It is important to understand and have through knowledge of numbering /dial plans from the CCNA voice exam perspective for deployment of a unified communications network and passing the test. Numbering plans define addressing schemes for endpoints in a VOIP networks. It also identifies each VOIP endpoint and applications with a unique telephone number. We will learn about different categories of numbering plans in the subsequent sections.
Private Numbering Plan – It is used to address endpoints and applications in a private network. Usually they don’t follow any standard addressing scheme. However, it is a good practice to follow a standard format because at some point of design they need to be connected to PSTN networks and at that point of time number translation would be required to connect private voice network to PSTN network.
While designing a private numbering plan, certain design considerations are required to be addressed described such as:
Number of Addressable Devices – Keep least number of digits for internal dialing, this will make it easier to remember extensions for users.
Number of Locations – Fewer digits for calls within a location and sites code + extensions.
Inbound Call Routing – can be addressed in two ways – allow DID to a block of numbers or single number for all external callers and routing all such calls to a receptionist of auto attendant.
PSTN Access Codes– used to differentiate internal and external calls. For example in US 9 is used as PSTN access code which means it cannot be used to begin an extension.
Depending on the usage of numbers following types of number classes are defined as:
Emergency Services – single number for all emergency services assigned by certain countries whereas some countries use different numbers for different emergency services. Examples of emergency services numbers include 911 – US, 999- UK and 000 – Australia.
Information or Directory Services – Directory and information services can be accessed using a single number like in US 411 is used for directory/ information services.
Local Calls – Local calls are free within the local area in certain countries they are chargeable.
Mobile Calls – In most of the countries caller is billed for calling on mobile phones. Except in case of US where the caller is billed both for incoming/outgoing calls and mobile phone numbers are assigned from same pool of traditional numbers.
Long Distance or Toll -These calls require access code, area code and subscriber number. These calls are charged on per minute basis or on a flat monthly rate.
Toll Free Calls – are free to caller but chargeable to called party. In US 1- 800 numbers are toll free.
Premium calls – are based on per-minute or per call charges. Used by businesses and used for “entertainment” purposes, in US 900 numbers are premium numbers
International – International calls are placed using an access code, country code and full subscriber number along with area code.
Public /PSTN Numbering Plans – These are unique for country where they are implemented.
The international public telecommunication numbering plan (E.164) – A valid E.164 standard address is used in telephone systems. It is an international numbering plan created by International Telecommunication Union (ITU) which defines the International public telecommunications numbering plan used in the PSTN and other data networks. It defines the format for numbers. E.164 numbers can have a maximum of fifteen digits and written with a prefix of a+. The North American numbering plan is 10 digit plan having 3 digit area code, and 7 digit telephone number. Telephone numbers belonging to a specific area code, 7 digit plan is used by Public Switched Telephone Network (PSTN). PBXs support variable length dial plans for numbers ranging from (3 – 11 digits). In H.323 specific dialing pattern is used to reach a particular phone number. Voice enable routers use dial plan to process calls. There is no standard protocol defined for dynamic routing of E.164 telephony addresses whereas H.323 Voice over IP (VOIP) dial plans are static and managed on gateway/ gateway keepers.
The E.164 numbering plan structure has following components:
- Country code
- National destination code
- Subscriber number
The numbering structure and digit analysis is required to route the calls. The number categories are based on a fifteen digit numbering space (excluding prefixes: only 12 digits were permissible before 1997) they are as under.
Number Structure for Geographic Area
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Country code |
National destination code (optional) |
Subscriber number |
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National (Significant) number |
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Cc = 1 to 3 digits |
Maximum = 15 – cc digits |
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International public telecommunication number for geographic areas (maximum 15 digits) |
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Number Structure for Global Services
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Country code |
Global subscriber number |
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Cc = 3 digits |
Maximum = 15 – cc digits |
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International public telecommunication number for global services (maximum 15 digits) |
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Number Structure for Networks
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Country code |
Identification code |
Subscriber number |
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Cc = 3 digits |
X = 1 to 4 digits |
Maximum = 15 – cc digits |
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International public telecommunication number for networks (maximum 15 digits) |
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National numbering plan – is used to define numbering structure for a group of countries.
Number structure for group of countries
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Country code |
Group Identification code |
Subscriber number |
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Cc = 3 digits |
gic = 1 digit |
Maximum = 15 – (cc + gic) = 11 digits |
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International public telecommunication number for group of countries (maximum 15 digits) |
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We will use example of the North American Numbering Plan (NANP) to explain how E-164 standard works in this scenario. The components of NANP plan are as noted below:
- Country code
- Area code
- Central office or exchange code
- Station code
We take an example and see how an NANP number breaks down (1-609-728-5668).
