1.6 Overview of Dial Plan Issues

This topic describes dial plan issues in multisite deployments.

Dial plans for multisite deployments must address several issues:

  • Direct inward dial versus attendants
  • Overlapping directory numbers
  • Nonconsecutive directory numbers
  • Variable-length numbering and interdigit timeout handling
  • Optimized call routing
    1. Toll bypass
    2. TEHO
    3. PSTN backup
  • Various PSTN requirements in various countries
    1. Access codes for PSTN, national, and international dialing
    2. Number presentation (ISDN TON)
  • Scalability

In a multisite deployment, dial plan design requires the consideration of several issues that do not exist in single-site deployments:

  • DID ranges and E.164 addressing: When you are considering integration with the PSTN, internally used directory numbers must be related to external PSTN numbers (E.164 addressing). Depending on the numbering plan (fixed-length or variable-length) and services that are provided by the PSTN, these solutions are common:
    1. Each internal directory number relates to a fixed-length PSTN number: In this case, each internal directory number has its own, dedicated PSTN number. The directory number can, but does not have to, match the least significant digits of the PSTN number. In countries with a fixed-length numbering plan, such as the NANP, the four-digit station codes, for instance, are used as internal directory numbers. If these numbers are not unique, digits of office codes or administratively assigned site codes might be added, resulting in five or more digits being used for internal directory numbers.Another solution is to not reuse any digits of the PSTN number, but to simply map each internally used directory number to any PSTN number that is assigned to the company. In this case, the internal and external numbers do not have anything in common.If the internally used directory number matches the least significant digits of its corresponding PSTN number, you can set significant digits at the gateway or trunk. Then you can also configure general external phone number masks, transformation masks, or prefixes, because all internal directory numbers are changed to fully qualified PSTN numbers in the same way. The internal directory number can be composed of parts of the PSTN number and administratively assigned digits. In that case, one or more translation rules must be used for incoming calls, and one or more calling-party transformation rules (transformation masks, external phone number masks, prefixes, and so on) must be configured.An internal directory number can be composed of site codes with PSTN station codes, site codes with various ranges (such as PSTN station codes 4100 to 4180 that map to directory numbers 1100 to 1180), or site codes with completely independent mappings of internal directory numbers to PSTN numbers.
    2. No DID support in fixed-length numbering plans: To avoid the requirement of one PSTN number per internal directory number when using a fixed-length numbering plan, it is common to not allow DID to an extension. Instead, the PSTN trunk has a single number, and all PSTN calls that are routed to that number are sent to an attendant (or an auto-attendant) from which the calls are transferred to the appropriate internal extension.
    3. Internal directory numbers are part of a variable-length number: In countries with variable-length numbering plans, a subscriber number, usually shorter, is assigned to the PSTN trunk. However, the PSTN routes all calls that start with this number to the trunk; the caller can add more digits to identify the extension. There is no fixed number of additional digits or total digits (there is a maximum, however), which provides the freedom to select the length of directory numbers. When placing a call to a specific user, a caller simply adds the appropriate extension to the company’s (short) PSTN number. If only the short PSTN number is dialed without an extension, the call is routed to an attendant within the company. Residential PSTN numbers are usually longer and do not allow additional digits to be added. The feature that is described here is available only on trunks.
  • Overlapping numbers: Users that are located at different sites can have the same directory numbers assigned. Because directory numbers are usually unique only within a site, a multisite deployment requires a solution for overlapping numbers.
  • Nonconsecutive numbers: Continuous ranges of numbers are important for summarization of call-routing information. Such blocks can be represented by one or a few entries in a call-routing table (route patterns, dial-peer destination patterns, voice translation rules, and so on) and can keep the routing table short and simple. If each endpoint requires its own entry in the call-routing table, the table becomes too large, much memory is required, and lookups take more time. Therefore, nonconsecutive numbers (some numbers at one site, and other numbers of the same block at a different site) are not optimal for efficient call routing.
  • Variable-length numbering: Some countries have fixed-length numbering plans for PSTN numbers, while others have variable-length numbering plans. A problem of variable-length numbers is that the length can be determined only by waiting for a timeout. If no more digits are dialed for the specified time, the number is considered to be complete. Waiting for the timeout adds to the postdial delay.
  • Optimized call routing: An IP WAN between sites with PSTN access allows PSTN toll bypass. Cisco Unified Communications Manager servers route calls between sites over the IP WAN instead of using the PSTN (toll bypass). In such scenarios, the PSTN should be used as a backup path only in the case of WAN failure. Another solution, which extends the idea of toll bypass, is to use the IP WAN also for PSTN calls: With TEHO, the IP WAN is used as much as possible, and the gateway that is closest to the dialed PSTN destination is used for the PSTN breakout. Finally, a backup path over the PSTN should be enabled for when a call cannot be sent over the IP WAN (for example, if the IP WAN is down or the maximum number of allowed calls is reached).
  • Various PSTN requirements: Various countries—and sometimes even various PSTN providers within the same country—can have different requirements regarding the PSTN dial rules. This situation can cause issues when calls are routed via multiple gateways. For example, if the requirements of a primary gateway are different from the requirements of a backup gateway, numbers must be transformed accordingly.The calling number ANI of calls that are being received from the PSTN can be represented in various ways: as a 7-digit subscriber number, as a 10-digit number including the area code, or in international format with the country code in front of the area code. To standardize the calling number for all calls, the format that is used must be known, and the number must be transformed accordingly. In countries where PSTN numbers do not have fixed lengths, it is impossible to detect the type of number (local, national, or international) by looking at only the length of the number. In such cases, the type of number must be specified in signaling messages (for example, by the ISDN TON).
  • Scalability: In large or very large deployments, there are dial plan scalability issues. When you interconnect multiple Cisco Unified Communications Manager clusters or Cisco Unified Communications Manager Express routers via trunks, it is difficult to implement a dial plan on an any-to-any basis, where each device or cluster needs to know the numbers or prefixes that are found at every other system. In addition to the need to enter almost the same dial plan at each system, a static configuration does not reflect true reachability. If there are any changes, the dial plans at each system must be updated. Although there are solutions that allow centralized dial plan configuration (for example, H.323 gatekeepers), in very large deployments, a dynamic discovery of directory number ranges and prefixes simplifies the implementation and provides a more scalable solution.