Implementing Cisco Unified Communications: Introducing Dial Plans

After reading this chapter, you should be able to perform the following tasks:

• Describe the characteristics and requirements of a numbering plan.
• Explain the components of a dial plan and their functions.

Dial plans are essential for any Cisco Unified Communications deployment. Whether you are implementing single-site or multisite deployments, having a thorough understanding of dial plans and the knowledge of how to implement them on Cisco IOS gateways is essential for any engineer who designs and implements a Cisco Unified Communications network. This chapter describes the characteristics of a dial plan and associated components (for example, a numbering plan).

Numbering Plan Fundamentals

To integrate VoIP networks into existing voice networks, you should have the skills and knowledge to implement call routing and design an appropriate numbering plan. A scalable numbering plan establishes the baseline for a comprehensive, scalable, and logical dial plan.

This section describes call-routing principles, discusses attributes of numbering plans for voice networks, addresses the challenges of designing these plans, and identifies methods of implementing numbering plans.

Introducing Numbering Plans

A numbering plan is a numbering scheme used in telecommunications to allocate telephone number ranges to countries, regions, areas, and exchanges, and to nonfixed telephone networks such as mobile phone networks. A numbering plan defines rules for assigning numbers to a device.

Types of numbering plans include the following:

• Private numbering plans: Private numbering plans are used to address endpoints and applications within private networks. Private numbering plans are not required to adhere to any specific format and can be created to accommodate the needs of a network. Because most private telephone networks connect to the PSTN at some point in a design, it is good practice to plan a private numbering plan to coincide with publicly assigned number ranges. Number translation might be required when connecting private voice networks to the PSTN.
• Public or PSTN numbering plans: PSTN or public numbering plans are unique to the country in which they are implemented. The most common PSTN numbering plans are explained in this section.

Different regions of the globe have different numbering plans. However, all of these national numbering plans must adhere to the international E.164 standard. As an example, the E.164 standard stipulates than no phone number can be longer than 15 digits.

North American Numbering Plan

The North American Numbering Plan (NANP) is an integrated telephone numbering plan that serves 19 North American countries that share its resources. Developed in 1947 and first implemented in 1951 by AT&T, the NANP simplifies and facilitates the direct dialing of long-distance calls. The countries that use the NANP include the United States and its territories, Canada, Bermuda, Anguilla, Antigua and Barbuda, the Bahamas, Barbados, the British Virgin Islands, the Cayman Islands, Dominica, the Dominican Republic, Grenada, Jamaica, Montserrat, St. Kitts and Nevis, St. Lucia, St. Vincent and the Grenadines, Trinidad and Tobago, and Turks and Caicos Islands.

NANP numbers are ten-digit numbers, usually formatted as NXX-NXX-XXXX, in which N is any digit from 2 through 9 and X is any digit from 0 through 9. This structure is depicted in Figure 4-1.

Figure 4-1 North American Numbering Plan

The first three digits of an NANP number (NXX) are called the Numbering Plan Area (NPA) code, often called the area code. The second three digits (NXX) are called the central office (CO) code, switched code, or prefix. The final four digits (XXXX) are called the line number or station number. The North American Numbering Plan Administration (NANPA) administers the NANP.

NANP Numbering Assignments

An area served by the NANP is divided into smaller areas, each identified by a three-digit NPA code, or area code. There are 800 possible combinations of area codes with the NXX format. However, some of these combinations are not available or have been reserved for special purposes, as shown in Table 4-1.

Table 4-1. NANP Numbering Codes

Eight N11 codes, called service codes, are not used as area codes. These are three-digit codes in the N11 format, as shown in Table 4-2.

Table 4-2. N11 Code Assignments

In some U.S. states, N11 codes that are not assigned nationally can be assigned locally, if the local assignments can be withdrawn promptly if a national assignment is made. There are no industry guidelines for the assignment of N11 codes.

