VoIP

Book Excerpt: Cisco Voice Gateways and Gatekeepers — continued

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

PSTN Numbering Plans

PSTN numbering plans are different for every country, but they share some common elements. Most PSTN numbering plans support the following types or classes of numbers:

Emergency services—Many countries provide a single number to reach all emergency services. Others have separate numbers for police, fire, ambulance, and so on.

Information or directory services—Most service providers have a number that can be used to reach directory services. In the United States, this is typically 411. Other services, such as current time or weather, might also be offered.

• Local—Local calls are calls within the same geographic area and typically can be dialed using fewer digits than long-distance calls. Local calls are typically free but might incur per-minute or per-call charges.
  
  
• Mobile—Most countries, with the notable exception of the United States, bill the caller for calls placed to mobile phones. This requires mobile phones to be assigned a distinct number range. In the United States, the owner of the mobile phone is billed for both incoming and outgoing calls, so mobile phones are assigned numbers from the same pool as traditional phones.
  
  
• Toll or long distance—Long-distance calls are directed to numbers outside the local area and commonly require an access code and area code in addition to the subscriber number. Historically, long-distance calls always incurred a per-minute toll. Many service providers now offer unlimited long-distance calls for a fixed fee.
  
  
• Toll free—Toll-free calls are free to the person who is making the call. The service provider bills the recipient.
  
  
• Premium—Premium calls are calls to businesses or services that invoke per-minute or per-call charges. Premium calls are classified distinctly from long-distance because they are frequently used for entertainment purposes, such as obtaining sports scores or using adult entertainment services. Most businesses block all outbound access to premium numbers, because these are rarely appropriate in a business environment.
  
  
• International—International calls are calls to other countries. They are placed using an access code followed by an International Telecommunications Union (ITU)-assigned country code and the full subscriber number, including any area or city codes.

The numbering plan for a country or region is typically complex. Over time, dramatic growth has caused most numbering plans to be revised, resulting in various exceptions or regional peculiarities. The following sections briefly introduce two common numbering plans for reference to show how various countries have implemented their numbering plans. For complete information on a particular country or regional numbering plan, please refer to the appropriate governing body.

As described in Chapter 5, "Circuit Options," the ITU Recommendation E.164 specifies how to build a numbering plan to allow interoperability between the numerous public networks. The E.164 segments a publicly assigned number into a country code (CC) and a national (significant) number, or N(S)N. The N(S)N is further segmented into a national destination code (NDC) and a subscriber number (SN). The DID numbers that a service provider assigns conform to the E.164 recommendation and are referred to as the E.164 number.

North American Numbering Plan
The North American Numbering Plan (NANP) is based on a ten-digit number assigned to each endpoint. This number is represented as XXX XXX-XXXX. The first three digits are the area code. The next three digits are the prefix and were originally used to route calls to the appropriate phone company switch. The last four digits are the subscriber number. Area codes are assigned geographically. Calls made within an area code typically use seven digits—the prefix and the subscriber number. Some densely populated areas have more than one area code assigned.

Determining whether a call is considered a local or toll call can be problematic. In sparsely populated areas, calls within a single area code might be considered long distance. In a densely populated area, calls between area codes might be considered local. To confuse matters more, some densely populated areas use ten-digit dialing for all local calls, whereas others use both seven-digit and ten-digit local dialing.

UK National Numbering Plan
The UK National Numbering Plan (NNP) also uses an area code system. The area codes are known as city codes. Unlike the NANP, the UK NNP does not have a fixed length subscriber number. The number of digits assigned to a subscriber varies based on the city. Because of the variable length of the subscriber number, the area codes are also a variable length. Larger cities have a three-digit area code, whereas smaller cities might have a four- or five-digit area code. For example, Liverpool subscribers are assigned a seven-digit subscriber number and a three-digit area code of 151, whereas subscribers in Coventry are assigned a six-digit subscriber number and a four-digit area code of 2476. This allows most calls between city codes to use 11 digits.

Table 9-1 illustrates the structuring of the UK NNP.

Table 9-1 UK NNP

Area Code Prefix	Service Type	Example Format
00	International	00+countrycode+number
01	Area codes	Liverpool 0151 xxx xxxx Leeds 0113 2xx xxxx
02	Area codes	London 020[378] xxx xxxx Coventry 0247 6xx xxxx
03	Area codes (expansion)	—
04	Reserved	—
05	Corporate	BT broadband voice 055 xxxx xxxx
06	Reserved	—
07	Mobile/pager/personal	Mobile 07[789]xx xxxxxx
08	Freephone (also shared cost)	0800 xxx xxx 0800 xxx xxxx 0808 xxx xxxx
09	Premium	09xx xxx xxxx

NOTE
The area code prefix is commonly depicted with a leading 0, as shown in Table 9-1. The 0 is actually an access code indicating that the call is a national call, similar to the 1 used to indicate a long-distance call in the NANP.

Table 9-2 lists the format for the various classes of numbers for the NANP and the UK NNP.

Table 9-2 NANP and UK NNP Numbering Plans

Call Type	NANP	UK NNP
Emergency	911	112 or 999
Services	[2-8]11	118 xxx
Local	[2-9]xx-xxxx [2-9]xx [2-9]xx-xxxx	Varies by area code
Long distance or national	1[2-9]xx [2-9]xx-xxxx	0+[1-3]xx xxx xxxx
International	011+country code+number	00+country code+number
Toll free	1[800,866,877,888]xxx-xxxx	0800 xxx xxx 0800 xxx xxxx 0808 xxx xxxx
Premium	1 900 xxx-xxxx 976-xxxx	09xxx xxxxxx
Mobile	N/A	07[7-9]xx xxxxxx

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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