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 elements.
To integrate VoIP (Voice over Internet Protocol) 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.
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 just as mobile phone networks. A numbering plan defines rules for assigning numbers to a device.
Different regions of the globe have different numbering plans. Nevertheless, all of these national numbering plans must adhere to the international E.164 standard. As an illustration, the E.164 standard stipulates than no phone number can be longer than 15 digits.
ANI II digits are two-digit pairs sent with an originating telephone number as part of the signaling that takes place while the setup phase of a call. These digits identify the type of originating station.
CICs are used to route and bill calls in the PSTN. CICs are four-digit codes in the format XXXX, where X is any digit from 0 through 9. There are separate CIC pools for different feature groups, just as line-side and trunk-side access.
A seven-digit number defines a local call. Some larger areas use ten-digit numbers instead of seven-digit numbers to define local calls. Notice that the first digit is in the range 2 through 9, during the remaining digits can be any number in the range of 0 through 9.
The European Telecommunications Office Administrative Council supervises the telecommunications work of the European Radiocommunications Office. This supervision includes the establishment, detailing, and change of ETNS conventions and the designation of European Service Identification for new ETNS 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, as shown in Table 4-3.
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 in other words followed by a national destination code and at the time by a subscriber number. An E.164 number can have as many as 15 digits. The ITU originally developed the E.164 plan.
In the E.164 plan, each address is in a class by itself 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, even though this does not include internal private automatic branch exchange extensions.
Scalable telephony networks require so then-designed, hierarchical telephone numbering plans. A hierarchical design has these five advantages:
A dial plan can be designed so that all extensions within the system are reached in a uniform way. In other words, 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.
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 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.
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 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 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.
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.
Figure 4-10 shows the enterprise has one large site with 7000 users and several smaller sites with less than 700 users each. The codes for all sites are two-digit numbers. The internal extensions in the large site are four digits long, during the extensions in the smaller sites are three digits long. 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.
Gateways provide a mapping between the DID and the internal number ranges. For instance, the PSTN DID range 200-555-3xxx can be easily converted to 1xxx and back when calls traverse the gateway. Complex mapping formulas are too complex to implement and should be avoided.
Inbound and outbound call legs. The gateway negotiates VoIP parameters with preceding and straightway gateways earlier a call is forwarded.
The entries that define where to forward calls are the dial peers. All dial peers at the same time 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. Earlier 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.
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 have to be manipulated so that they are reachable within the PSTN. If not, the PSTN switches will not recognize the called number, and the call will fail.