Additional NANP reserved area codes include the following:

• 456-<2–9>XX-XXXX numbers: These codes identify carrier-specific services by providing carrier identification within the dialed digits. The prefix following 456, <2–9>XX, identifies the carrier. Use of these numbers enables the proper routing of inbound international calls, destined for these services into, and between, NANP area countries.
• 555-01XX line numbers: These numbers are fictitious telephone numbers that can be used, for example, in the film industry, for educational purposes, and for various types of demonstrations. If anyone dials one of these numbers, it does not cause a nuisance to any actual person.
• 800-XXXX through 855-XXXX line numbers: These numbers are in the format 800-855- XXXX and provide access to PSTN services for deaf, hard-of-hearing, or speech-impaired persons. Such services include Telecommunications Relay Service (TRS) and message relay service.
• 900-<2–9>XX-XXXX numbers: These codes are for premium services, with the cost of each 900 call billed to the calling party. 900-<2–9>XX codes, each subsuming a block of 10,000 numbers, are assigned to service providers who provide and typically bill for premium services. These service providers, in turn, assign individual numbers to their customers.

European Telephony Numbering Space

The European Telephony Numbering Space (ETNS) is a European numbering space that is parallel to the existing national numbering spaces and is used to provision pan-European services. A pan-European service is an international service that can be invoked from at least two European countries.

The European Telecommunications Office (ETO) Administrative Council supervises the telecommunications work of the European Radiocommunications Office (ERO). This supervision includes the establishment, detailing, and change of ETNS conventions and the designation of European Service Identification (ESI) for new ETNS services.

The main objective of ETNS is to allow effective numbering for European international services for which national numbers might not be adequate and global numbers might not be available. The designation of a new European country code, 388, allows European international companies, services, and individuals to obtain a single European number for accessing their services.

Four ETNS services are now available: Public Service Application, Customer Service Application, Corporate Networks, and Personal Numbering. An ESI code is designated for each ETNS service. The one-digit code follows the European Country Code 388 and European Service Code 3 (3883), as shown in Table 4-3.

Table 4-3. ETNS Service and ESI Codes

Figure 4-2 shows the structure of a standard international number. The initial part that is known as the ESI consists of the country code and group identification code that identifies the ETNS (3883), followed by a European Service Code that identifies a particular ETNS service. The European Subscriber Number is the number that is assigned to a customer in the context of the specific service. The maximum length of a European Subscriber Number is 15 digits; for example, 3883 X XXXXXXXXXX.

Figure 4-2 European Numbering Structure

Fixed and Variable-Length Numbering Plan Comparison

A fixed numbering plan, such as found in North America, features fixed-length area codes and local numbers. An open numbering plan, as found in countries that have not yet standardized on numbering plans, features variance in the length of the area code or the local number, or both.

A numbering plan can specify parameters such as the following:

• Country code: A country code is used to reach the particular telephone system for each country or special service.
• Area code: An area code is typically used to route calls to a particular city, region, or special service. Depending on the region, it might also be referred to as a Numbering Plan Area, subscriber trunk dialing code, national destination code, or routing code.
• Subscriber number: A subscriber number represents the specific telephone number to be dialed, but does not include the country code, area code (if applicable), international prefix, or trunk prefix.
• Trunk prefix: A trunk prefix refers to the initial digits to be dialed in a domestic call, prior to the area code and the subscriber number.
• International prefix: An international prefix is the code dialed prior to an international number (the country code, the area code if any, and then the subscriber number).

Table 4-4 contrasts the NANP and a variable-length numbering plan (Germany's numbering plan in this example).

Table 4-4. Fixed and Variable-Length Numbering Plan Comparison

E.164 Addressing

E.164, as illustrated in Figure 4-3, is an international numbering plan for public telephone systems in which each assigned number contains a one-, two-, or three-digit country code (CC) that is followed by a national destination code (NDC) and then by a subscriber number (SN). An E.164 number can have as many as 15 digits. The ITU originally developed the E.164 plan.

