call-processing model with three remote sites. Each remote site has a location name that all IP endpoints in that location are associated with and a region that determines which codec is be used for voice conversations to IP endpoints in this location.

Figure 8-21 CallManager Centralized Call-Processing Model with Regions and Locations Defined

Each IP endpoint uses the G.711 codec for voice conversations with other endpoints in the same region. When an IP endpoint establishes a voice conversation with an IP endpoint in another region, the G.729 codec is used. The following examples illustrate this:

•HQ IP Phone A calls HQ IP Phone B—Because both IP Phones reside in the same region, the G.711 codec is used.

•HQ IP Phone A calls Atlanta IP Phone A—Because these IP Phones reside in different regions, the G.729 codec is used.

594 Chapter 8: Call Admission Control and QoS Signaling

Calculation of Resources

After the regions have been defined and all devices have been configured to reside in the desired location, you can begin to allocate the desired bandwidth between the locations to use for CAC. This allocated bandwidth does not reflect the actual bandwidth required to establish voice conversations; instead, the allocated bandwidth is a means for CallManager to provide CAC. CAC is achieved by defining a maximum amount of bandwidth per location to use and then subtract a given amount, dependent upon the codec used, from that maximum for each established voice conversation. In this way, CallManager has the capability to deny or reroute call attempts that exceed the configured bandwidth capacity.

Table 8-14 shows the amount of bandwidth that is subtracted, per call, from the total allotted bandwidth for a configured region.

Table 8-14 Location-Based CAC Resource Calculations

In Figure 8-21, suppose that you need to protect six simultaneous calls between the HQ and Atlanta locations, but allow only four between the HQ and Dallas location and three between the HQ and San Jose location. Because each region has been configured to use the G.729 codec between regions, each of these voice conversations represent 24 kbps to the configured location. Remember that the location bandwidth does not represent the actual bandwidth in use.

For the Atlanta location to allow 6 simultaneous calls, the Atlanta Location Bandwidth needs to be configured for 144 kbps, as shown in the following calculation:

6 calls = 6 * 24 kbps = 144 kbps

For the Dallas location to allow 4 simultaneous calls, the Dallas location bandwidth needs to be configured for 48 kbps, as shown in the following calculation:

4 calls = 4 * 24 kbps = 48 kbps

Finally, for the San Jose location to allow 3 simultaneous calls, the San Jose location bandwidth needs to be configured for 72 kbps, as shown in the following calculation:

3 Calls = 3 * 24 kbps = 72 kbps

The link to the Atlanta location, configured for 144 kbps, could support 1 G.711 call at 80 kbps, 6 simultaneous G.729 calls at 24 kbps each, or 1 G.711 call and 2 G.729 calls simultaneously.