VoIP

Book Excerpt:
Cisco Voice Gateways and Gatekeepers

In this book excerpt, the authors explain the elements to consider when building your private numbering plan.

by Denise Donohue, David Mallory, and Ken Salhoff in Cisco Voice Gateways and Gatekeepers, published by Cisco Press [October 31, 2006]

This is excerpt is from Chapter 9: Dial Plans, pp. 275-282 Cisco Voice Gateways and Gatekeepers, by Cisco Press

Numbering Plans

A numbering plan describes the endpoint addressing used in a voice system and is analogous to the IP addressing used in a data network. Unlike a data network, today the end user must know and enter the endpoint address to communicate with another endpoint. Because the end user must know and enter the address of the endpoint he wants to communicate with, ease of use is a major focus of numbering plans.

Numbering plans are classified as private or public. A private numbering plan is used within a single organization, and the organization defines and administers it. A public, or PSTN, numbering plan is defined for a single country or group of countries. Government agencies or service providers typically administer PSTN numbering plans, subject to governmental oversight.

Private Numbering Plans

When you are designing a private numbering plan, you must consider numerous factors. The most important factor is the number of devices requiring a number. This includes devices such as end-user phones, common area phones, fax machines, modems, and security systems. You should select an extension length that accommodates the existing number of devices and allows for growth. If you have 700 devices, you would probably want to use four-digit extensions, which allow for 10,000 numbers, even though you could assign a three-digit extension, allowing for 1000 numbers, to all existing devices. Increasing the extension length of an existing system to accommodate additional devices requires substantial planning to ensure that all devices are reachable during the changeover period. It also requires that end users are informed and possibly retrained.

Another important factor is how many locations you have. Consider the following two companies. Company A has a single location with 4000 devices. Company B has 200 locations, each with 20 devices. Both companies have to address 4000 devices, but Company B has to consider more factors than Company A. Calling patterns, PSTN connections, and alternate routing requirements impact the Company B numbering plan.

Private Numbering Plan Design Considerations
When you are designing a private numbering plan, you need to address the following considerations:

• Number of addressable devices—Most organizations try to use the fewest possible digits for internal calling to make it easy for users to remember and dial extensions. To allow for growth, use at least one more digit than the minimum required. For example, if you have 7,000 addressable stations, it is possible to use a four-digit numbering plan, but a five-digit numbering plan, which allows for 100,000 numbers, supports significantly more growth.
  
  
• Number of locations—Organizations typically use fewer digits for calls within a location, and site codes plus extensions for calls between locations. The number of digits in the site code must support the number of sites.
  
  
• Methods for assigning site codes—Some organizations use a mnemonic, assigning site codes based on the location. For example, New York City (NYC) would be assigned a site code of 692, and Tampa (TPA) would be assigned a site code of 872. Other organizations use existing numeric codes assigned to branch offices.
  
  
• Inbound call routing—The method that is used to accept calls from the PSTN determines how numbers are assigned to devices.
  
  Service providers offer direct inward dial (DID or DDI) services to provide a range of PSTN numbers. Organizations can pay for a sequential block of these DIDs and allow external callers to place calls directly to the intended recipients. In this case, organizations use the DID number to assign the extension.
  
  Many organizations opt to publish a single number for external callers. This number is routed to an auto attendant (AA) or receptionist who forwards the call to the intended recipient. This option is less expensive than DID services and allows more flexibility in assigning extensions.
  
  Some organizations use a combination of DID and AAs. They might assign DIDs to employees who frequently receive outside calls, such as salespeople. Calls to all other employees are routed through the AA.
  
  
• PSTN access codes—A PSTN access code distinguishes internal calls from external calls. This eliminates interdigit timeout when an internal numbering plan uses fewer digits than a PSTN numbering plan. It is customary to use 9 as a PSTN access code in the United States. In many other countries, 8 is used as the PSTN access code.
  
  One effect of using a PSTN access code is that you cannot use extensions beginning with this number. Consider a company using a four-digit numbering plan and a PSTN access code of 9. The company has purchased a 10,000 DID range from the service provider of 813 555-0000 thru 813 555-9999. If the company assigns extensions in the 9XXX range, the call processing system will not be able to distinguish internal calls from external calls. When a call is placed to extension 9500, the system will have to wait to see if additional digits will be dialed to determine if this is an internal call or an external call.

Reproduced from the book Cisco Voice Gateways and Gatekeepers. Copyright 2006, Cisco Systems, Inc.. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240.

Visit www.ciscopress.com for a detailed description and to learn how to purchase this title.

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