To integrate VoIP networks into existing voice networks, network administrators must have the skills and knowledge to implement a scalable numbering plan and a comprehensive, scalable, and logical dial plan. This section describes the attributes of numbering plans and scalable dial plans for voice networks, addresses the challenges of designing these plans, and identifies the methods of implementing dial plans.
Hierarchical Numbering Plans
The previous section discussed the Public International Telecommunications Numbering Plan (E.164), national numbering plans, and private numbering plans. Each of the numbering plans can benefit, in terms of scalability, from a hierarchical design. A hierarchical design has the following advantages:
- Simplified provisioning Refers to the ability to easily add new groups and modify existing groups
- Simplified routing Keeps local calls local and uses a specialized number, such as an area code, for long-distance calls
- Summarization Establishes groups of numbers in a specific geographical area or functional group
- Scalability Provides additional high-level number groups
- Management Controls number groups from a single point in the overall network
The North American Numbering Plan (NANP) serves as a good role model for a scalable numbering plan. Consider how the NANP might be adapted to your environment. To illustrate the operation of the NANP, examine Figure 5-11.
In Figure 5-11, the calling party dials 1-703-555-0123. The calling party's local central office (CO) forwards the call to a long-distance carrier because the first digit (that is, the 1) indicated the call was a long-distance call. The long-distance carrier then forwards the call, based on the dialed area code, to a Virginia long-distance office. The Virginia long-distance office forwards the call, based on the CO code (that is, the NXX code), to an Alexandria CO. Finally, the Alexandria CO, based on the last four digits, forwards the call out to the called party.
While the NANP acts as a good starting point in designing a numbering plan, it is not always easy to design a hierarchical numbering plan. Existing numbering plans in the network might include proprietary PBXs, key systems, and telephony services such as a Centrex service. The necessity to conform to the PSTN at the gateways also contributes to the complexity of the design. Translation between these systems is a difficult task. If possible, avoid retraining system users. The goal is to design a numbering plan that has the following attributes:
- Minimal impact on existing systems
- Minimal impact on users of the system
- Minimal translation configuration
- Consideration of anticipated growth
Scalable Dial Plans
The North American telephone network is designed around a ten-digit numbering plan that consists of three-digit area codes and seven-digit telephone numbers, as shown in Figure 5-13. For telephone numbers that are located within an area code, the PSTN often uses a seven-digit dial plan. Features within a CO-based PBX, such as Centrex, allow the use of a custom five-digit dial plan for customers who subscribe to that service. PBXs are more flexible and allow for variable-length dial plans containing 3 to 11 digits.
Dial plans contain specific dialing patterns for a user who wants to reach a particular telephone number. Dial plans also contain access codes, area codes, specialized codes, and combinations of the numbers of digits dialed.
Dial plans require knowledge of the customer network topology, current telephone number dialing patterns, proposed router and gateway locations, and traffic-routing requirements. If the dial plans are for a private internal voice network that is not accessed by the outside voice network, the telephone numbers can be any number of digits.
Typically, companies that implement VoIP networks carry voice traffic within the least expensive systems and paths. Implementing this type of system involves routing calls through IP networks, private trunks, PBXs, key systems, and the PSTN. The numbering plan to support the system is scalable, easily understood by the user, and transportable between all of the system components. The use of alternate path components reduces instances of call failure. Finally, the numbering plan conforms to all applicable standards and formats for all of the systems involved.
Scalable Dial Plan Attributes
When designing a large-scale dial plan, Cisco recommends you adhere to the following attributes:
- Logic distribution Good dial plan architecture relies on the effective distribution of the dial plan logic among the various components. Devices that are isolated to a specific portion of the dial plan reduce the complexity of the configuration. Each component focuses on a specific task accomplishment. Generally, the local switch or gateway handles details that are specific to the local point of presence (POP). Higher-level routing decisions are passed along to the gatekeepers and PBXs. A well-designed network places the majority of the dial plan logic at the gatekeeper devices.
- Hierarchical design (scalability) You should attempt to keep the majority of the dial plan logic (routing decisions and failover) at the highest-component level. Maintaining a hierarchical design makes the addition and deletion of number groups more manageable. Scaling the overall network is much easier when configuration changes are made to a single component.
- Simplicity in provisioning Keep the dial plan simple and symmetrical when designing a network. Try to keep consistent dial plans on the network by using translation rules to manipulate the local digit dialing patterns. These number patterns are normalized into a standard format or pattern before the digits enter the VoIP core. Putting digits into a standard format simplifies provisioning and dial-peer management.
- Reduction in postdial delay Consider the effects of postdial delay in the network when you design a large-scale dial plan. Postdial delay is the time between the last digit dialed and the moment the phone rings at the receiving location. In the PSTN, people expect a short postdial delay and to hear ringback within seconds. The more translations and lookups that take place, the longer the postdial delay becomes. Overall network design, translation rules, and alternate pathing affect postdial delay. Therefore, you should efficiently use these tools to reduce postdial delay.
- Availability and fault tolerance Consider overall network availability and call success rates when you design a dial plan. Fault tolerance and redundancy within VoIP networks are most important at the gatekeeper level. By using an alternate path you help provide redundancy and fault tolerance in the network.
- Conformance to public standards Different geographical locations might impose restrictions to your dial plan. Therefore, familiarize yourself with any such limitations prior to designing your dial plan.