- 1 – Stands for country code
- 602 – Stands for area code (E.164 National destination number)
- 728 – Central office code (E.164 National destination number)
- 5668 – Station code (E.164 Subscriber number)
Under NANP though specific categories are defined that a standard E.164 number don’t have but the number still complies as per the broad category of E.164 scheme.
The UK National Numbering Plan – UK national numbering plan is regulated by office of communications (Ofcom). UK NNP don’t use fixed length subscriber number instead number of digits varies from city to city. So subscriber number as well as area code both is variable and most of the numbers are 10 – 11 digits. UK national numbering plan is as under:
- 01 – Geographic area codes
- 02 – Geographic area codes (introduced in 2000)
- 03 – Nationwide non-geographic code, charged to caller at geographic area code rates (in 2007)
- 04- Reserved
- 05 – Corporate numbering and VoIP services
- 06 – Reserved for possible use by Personal Numbering instead of 070 following consumer confusion with mobile phones
- 07 – Personal Numbering on 070, Pagers on 076, mobile phones on 075, 07624, 077, 078, and 079, Wi-Fi numbers on 079112 and 079118
- 08 – Freephone (toll free) on 080, and Special Services (formerly known as local and national rate) on 084 /087
- 09 – Premium Rate services
A dial plan is a numbering plan for a voice enabled network; we use it in our day to day lives though we are not aware of them by name. It is the assignment of individual or block of telephone numbers (E.164 addresses) to physical line or circuits. With E.164, each address is globally unique and with 15 digits maximum limit there can be 100 trillions combinations possible. A dial plan defines how calls are routed and interconnected in a telephony solution.
To understand a dial plan first we need to understand its components which are described below:
Endpoint Addressing (Numbering Plan) – To access internal/external destination each endpoint in a voice network is assigned a directory number. Endpoint may include – IP phones, fax machines, analog phones) and applications such as Instant messenger client, voice mail systems, auto attendants, IVRs and so on.
Call Routing and Path Selection – Different paths can be defined to reach a final destination, so that in the event of failure a redundant path can be taken. A voice gateway is used for call routing and path selection depending on choice of protocol.
Digit Manipulation – At times it is required to dialed string need to be manipulated before routing. For example a call from a PSTN uses the DID number as 4085557001 and internal call pattern is 7001, thus the 408555 needs to be stripped off to route a call.
Calling Privileges – Access privileges can be assigned to grant or deny the access to different group of devices such as phones in visitor areas have restricted access only limited to internal/ local PSTN destinations. In PBX systems this service is referred as class of service (CoS), which defines the destinations which can be dialed.
Call Coverage – For certain service requirements like circular hunt, longest idle or broadcast special groups of devices can be created to handle incoming calls so that calls don’t dropped without answering. A common example of this is a user trying to reach other user or to a voicemail box when intended user do not answer the call. Also for customer services requirements a set of numbers could be associated with customer care representatives or helpdesk, so that incoming calls can be distributed to assigned phone numbers.
To implement scalable and complex dial plans certain attributes needs to be adhered do as described below:
Dial Plan Logic Distribution – A good and logical dial plan is configured in a manner that each specific requirement of task processing should be handled by specific set of devices like local switch / gateway is dedicated to handle details specific to local point of presence (POP), higher level routing decisions are handled by gatekeeper / PBXs.
Hierarchical Numbering Plan – An ideal dial plan is hierarchical in nature to avoid overlapping of number ranges. It makes it easier to add/ delete numbers and manageable.
Simplicity in Provisioning – Dial plan need to be simple, symmetrical and consistent in nature and use translation rules to manipulate local digit pattern. Use of standard digit format facilitates provisioning and dial-peer management in an easier manner.
Post Dial Delay – Post dial delay indicates the lag between the last digit dialed and ringing of phone at receiving end. Incase more routing rules, digit manipulations need to be looked at post dial delay becomes long, so number of dial peers and translation rules need to be less to reduce the delay. VPNs and NAT related processor concentrated functions also can impact the capability of gateway to process calls.
The ITU and Internet engineering task force (IETF) are working together on a new plan named as ENUM which will expand current E.164 to cover both analogue and digital devices such as systems and all other type of communication devices analogue phone sets, fax machines, IP phones, video terminals, wireless communications devices, modems and all VOIP devices.
This concludes the lesson topic on CCNA voice exam related to how numbering works in case of PSTN networks.