Extensible Markup Language (XML) services: Cisco Unified CallManager provides access to directories. IP phones can be used to perform lookups in the available directories. Also, IP phones can use XML-based applications that are accessible and configured as IP phone services.

Programming interface to external applications: Through a programming interface, external applications can be integrated with the Cisco Unified CallManager IP telephony solution. Examples of such applications are Cisco IP Communicator, Cisco IP Interactive Voice Response (IVR), Cisco Personal Assistant, and Cisco Unified CallManager Attendant Console. Cisco IP Communicator gets special mention. As seen in Figure [1], this product is a virtual phone, represented by an interactive screen on a PC. A variety of third-party products use Cisco Unified CallManager programming interfaces.

Figure shows a company using Cisco Unified CallManager. The company has two sites: the headquarters and a branch. A Cisco Unified CallManager cluster is located in the headquarters. Each site has a voice gateway for PSTN access. In the example, a user in the branch office wants to place a call to a user located at headquarters. The call follows these steps: Step 1 When the branch user dials the telephone number, the IP phone sends signaling messages to a member of the Cisco Unified CallManager cluster. Step 2 The Cisco Unified CallManager server processes the call by looking up the called number in the Cisco Unified CallManager’s call routing table. Step 3 When the Cisco Unified CallManager server determines the IP address of the destination telephone, the Cisco Unified CallManager sends a signaling message to the destination telephone. The destination telephone starts ringing, and the user who is being called can accept the call. Step 4 After the call is accepted, the telephones start sending and receiving RTP packets that carry audio signals.

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Content 2.5 Implementing VoIP in an Enterprise Network 2.5.5 Enterprise IP Telephony Deployment Models Enterprises have these choices for deploying Cisco Unified CallManager as listed on Figure :

• Single site: An enterprise can deploy the IP telephony solution at a single site only. In this deployment, there is a local Cisco Unified CallManager cluster that serves local telephones only.
• Multisite with centralized call processing: The enterprise can deploy the IP telephony solution at multiple sites with centralized call processing. With centralized call processing deployment, there is a single Cisco Unified CallManager cluster that is located at one of the sites. The Cisco Unified CallManager cluster serves local and remote IP phones.
• Multisite with distributed call processing: The enterprise can deploy the IP telephony solution at multiple sites with distributed call processing. With distributed call processing deployment, there is a Cisco Unified CallManager cluster at each site. Each Cisco Unified CallManager cluster serves local IP phones.
• Clustering over WAN: An enterprise can deploy the IP telephony solution at multiple sites. In this deployment, Cisco Unified CallManager servers are located at more than one site. However, all of the Cisco Unified CallManager servers belong to a single cluster. The members of the cluster are separated by an IP WAN. IP phones usually use the local servers as their call agents.

Caution
The clustering-over-WAN deployment model requires the round-trip delay between any pair of servers to be less than 40 ms. Hybrid deployments are also possible. For example, Cisco Unified CallManager clusters can reside at multiple sites, but there can also be sites in the enterprise that do not have a Cisco Unified CallManager cluster and are served by a cluster at a remote site. Figures to show examples of possible deployments of the Cisco Unified CallManager.

• Single Site: In Figure , IP telephony is deployed at a single building with a local Cisco Unified CallManager cluster and application servers.
  • Local voice gateways offer PSTN access and provide DSP resources.
  • The IP WAN is not used for voice; the PSTN is used for all external calls.
• Multisite with Centralized Call Processing: Figure shows a company with two sites that is using a centralized call-processing model.
  • There is one Cisco Unified CallManager cluster, and all Cisco Unified CallManager servers are located at the headquarters site.
  • Application servers are also centralized, but each site has local DSP resources.
  • Because there are local DSPs at each site, conference calls using the DSP resources do not have to cross the IP WAN if all participants of the conference are at the same site.
  • The IP WAN carries data and voice. All signaling messages from branch IP phones have to cross the IP WAN. Media streams cross the IP WAN only for intersite calls.
  • Each site has local PSTN access.
  • CAC is used to limit the number of VoIP calls between the two sites. If CAC denies calls, the Automated Alternate Routing (AAR) feature of the Cisco Unified CallManager reroutes the calls through the PSTN.
  • If the IP WAN goes down, the branch router uses Cisco SRST to maintain IP telephony operation for calls within the branch. All other calls are routed through the PSTN.
• Multisite with Distributed Call Processing: Figure shows the distributed call processing deployment at two sites.
  • Each site has its own Cisco Unified CallManager cluster with local servers.
  • Local application servers and DSP resources are available at each site.
  • The IP WAN carries data and voice. Only intersite calls use the IP WAN.
  • Both the signaling messages and media streams of intrasite calls remain local to each site because of the presence of a call agent in each site.
  • Both sites have local PSTN access.
  • CAC is used to limit the number of VoIP calls between the two sites. If CAC denies a call, the call is rerouted through the PSTN using AAR.
  • If the IP WAN goes down, the local Cisco Unified CallManager clusters reroute intersite calls over the PSTN via the cluster’s local gateways.
• Clustering over WAN: Figure shows clustering over an IP WAN.
  • Each site has local Cisco Unified CallManager servers, but the servers all belong to a single cluster. Application servers and DSP resources are locally available at each site.
  • IP phones at the Los Angeles office use local Cisco Unified CallManager servers as the primary call agents, while the servers in San Diego are used as backup call agents. IP phones in San Diego are configured in a similar way to the phones in Los Angeles using the local Cisco Unified CallManager servers as the primary call agents and the servers in Los Angeles as backup.
  • The IP WAN carries data and voice. Only intersite calls use the IP WAN. Both the signaling messages and media streams of intrasite calls remain local to each site because of the presence of a call agent at each site. Both sites have local PSTN access. CAC is used to limit the number of VoIP calls that can occur between the two sites. If calls are denied by CAC, the calls are rerouted through the PSTN by AAR. If the IP WAN goes down, the local Cisco Unified CallManager clusters reroute intersite calls over the PSTN using the local gateways.
  • All intracluster communication between Cisco Unified CallManager servers (for example, database synchronization or exchange of account data) goes over the IP WAN. The company must guarantee a required round-trip time of less than 40 ms for the intracluster traffic between the two sites.

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Content 2.5 Implementing VoIP in an Enterprise Network 2.5.6 Cisco IOS Configurations for VoIP Cisco IOS routers can be used as VoIP gateways. For a basic VoIP configuration, two gateways are needed. Both need a connection