Telephone and Device Numbering

Each device within a network must have its own unique address. Some of the different types of addresses that are available include telephone numbers and data network addresses.

International Numbering Plan (ITU)
The International Telecommunications Union (ITU), a division of the United Nations, has defined a world numbering plan recommendation, “E.164.” The E.164 numbering plan defines the use of a country code (CC), national destination code (NDC), and subscriber number (SN) for telephone numbering. The CC consists of one, two or three digits. The first digit identifies the world zone. The number of digits used for telephone numbers throughout the world varies. However, no portion of a telephone number can exceed 15 digits. There are several “E” series of ITU numbering recommendations that assist in providing unique identifying numbers for telephone devices around the world.

North American Numbering Plan (NANP)
An 11 digit-dialing plan is used within North America. It contains 5 parts: international code, optional intersystem code (1 +), geographic numbering plan area (NPA), central office code (NXX), and station number (XXXX). The NPA code defines a geographic area for the serving telephone system (such as a city). The NXX defines a particular switch that is located within the telephone system. Finally, the station code identifies a particular line (station) that the switch provides service to. Figure below shows the telephone numbering systems.



Internet and Data Network Numbering
Most data network addresses are hierarchical where the beginning of the address identifies the entire network and each progressive address number (or group of numbers) identifies more specific parts within the network.

Data networks are usually composed of several interconnected links. These links can be of different technologies with each of their end points identified by a unique numbering system. Figure 1.14 shows how different types of data network addressing systems can co-exist. This diagram shows a data connection that is composed of several parts. An end-user is connected to an application server through a company Ethernet network. The computers network interface card (NIC) has an address unique to the Ethernet hub. The Internet address is included as part of the data message after the Ethernet address. The company’s network is connected to an ISP by a high-speed frame relay connection. The frame relay access device (FRAD) has a unique identifier to the ISP. The ISP connects the data connection via asynchronous transfer mode (ATM) to the ASP.



Number Portability
Number portability involves the ability for a telephone number to be transferred between different service providers. This allows customers to change service providers without having to change telephone numbers. Number portability involves three key elements: local number portability, service portability and geographic portability.

The first part of the telephone number (NPA-NXX) usually identifies a specific geographic area and specific switch where the customer subscribes to telephone service. If a telephone number is assigned to another system (different NXX) in the same geographic area (same NPA), the interconnecting carriers (IXCs) connecting to that system must know which local system to route the calls based on the selected local service providers. In this case, the IXC must look up the local telephone number in a database (called a database dip) prior to delivering the call to the end customer.

Figure below shows an example of local number portability (LNP). In this diagram, a caller in Los Angeles is calling to someone in Chicago. The call is routed through the LEC in Los Angeles and routed through a long distance provider who needs to connect the call into a local telephone company in Chicago. Because there are several local providers in Chicago, the IXC must look into a database to see if the number has been ported to a different service provider. This is the next to last switch before the call reaches the end office switch (called “N-1”). This switch uses the dialed digits to find which local carrier is providing service in the Chicago area. When the IXC finds which exchange is serving the number, the call is routed to the correct local switching office and the call is completed.


example of local number portability

Service portability allows a customer to take their telephone number to a different type of service provider. Service portability involves determination of the type of service provider (e.g., wireless or wired) who is responsible for completing the call using the area code and NXX. The interconnection and call processing for different types of service providers varies.

Geographic portability involves the transfer of telephone numbers outside the normal geographic boundaries of the service provider’s area. Geographic portability allows a customer to keep their same area code when they move to new cities or other distant geographic regions...