When checked, Shutterstock's safe search screens restricted content and excludes it from your search results. The dial plan is one of the key elements of a Unified Communications system, and an integral part of all call processing agents.
Depending on the calling device, different paths can be selected to reach the same destination.
Different groups of devices can be assigned to different classes of service, by granting or denying access to certain destinations. In some cases, it is necessary to manipulate the dialed string before routing the call; for example, when rerouting over the PSTN a call originally dialed using the on-net access code, or when expanding an abbreviated code (such as 0 for the operator) to an extension.
Special groups of devices can be created to handle incoming calls for a certain service according to different rules (top-down, circular hunt, longest idle, or broadcast). This chapter presents information intended to guide the system designer toward a dial plan that accommodates the legacy dialing habits of telephony users, while also taking advantage of new functionality afforded by the increasing integration between computing technology and telephony, such as dialing from contacts, click-to-call actions from computers and smart phones, and adoption of mobility-related features. This section analyzes the thought process involved in planning an IP Telephony dial plan, ranging from the number of digits used for internal extensions to the overall architecture of a company's internal dial plan. This section contains design and deployment guidelines related to multisite IP Telephony networks, endpoint addressing methods, approaches to building classes of service, and call coverage functionality. This section provides detailed background explanations of the elements of a Cisco Unified Communications dial plan. Table 9-1 lists the topics that are new in this chapter or that have changed significantly from previous releases of this document.
The digit analysis function controls which calls are allowed to a user, to a gateway, or to an application. Patterns are numerical representations of telephone numbers which, when matched, trigger call routing. Calling search spaces allow control over what groups of patterns a device (such as a phone) can access. For both of these functions, Unified CM offers the system designer many tools with which digit manipulation can be effected and with which control over the call processing can be performed for different situations.
The fundamental elements of the individual features of this architecture are presented in multiple documents.
For call paths relying on external trunks, an additional level of availability is afforded by the use of alternate routes. For calls between on-cluster endpoints, such as calls between two IP phones, if the IP path is not available due to a network failure, the Call Forward Unregistered feature is invoked, allowing the routing of the call through an alternate network such as the PSTN.
External dial plan resolution subsystems such as H.323 gatekeepers or SIP proxies should also be provisioned with applicable redundancy capabilities, to offer yet another level of high availability. The configuration of dial plans is typically not onerous on the Unified CM configuration when compared to other capacity-affecting aspects such as the number of gateways, the number of CTI connections, or the rate of call attempts (BHCA). A dial plan suitable for an IP telephony system is not fundamentally different from a dial plan designed for a traditional TDM telephony system; however, an IP-based system presents the dial plan architect with some new possibilities.
The system administrator must plan the system's recognition of these patterns to ensure that the system will act promptly upon detection of a string that corresponds to a predetermined pattern so that users experience no (or minimal) post-dialing delay. For phones using the Skinny Client Control Protocol (SCCP) and for SIP phones using the Key Press Markup Language (KPML) during dialing, you can implement pattern recognition in Cisco Unified Communications Manager (Unified CM) by configuring route patterns, translation patterns, phone DNs, and so forth. Once digit dialing is completed, Unified CM provides user feedback in the form of call progress tones, such as ringback tone if the destination is in the alerting stage or reorder tone if the destination is invalid. IP phones running the Session Initiation Protocol (SIP) can be configured with pattern recognition instructions called SIP dial rules. Calls that originate and terminate on the same telephony network are considered to be on-network (or on-net). In TDM systems, the on-net boundaries of a telephony system are established by the PBX or Centrex system, and they typically do not extend outside of a single site. One of the key attributes of IP telephony is its ability to expand the boundaries of calls that can be considered on-net. Consider an extension with direct inward dial (DID) capability, which can be reached directly from the PSTN.