Figure 4-3 E.164 Address Structure

In the E.164 plan, each address is unique worldwide. With as many as 15 digits possible in a number, there are 100 trillion possible E.164 phone numbers. This makes it possible, in theory, to direct-dial from any conventional phone set to any other conventional phone set in the world by dialing no more than 15 single digits.

Most telephone numbers belong to the E.164 numbering plan, although this does not include internal private automatic branch exchange (PABX) extensions.

The E.164 numbering plan for telephone numbers includes the following plans:

• Country calling codes
• Regional numbering plans, such as the following:
  • ETNS
  • NANP
• Various national numbering plans, such as the U.K. National Numbering Scheme

Scalable Numbering Plans

Scalable telephony networks require well-designed, hierarchical telephone numbering plans. A hierarchical design has these five advantages:

• Simplified provisioning: Ability to easily add new numbers and modify existing numbers
• Simplified routing: Keeps local calls local and uses a specialized number key, such as an area code, for long-distance calls
• Summarization: Allows the grouping of numbers in number ranges
• Scalability: Leaves space for future growth
• Management: Control from a single management point

When designing a numbering plan, consider these four attributes to allow smooth implementation:

• Minimal impact on existing systems
• Minimal impact on users of the system
• Minimal translation configuration
• Consideration of anticipated growth

Although a non-overlapping numbering plan is usually preferable to an overlapping numbering plan, both plans can be configured to be scalable.

Non-Overlapping Numbering Plan

A dial plan can be designed so that all extensions within the system are reached in a uniform way. That is, a fixed quantity of digits is used to reach a given extension from any on-net origination point. Uniform dialing is desirable because of its simplicity. A user does not have to remember different ways to dial a number when calling from various on-net locations.

Figure 4-4 shows an example of a four-digit uniform dial plan, described here:

• The 0xxx and 9xxx number ranges are excluded due to off-net access code use and operator dialing. In such a system, where 9 and 0 are reserved codes, no other extensions can start with 0 or 9.
• Site A has been assigned the range 1xxx, allowing for as many as 1000 extensions.
• Site B has been assigned the range 2xxx, allowing for as many as 1000 extensions.
• Sites C and D were each assigned 500 numbers from the 4xxx range.
• The ranges 6xxx, 7xxx, and 8xxx are reserved for future use.

Figure 4-4 Non-Overlapping Numbering Plan

After a given quantity of digits has been selected, and the requisite ranges have been excluded (for example, ranges beginning with 9 or 0), the remaining dialing space has to be divided between all sites. Most systems require that two ranges be excluded, thus leaving eight different possibilities for the leading digit of the dial range. The table in Figure 4-4 is an example of the distribution of dialing space for a typical four-digit uniform dial plan.

Scalable Non-Overlapping Numbering Plan Considerations

In a non-overlapping numbering plan, all extensions can be addressed using the same number of digits, making the call routing simple and making the network easily manageable. The same number length is used to route the call to an internal user and a remote user.

The disadvantage of the non-overlapping numbering plan is that it is often impractical in real life. It requires a centralized numbering approach and a careful design from the very beginning.

Overlapping Numbering Plans

In Figure 4-5, Site A endpoints use directory numbers 1001 through 1099, 3000 through 3157, and 3365 through 3985. At Site B, 1001 through 1099 and 3158 through 3364 are implemented. Site C uses ranges 1001 through 1099 and 3986 through 3999. There are two issues with these directory numbers: 1001 through 1099 overlap. These directory numbers exist at all three sites, so they are not unique throughout the complete deployment. In addition, the poor structure of splitting the range 3000 through 3999 requires many entries in call-routing tables, because the ranges cannot be summarized by one or a few entries.

