Building a Scalable Dial Plan

Most of the dial plan decisions you make will be greatly influenced by business and political considerations. Thus, keep the following practical considerations in mind when designing and implementing your scalable dial plan:

• Hierarchical numbering plan A hierarchical numbering plan is required to scale the number of devices without introducing interdigit timeout issues or call routing issues because of overlapping extension ranges. A hierarchical numbering plan also reduces the complexity of digit manipulation required to direct calls over alternate paths.
• Dial plan distribution Components of the dial plan are configured in the call processing system, the voice gateways, and, if implemented, the gatekeepers. Isolating the gateway to a specific portion of the dial plan simplifies the dial plan provisioning by reducing the number of required dial peers. For H.323 gateways, consider configuring only local PSTN details and using gatekeepers for higher-level routing decisions.
• Post dial delay To enhance the user experience, minimize the processing post dial delay. Post dial delay is the time between the last digit being dialed and the phone ringing at the remote end. Digit manipulations, multiple paths, and gateway processing affect post dial delay. To reduce post dial delay, try to minimize the amount of dial peers and voice translations. Also consider other process-intensive functions that the router might be performing, such as virtual private networking (VPN) or Network Address Translation (NAT), because these might impact the ability of the gateway to process calls.
• Fault tolerance One of the advantages of VoIP is the ability to provide fault tolerance. Calls are automatically routed over redundant IP paths if available. You can configure additional dial peers to route calls over the PSTN if the IP path has failed completely. For critical sites, you can deploy redundant gateways.

  Cisco Survivable Remote Site Telephony (SRST) is a gateway feature that provides call processing redundancy, allowing an IP phone to register with the gateway if no CallManager is available. SRST is discussed in detail in Chapter 13, "SRST and MGCP Gateway Fallback."