For example, Cisco employees at any Cisco office can dial the San Jose reception number as 8 526 4000. This form is shorter than the alternative of using an off-net form to route the calls on-net (for example, allowing Cisco employees in Canada to reach the Cisco reception in San Jose by dialing 9 1 408 526 4000 while routing the call on-net). For calls where the destination is off-net and outside of the company's dial plan, the Unified Communications system must offer a simple, locally significant dialing form to the users. A telephony system must be configured so that any extension can be reached in an unambiguous manner. The number of dialable extensions determines the quantity of digits needed to dial extensions. 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.
For example, if phone A is reached by dialing 1234 from any on-net location, whether the calling phone is in the same office or at a different site, the enterprise's dial plan is deemed uniform.
When the enterprise consists of few sites, this approach can be used with few complications. Once 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.
When implementing a new dial plan, one of the main desires of any planner is to avoid having to change phone numbers. The use of access and site codes (see Table 9-3) enables the on-net dial plan to differentiate between extensions that would overlap if a uniform abbreviated dial plan were implemented. From site B, merely dialing the combination of site A's code with the desired extension is not feasible: in this case because 11234 would partially overlap with site B's extension 1123, thus causing interdigit timeout issues.
For example, a system with 75 sites which each use four-digit abbreviated dialing would require a format of 8 + SS + XXXX, where 8 is the on-net access code, SS is a two-digit site code, and XXXX is a four-digit extension number, giving a total of seven digits. It is common practice in most enterprise telephony systems to dedicate a digit (for example, 9) as an off-net access code to steer calls to an off-net destination.


In systems where the same off-net access code (for example, 9) is already in use by all sites, it might be desirable to use the same code for both off-net and on-net off-site destinations.
This section describes dial plan features used to implement simplified call routing based on globalized numbers. The Local Route Group offers the ability to create patterns that route off-net calls to a gateway chosen for its proximity to the originating party.
Telephone numbers can use the + sign to represent the international dialing access code needed to reach a destination from different countries. This book may have occasional imperfections such as missing or blurred pages, poor pictures, errant marks, etc. Generally, the dial plan is responsible for instructing the call processing agent on how to route calls.
Moreover, a secondary path can be used when the primary path is not available (for example, a call can be transparently rerouted over the PSTN during an IP WAN failure). For example, lobby phones might be allowed to reach only internal and local PSTN destinations, while executive phones could have unrestricted PSTN access.
Digit manipulation is also used to adapt the local dialing habits of a user to the global routes used to select a path for a call.
The dial plan information covered in this chapter applies to any Unified Communications deployment model; in particular, when deploying multi-site systems, the system designer should pay close attention to the site-specific dialing habits as well as site-specific routing of calls, such as the use of a gateway co-located with a specific group of users. Covered topics include call routing logic, calling privileges, and digit manipulation techniques for various Cisco products. For instance, devices at one site can be given access to the local partition containing extension 1000, without having access to a different site's extension 1000. This function is where IP trunks, PSTN trunks, or even connections to legacy PBXs, are chosen to carry a particular call.
For example, the system administrator can configure the types of calls allowed from a phone when it is roaming between different sites, how a call is processed when the phone is busy or when it rings with no answer, or which destinations a phone can use when call-forwarding all calls. The product-specific documentation, along with the help files in Unified CM, offer the most fundamental descriptions of the features. All dial plan configuration is made redundant by the same mechanisms that allow other Unified CM services to be redundant. For instance, a route list can be used to establish a primary path to a given off-cluster destination, followed by a secondary or even a tertiary choice.
If the IP path is not available due to lack of bandwidth on the network, the Automated Alternate Routing (AAR) feature is invoked to route the call through an alternate network. The Cisco Unified Communications Sizing Tool does incorporate dial plan information in its calculations for system provisioning.
It is at the very core of the user experience because it defines the rules that govern how a user reaches any destination.
For example, because of the flexibility of IP-based technology, telephony users in separate sites who used to be served by different, independent TDM systems can now be included in one, unified IP-based system.
For instance, many enterprises implement a five-digit abbreviated dialing pattern for calls made within the same office location.