Figure 4-5 Overlapping and Poorly Structured Numbering Plan

A sampling of ways to solve overlapping and poorly structured directory number problems includes the following:

• Redesign the directory number ranges to ensure non-overlapping, well-structured directory numbers.
• Use an intersite access code and a site code that will be prepended to a directory number to create unique dialable numbers. For example, you could use an intersite code of 8, assigning Site A the site code 81, Site B the site code 82, and Site C the site code 83.
• Do not assign direct inward dialing (DID) numbers. Instead, publish a single number, and use a receptionist or auto-attendant.

Overlapping Numbering Plan Example

Figure 4-6 illustrates the most common solution to the overlap problem in numbering plans.

Figure 4-6 Overlapping Numbering Plan Example

The principle of site-code dialing introduces an intersite prefix (8, in this example) and a site code (1x, in this example) that must be prepended when dialing an internal extension in another site. With this solution, a Site A user dials a four-digit number starting with 1 to reach a local extension, and enters a seven-digit number starting with 8 to reach an endpoint in a remote site. The intersite prefix and the site code that are used in this scenario show sample values and can be set differently according to enterprise requirements. For example, the intersite prefix is commonly set to 8 and the access code to 9 in an NANP region, while the intersite prefix is typically 9 and the access code 0 in Europe.

Scalable Overlapping Numbering Plan Considerations

The site-code dialing solution of the overlap issue in numbering plans is useful in real life, as it allows a decentralized approach to the numbering effort. Even various departments within an organization can manage their own addressing space, and the site codes can interconnect them into a manageable unified communications network. Site code dialing does not require a careful design from the beginning and can be implemented as the enterprise grows.

Internal extensions should not start with the intersite prefix (for example, 8), because such entries could cause ambiguity in the dial plan. The intersite prefix notifies the call-routing device that the call is destined for a remote location and therefore should not overlap with any internal number.

Private and Public Numbering Plan Integration

Figure 4-7 illustrates an enterprise with four locations in the NANP region.

Figure 4-7 Private and Public Numbering Plan

Site-code dialing has been designed to allow calls between the enterprise locations. Each site has a trunk connection to the PSTN, with the PSTN DID range provided by the telephone company (telco) operator. Sites A and B have DID ranges that allow public addressing of each internal extension. Site C has a single DID number with an interactive voice response (IVR) solution that prompts the callers for the number of the internal extension for forwarding inbound calls to the intended callee. The DID range of Site D covers some internal extensions and must be combined with an IVR to provide inbound connectivity to others.

Access code 9 identifies a call that is destined to an external PSTN recipient. In this example, internal users dial 9-600-555-6666 to reach the PSTN endpoint.

The following are a few challenges that you might face with numbering plan integration:

• Varying number lengths: Within the IP network, consideration is given to varying number lengths that exist outside the IP network. Local, long-distance, and international dialing from within the IP network might require digit manipulation.
• Necessity of prefixes or area codes: It can be necessary to strip or add area codes, or prepend or replace prefixes. Rerouting calls from the IP network to the PSTN for failure recovery can require extra digits.

Private and Public Numbering Plan Integration Functions

The three basic features, as illustrated in Figure 4-8, that are provided by the integrated private and public numbering plans include the following.

Figure 4-8 Private and Public Numbering Plan Integration Functions

• Reachability to external PSTN destinations: Internal users get access to external destinations over a gateway, which acts like a junction between the private and public addressing scheme.
• Auto-attendant: An IVR system is required to provide connectivity to internal extensions when a sufficient DID range is not available.
• PSTN acts a backup path in case the IP WAN fails or becomes congested: In such cases, the gateways redirect the intersite calls over the PSTN to provide uninterrupted service.

