Simple Multisite IP Telephony Network

Simple Multisite IP Telephony Network

To help solidify the concepts of the various call-processing models, consider a case study company, Span Engineering LLC. Span Engineering LLC is migrating to a Cisco IP telephony solution. It has three sites in the United States and a fourth site in Brazil. Figure 5-5 shows the distributed architecture implemented by Span Engineering. Notice in the figure that Post, Telephone, and Telegraph (PTT) is the international term for PSTN. Also, the term LD refers to a long-distance network.

Figure 5-5. Distributed Architecture: Span Engineering Case Study

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Span Engineering is a company based in the United States, with headquarters in Chicago and branches in Dallas and on the west coast of the United States. It has recently expanded internationally to Sao Paulo, Brazil. The plan for migrating to Cisco IP telephony includes eliminating all PBXs and migrating to full VoIP in the near future. The overall deployment model used by Span Engineering is multisite distributed and contains both centralized and single-site locations:

• Centralized call control (Chicago) Span Engineering has three separate locations in Illinois, with the Chicago location serving as the headquarters. A centralized network architecture is used for the Illinois sites. The Cisco CallManager cluster in Chicago manages call address resolution and CAC for the three Illinois locations.

• Centralized call control (West Coast) Span Engineering has recently acquired three West Coast companies. The companies consist of a 12-story location in San Francisco, a 2-story location in Oakland, and a single-story location in San Jose. The San Francisco location houses the Cisco CallManager cluster that serves all three West Coast locations.

• Single-site call control (Sao Paulo and Dallas) Span Engineering currently has no plans to expand in these locations but wants to ensure high availability of voice communication capabilities. The company has chosen to put Cisco CallManager clusters in each of these locations as single-site locations. Both the Dallas and Sao Paulo sites have local connections to telephone service providers for local dial tone. In nonNorth American countries, the telephone service provider is referred to as the PTT provider.