With each digit dialed by the user, the phone sends a signaling message to Unified CM, which performs the incremental work of recognizing a matching pattern. These are composed of various dialing rules applicable to calls placed to destinations within an office location (intra-site calls), destinations within a company but between different sites (inter-site calls), and destinations outside the company (off-net calls). By contrast, if a call originates in company A and terminates at company B, it probably has to be routed through different telephony networks: first company A's network, followed by the PSTN, and finally into company B's network. When they do, they typically do not include sites not immediately on the periphery of a large system hub. For instance, telephony users in an enterprise with six dispersed branch offices might be used to reaching one colleague with abbreviated dialing (for example, four-digit dialing) if the called party is located in the same site but dialing a full PSTN number to reach another colleague located in a different site.
An off-net PSTN caller has to dial the fully qualified PSTN number (for example, 1 415 555 1234) to reach a DID extension.
For example, Cisco employees located in San Jose, California can call the main Cisco reception number using the five-digit string 64000. The digit 8 serves as the inter-site access code, and 52 serves as the site code for San Jose.
Even though the dialing form is similar to that used to reach an off-net destination, the system is configured to keep calls to on-net destinations dialed in the off-net form within the system. For example, calls made from one site in San Francisco to another site in New York can be dialed either in the on-net or off-net form described above, but routed off-net through the PSTN. For instance, in a system using an abbreviated four-digit on-net dial plan, there cannot be an extension 1000 in site A and another extension 1000 in site B if the requirement is to reach either of them by dialing only four digits from site C.
For example, a system with extensions 1000 and 10000 would force users to wait for the interdigit timeout when they dial 1000. For example, a four-digit abbreviated dial plan cannot accommodate more than 10000 extensions (from 0000 to 9999). For instance, if more than 8000 extensions are required (considering the exclusions of the 0XXX and 9XXX ranges), the system may require that an abbreviated dial plan use more than four digits. For example, if the DID range of 415 555 1XXX exists in a system using a four-digit uniform abbreviated dial plan, and DID range 650 556 1XXX is also being considered, it might be desirable to increase the quantity of digits for on-net dialing to five. In such a system where 9 and 0 are reserved codes, no dial plan (uniform or not) could accommodate on-net extension dialing that begins with 9 or 0. Table 9-2 exemplifies the distribution of the dialing space for a typical four-digit uniform dial plan. Note that the corresponding DID ranges must also be retained from the site's local exchange carrier.
If growth requires more extensions for DID, any unassigned numbers from the non-DID range could be used. Note that their corresponding DID ranges must match each of the site's respective portions of the 4XXX range. In addition, the extension ranges of any existing phone systems may have overlapped without any problems in the past, but they could be incompatible with a uniform dial plan. For calls from site A to site B under the old telephony system, the calls had to be routed as off-net calls.


But from site B, the dial plan must accommodate the ability to reach extension 1234 at site A without conflicting with site B's own extension 1234.
If, instead, we assign 8 as an inter-site on-net access code, this would allow site B to dial 81234 to reach site A's extension 1234. In the variable-length on-net dial plan, another steering digit (for example, 8) is also required as an on-net access code to dial calls to on-net extensions at other sites.
For instance, if a site already uses an abbreviated dialing range beginning with 8, the use of that same digit as an access code would require a change.
Although it is preferable to plan a new system so that the implementation is as transparent as possible to users, dialing habits might have to be adapted to accommodate the integration of multiple sites that used to be on separate telephony systems. The simplification is primarily obtained through the use of a single routing structure for off-net calls, no matter the source of the call. For example, a single pattern can be defined to route off-net, intra-country calls for all sites within a given country.
The call routing function also allows for the automated failover of calls from, for example, an IP connection as a first choice to a PSTN connection as a backup choice, in case the first choice is not available because no bandwidth is available or because a particular portion of the network is not available.
The section on Dial Plan Elements, in this chapter offers the next level of functionality description.