Private and Public Numbering Plan Integration Considerations

When integrating private and public numbering plans, give special consideration to these aspects:

• No ambiguity with the internal and intersite dialing: The prepended access code should uniquely identify all calls that should break out to the PSTN.
• Path selection transparent to the user: Users dial site codes to reach remote locations, and the intersite calls select the IP network as the primary path. If the IP WAN is unavailable, the call should be redirected over the PSTN. The user does not need to take any action for the secondary path to be chosen.
• Auto-attendant for non-DID numbers: When the DID range does not cover all internal extensions, an auto-attendant is needed to allow inbound calls.
• Number adjustment: The voice gateway needs to adjust the calling and called numbers when a call is set up between the sites or via the PSTN. One manipulation requirement arises when an intersite call is rerouted over the PSTN. The intersite prefix and site code (for example, 8-12) must then be replaced with a public number identifying the location (for example, 300-555). Another type of manipulation is needed to map the internal ranges to DID scopes, for example, 1xxx through 0-555-3xxx.

Number Plan Implementation Overview

The implementation of the private numbering plan takes into account the number of users per site and the number of sites. The length of the internal numbers and the site codes must match the size of the environment and at the same time allow space for future growth. Figure 4-9 illustrates that the internal extensions can consist of two, three, or four digits, and the site codes can consist of one, two, or three digits. Note that extension length should be consistent for each site to avoid interdigit timeout or reachability issues.

Figure 4-9 Private Number Plan Implementation

Call routing to local endpoints is achieved automatically, because the registering endpoints have virtual dial peers that are associated with them. The dial peers ensure that calls are routed to the registered phones based on their directory numbers.

Call routing to remote locations is enabled by VoIP dial peers that describe the primary path over an IP WAN.

Private Number Plan Implementation Example

Figure 4-10 shows the enterprise has one large site (Site A) with 7000 users and several smaller sites with less than 700 users each. The codes for all sites are two-digit numbers (21 through 40). The internal extensions in the large site are four digits long (1001–7999), while the extensions in the smaller sites are three digits long (101–799). To implement the dial plan, VoIP dial peers are configured with destination patterns that match seven-digit numbers in the large site and six-digit numbers in the remaining sites, starting with the intersite prefix 8.

Figure 4-10 Private Number Plan Implementation Example

Public Number Plan Implementation

The enterprise does not design its public numbering plan. It is imposed by the telco operator. The enterprise might influence the size of the DID range, which is often related to a financial decision.

Gateways provide a mapping between the DID and the internal number ranges. For example, the PSTN DID range 200-555-3xxx can be easily converted to 1xxx and back when calls traverse the gateway. Complex mapping formulas (for example, mapping of 200-555-3xxx to 1xxx + 50) are too complex to implement and should be avoided.

Call Routing Overview

The most relevant properties of call routing can be compared to the characteristics of IP packet routing, as shown in Table 4-5.

Table 4-5. Call Routing Refresher

The entries that define where to forward calls are the dial peers. All dial peers together build the dial plan, which is equivalent to the IP routing table. The dial peers are static in nature.

Hop-by-hop call routing builds on the principle of call legs. Before a call-routing decision is made, the gateway must identify the inbound dial peer and process its parameters. This process might involve VoIP parameter negotiation.

A call-routing decision is the selection of the outbound dial peer. This selection is commonly based on the called number, when the destination-pattern command is used. The selection might be based on other information, and that other criteria might have higher precedence than the called number. When the called number is matched to find the outbound dial peer, the longest match rule applies.

If more than one dial peer equally matches the dial string, all of the matching dial peers are used to form a so-called rotary group. The router attempts to place the outbound call leg using all of the dial peers in the rotary group until one is successful. The selection order within the group can be influenced by configuring a preference value.

A default call route can be configured using special characters when matching the number.

Call Routing Example

The voice gateways in this example are faced with the task of selecting the best path for a given destination number. Such a requirement arises when the preferred path goes through an IP WAN. A backup PSTN path should be chosen when an IP WAN is either unavailable or lacks the needed bandwidth resources.

Figure 4-11 illustrates a scenario with two locations that are connected to an IP WAN and PSTN. When the call goes through the PSTN, its numbers (both calling and called) have to be manipulated so that they are reachable within the PSTN. Otherwise, the PSTN switches will not recognize the called number, and the call will fail.

Figure 4-11 Call Routing Example