Specifically, Unified Communications Manager groups are used to control phones, route lists, and gateways, so that a single failure of a Unified CM server does not render any dial plan function unavailable. If the higher-order choice (such as an IP trunk) is not available, the next order choice will be attempted, until either the call is established successfully or all preconfigured choices have been exhausted. These new possibilities afforded by IP-based systems require some rethinking of the way we look at dial plans. Also, many enterprises rely on a single-digit access code to represent outside dialing, followed by some quantity of digits to reach a local PSTN number or a long-distance PSTN number (for example, 9 followed by seven digits to reach a local number, or 9 followed by 1 and ten digits to reach a long-distance destination). Once a pattern is recognized, the SIP phone sends an invitation to Unified CM to place a call to the number corresponding to the user's input.
The form used in dialing these different types of calls varies according to user preferences and local PSTN dialing requirements.
From the caller's perspective, the call was routed off-network (or off-net); from the called party's perspective, the call originated off-net. With an IP-based system where all users are served by the same IP network, it now becomes economically feasible to unite all six branches under a four-digit abbreviated dialing plan, with the IP network as the preferred path and automated overflow to the PSTN as a secondary path if the IP network is congested.
An on-net caller might, however, prefer the ability to reach that same extension by simply dialing the last few digits of the DID number. For example, if a user dialed 9141, the system would have to wait for either more digits (if the user were dialing 9 1 415 555 1234, for example) or the expiration of the interdigit timeout before routing the call to extension 9141. If 0 and 9 are reserved as operator code and off-net access code, respectively, the number range is further reduced to 8000 (1000 to 8999).
A user does not have to remember different ways to dial a number when calling from various on-net locations. This means that DID ranges could not be used if they would force the use of 9 or 0 as the first digit in the dial plan.
Because the DID ranges are for different sites (probably from different PSTN service providers), more coordination effort is required to split ranges between sites. These two access codes, along with the use of an operator access code (for example, 0), implicitly exclude three of the ten possible leading digits of any dialed string. For instance, adapting to a new global, enterprise-wide dial plan might require changing the way a user reaches another user at a different site, the quantity of digits used to make intra-site calls and, in some cases, the extension numbers.
For example, two users in separate countries could use the same route patterns to carry calls to their respective local gateways, instead of requiring site-specific route patterns, each configured to match their respective dialing habits. Phones at every site can be configured to match this pattern, which then would route the call based on the Local Route Group associated with the calling phone. We believe this work is culturally important, and despite the imperfections, have elected to bring it back into print as part of our continuing commitment to the preservation of printed works worldwide.
It does highlight some fundamental functionality elements essential to the understanding of design-related concepts presented herein. The section on Design Considerations in this document offers the system designer top-down architectural information to be considered when designing a dial plan. This section examines some of the elements that the system planner must consider to properly establish the requirements that drive the design of the dial plan. In a four-digit abbreviated dial plan, the on-net caller would dial only 1234 in this example to reach the same extension.
Likewise, if an operator code is used (for example, 0), the entire 0XXX extension range would have to be excluded from a four-digit uniform dial plan. For instance, in a five-digit abbreviated dial plan, the DID range 415 559 XXXX (or any subset thereof) could not be used. As the quantity of sites assigned within a given range increases, it becomes increasingly difficult (sometimes impossible) to make full use of an entire range.
To avoid exposing users to multiple generations of dial plan changes, try to anticipate expansion of the enterprise, which could result in the addition of sites in different dialing regions, an increase in the required number of on-net extensions, PSTN renumbering (for example, an area code split), or business expansion into different countries. We appreciate your understanding of the imperfections in the preservation process, and hope you enjoy this valuable book. Both localized user inputs can be translated to a global form of +1 212 555 1234, so that a single route is used to select a path for the call.
In this mode of operation, user feedback during the dialing of the digit string is limited to what the phone can provide (see SIP Dial Rules). In this example, alternatives include increasing the length of the abbreviated dialing to six or more digits, or avoiding any DID range whose last five digits start with 9, or even not requiring that the DID numbers match the on-net abbreviated extensions. Once the string has been composed, user feedback can still be provided by Unified CM in the form of call progress tones.



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